New heterodont odontocetes from the Oligocene Pysht Formation in Washington State, U.S.A., and a reevaluation of Simocetidae (Cetacea, Odontoceti)

Odontocetes first appeared in the fossil record by the early Oligocene, and their early evolutionary history can provide clues as to how some of their unique adaptations, such as echolocation, evolved. Here, three new specimens from the early to late Oligocene Pysht Formation are described further increasing our understanding of the richness and diversity of early odontocetes, particularly for the North Pacific. Phylogenetic analysis shows that the new specimens are part of a more inclusive, redefined Simocetidae, which now includes Simocetus rayi, Olympicetus sp. 1, Olympicetus avitus, O. thalassodon sp. nov., and a large unnamed taxon (Simocetidae gen. et sp. A), all part of a North Pacific clade that represents one of the earliest diverging groups of odontocetes. Amongst these, Olympicetus thalassodon sp. nov. represents one of the best known simocetids, offering new information on the cranial and dental morphology of early odontocetes. Furthermore, the inclusion of CCNHM 1000, here considered to represent a neonate of Olympicetus sp., as part of the Simocetidae, suggests that members of this group may not have had the capability of ultrasonic hearing, at least during their early ontogenetic stages. Based on the new specimens, the dentition of simocetids is interpreted as being plesiomorphic, with a tooth count more akin to that of basilosaurids and early toothed mysticetes, while other features of the skull and hyoid suggest various forms of prey acquisition, including raptorial or combined feeding in Olympicetus spp., and suction feeding in Simocetus. Finally, body size estimates show that small to moderately large taxa are present in Simocetidae, with the largest taxon represented by Simocetidae gen. et sp. A with an estimated body length of 3 m, which places it as the largest known simocetid, and amongst the largest Oligocene odontocetes. The new specimens described here add to a growing list of Oligocene marine tetrapods from the North Pacific, further promoting faunistic comparisons across other contemporaneous and younger assemblages, that will allow for an improved understanding of the evolution of marine faunas in the region.

In this work three additional specimens of stem odontocetes collected from the early to late Oligocene Pysht Formation of Washington State are described. The morphology of these new specimens shows similarities with Simocetus and Olympicetus and provides further insight into the diversity of early odontocetes in the North Pacific. In addition, cranial and dental features of simocetids hint at different modes of prey acquisition within members of the clade, with some taxa using suction feeding, while others being raptorial or combined feeders. The Pysht Fm. has a rich fossil record of marine tetrapods, including plotopterids (Olson, 1980;Dyke, Wang & Habib, 2011;Mayr & Goedert, 2016), desmostylians (Domning, Ray & McKenna, 1986;Ray, Domning & McKenna, 1994), aetiocetids Shipps, Peredo & Pyenson, 2019), stem mysticetes (Peredo & Uhen, 2016), pinnipeds (Everett, Deméré & Wyss, 2023) and many others still remaining to be described (Whitmore Jr & Sanders, 1977;Hunt Jr & Barnes, 1994;Barnes, Goedert & Furusawa, 2001;Marx et al., 2016b). The fossils described in this work demonstrate that stem odontocetes were more diverse in the North Pacific Region during the Oligocene and hint at the presence of clade of stem odontocetes that were geographically confined to this region in a pattern that parallels aetiocetid mysticetes (Hernández Cisneros & Vélez-Juarbe, 2021).

Phylogenetic analysis
The phylogenetic analysis was performed using the morphological matrix of Albright III, Sanders & Geisler (2018) as modified recently by Boessenecker et al. (2020), with modification of two characters and addition of four new ones (see Files S1-S2). Characters 328 and 329 are modified to be specific to the upper molars, while new characters 330 and 331 are related to the number of denticles on the mesial and distal edges, respectively, on the main lower molars. The third new character (c.337) refers to the presence of a transverse cleft on the apex of the zygomatic process of the squamosal (first noted by (Racicot et al., 2019), for CCNHM 1000). The fourth new character (c.338) relates to the morphology of the thyrohyoid/thyrohyal, adding up to a total of 338 characters (see Files S1-S2). Besides LACM 124104, LACM 124105 and LACM 158720, one additional odontocete from the Pysht Fm. was added, CCNHM 1000 (collected from the same locality as the specimens described here), based on the description from Racicot et al. (2019: S1). All otherwise undescribed specimens in earlier versions of this matrix were removed from this analysis because their character states cannot be independently corroborated, resulting in a total of three outgroup and 107 ingroup taxa. The matrix was analyzed using PAUP* (version 4.0a169;Swofford, 2003); all characters were treated as unordered and with equal weights. A heuristic search of 10,000 replicates was performed using the tree bisection-reconnection (TBR) algorithm and using a backbone constraint based on the phylogenetic tree of extant cetaceans from McGowen et al. (2020); bootstrap values were obtained by performing 10,000 replicates. The terminology used for the descriptions follows Mead & Fordyce (2009).

Taxonomy
The electronic version of this article in portable document format will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is LSIDurn:lsid:zoobank.org:pub:D190F6B6-FB67-4F2B-AC24-145DF06D3FD3. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central, and CLOCKSS. Abbreviations: as, alisphenoid; cp, coronoid process; eo, exoccipital; f, frontal; oc, occipital condyle; oi, optic infundibulum; pa, parietal; pp, paroccipital process of exoccipital; pt, pterygoid; smf, sternomastoid fossa; so, supraoccipital; sq, squamosal; zps, zygomatic process of squamosal.

Description
As preserved, the partial skull (LACM 124104; Figs. 1-4) has a pachyostotic appearance, in comparison with the other described simocetids. Based on the fused/closed sutures and heavily worn tooth, the specimen is considered to belong to an adult individual. The estimated bizygomatic width, 322 mm (c.335[2]), suggests a body length of around 3 m (based on equation ''i'' for stem Odontoceti from Pyenson & Sponberg, 2011), which is larger than any of the other described simocetids.  Vomer-Most of the palatal surface of the vomer is missing, as is much of the rostrum. Posteriorly, it seems to have been exposed ventrally along an elongated, diamond-shaped, window between the palatines and pterygoids as in other simocetids (Figs. 2C-2D;Fordyce, 2002;Vélez-Juarbe, 2017;see below).     , 2002;Vélez-Juarbe, 2017). An elongated groove along the ventrolateral end of the left palatine seems to have been part of the palatine foramen/canal.
Frontal-Only the posteriormost portions of the frontals are preserved, but they are eroded (Fig. 1). Dorsally, the interfrontal suture seems to have been completely fused, and it posteriorly formed a broad V-shaped contact with the parietals, which continues as a vertical contact along the temporal surface (Fig. 3). Parietal-As in other simocetids, the parietals are broadly exposed dorsally, and the interparietal is either absent or fused early in ontogeny (c.135[0], 136[1]; Fig. 1). The parietals do not extend anterolaterally, resembling Simocetus rayi, and differing from Olympicetus where the parietals extend into the base of the supraorbital processes. The . Ventrally, the postglenoid process is incompletely preserved, but seems to have been anteroposteriorly broad as in other simocetids. Posterior to the base of the postglenoid process, the external auditory meatus seems to have been broad (c.190[?0]; the posttympanic process is not preserved). The glenoid fossa is shallowly concave with nearly indistinct borders. Medial to the glenoid fossa is a shallow, oval tympanosquamosal recess ( Orbitosphenoid/Optic Infundibulum-The orbitosphenoid is exposed within the optic infundibulum where it is in contact with the parietal dorsally and palatine ventrally, and forms the dorsal, medial and ventral walls of the optic canal. A sulcus along the ventrolateral portion of the orbitosphenoid, close to its suture with the palatine, is likely the groove for the maxillary nerve (V2). Anteromedially, the bones are eroded, while more posteriorly they are obscured by sediment; therefore additional features of the optic infundibulum cannot be properly interpreted.
Mandible-The mandible is missing for the most part, with the exception of the left coronoid process (Fig. 1). The process has a subtriangular outline, as preserved being about as long as high, with the dorsal edge slightly recurved medially. The general outline resembles the coronoid process of Olympicetus avitus (Vélez-Juarbe, 2017). Dentition-Only a double-rooted upper right molariform tooth is preserved in association with the specimen (Figs. 5A-5C). The mesial root is mostly missing, but seems to have been buccolingually broader than the distal root, which is more cylindrical and slightly recurved buccally. The crown (mesiodistal length = 10 mm; height = seven mm; maximum buccolingual width = 8 mm) is worn, is longer than tall, and is buccolingually broader on its anterior half due to the presence of a lingual bulge, somewhat resembling tooth 'mo3' of Olympicetus avitus ( Fig. S1E ; Vélez-Juarbe, 2017), but differing by lacking a well-defined secondary carina with denticles. The crown has three denticles, with the apical one slightly larger than the two on the distal carina, but there are no denticles on the blunter, mesial carina (Figs. 5A-5C). There is no buccal cingulum, and only a nearly inconspicuous cingulum occurs on the distolingual corner of the base of the crown. The outline of the crown, as well as the presence of a buccolingually broad mesial root, or alternatively a third, lingual root, is similar to the condition observed in the P4 of Simocetus rayi, and is tentatively assigned to that position (Fordyce, 2002).  (Fig. 5E). The ventral arch has a more prominent hypapophysis than that observed in Olympicetus spp. (Fig. 5E). The base of the transverse processes flares posterolaterally. The axis is missing most of the apex and left half of the dorsal arch, as well as the left transverse process (Figs. 5F-5G). The pedicle is anteroposteriorly broad and flattened transversely. The postzygapophysis is oriented posterolateroventrally, forming a flat, smooth surface (Fig. 5G). The anterior articular surface is broad, with a suboval outline, and raised edges; the surface is shallowly concave, merging ventromedially with the ventral surface of the odontoid process (Fig. 5F). The odontoid process is short, broad and blunt, with a mid-dorsal ridge that extends along the dorsal surface of the centrum, reaching the distal end (Fig. 5F). Posteriorly, the centrum has a cardiform outline. The epiphysis is fused, and its surface is concave, with a mid-ventral cleft that slightly bifurcates towards its posteroventral end. The ventral surface of the centrum has a mid-ventral keel that becomes broader and more prominent towards the posterior end of the centrum. The transverse process is anteroposteriorly flat and oriented mainly laterally. There are no transverse foramina (Figs. 5F-5G).
The third cervical preserves only a portion of the right side of the neural arch; the pedicle is anteroposteriorly flattened and transversely broad. Both anterior and posterior epiphyses are fused (Figs. 5H-5I). The prezygapophysis consists of a rounded, flat surface that is oriented anterodorsomedially, complementing its counterpart in the axis. The transverse foramen is large, being slightly broader than tall (16 mm × 11 mm). The transverse process is mainly oriented laterally; its posterior surface forms a low keel that extends from the base to the apex, and its anteroventral edge is flared (Fig. 5I). The centrum is rounded, anteroposteriorly short, with shallowly concave proximal and distal articular surfaces. Low midline keels are present along the ventral and dorsal surfaces of the centrum. A pair of small (∼4 mm) nutrient foramina occur on each side of the mid-dorsal keel. Remarks-LACM 124104 represents the largest known simocetid, with an estimated bizygomatic width of 322 mm, in comparison with that of Simocetus rayi (238 mm), which (using equation ''i'' from from Pyenson & Sponberg, 2011)  ). This specimen does preserve a remarkable amount of details of the size and morphology of the pterygoid sinus fossa, which together with other simocetids, suggest that they had well developed, large fossae, particularly when compared to those of other early diverging odontocetes, such as Archaeodelphis patrius, which seems to have much shorter fossae (LACM 149261, cast of type). LACM 124104 resembles, and may be congeneric with an odontocete skull from the early Oligocene Lincoln Creek Formation of Washington State, briefly described by Barnes, Goedert & Furusawa (2001), sharing many characters of its morphology, including its large size (bizygomatic width = 265 mm) and the pachyostotic appearance of some of the cranial bones; this will be addressed in more detail in a follow-up study.
Posterior to this surface, the frontals are shallowly depressed towards their contact with the parietals, forming a saddle-like outline of the skull roof in lateral view, resembling the condition observed in O. avitus (Fig. 8). The interfrontal suture is completely fused; dorsally the frontals form a broad, V-shaped contact with the parietals, whereas their contact along the temporal surface is nearly vertical. The supraorbital processes gently slope ventrolaterally from the midline (c.47[0]), and only their anterior half is covered by the ascending process of the maxillae (Figs. 6, 8). The preorbital processes are rounded and only partially covered by the maxillae and are thus exposed dorsally; anteriorly they contact the maxillae and anteroventrally the lacrimals. The postorbital process is blunt, long, and oriented  (Fig. 10C), resembling the condition observed in Simocetus rayi, Archaeodelphis patrius and basilosaurids (Allen, 1921;Luo & Gingerich, 1999;Fordyce, 2002;Uhen, 2004). In posterior view, the squamosal has a relatively narrow exposure lateral to the exoccipitals (c.146[1]; Figs. 7A-7B). Pterygoid-In ventral view, the pterygoids form robust, cylindrical hamular processes that are not excavated by the pterygoid sinuses (c.173[1], 174[0]) and are separated anteriorly along the midline by a diamond-shaped exposure of the vomer, resembling the condition observed in Simocetus rayi ( Fig. 7; Fordyce, 2002:fig: 4) Alisphenoid-Only small portions of the alisphenoid can be observed on both sides. In lateral view, only a small portion of the alisphenoid is exposed on the temporal fossa, where it forms the posteromedial part of the subtemporal crest (c.142[1], 166[0]) as in other Olympicetus (Vélez-Juarbe, 2017; see below). Orbitosphenoid/Optic Infundibulum-The orbitosphenoid is fused with surrounding bones, unlike the ontogenetically younger specimen of Olympicetus avitus. Within the optic infundibulum, the foramen rotundum and orbital fissure seem to have a similar diameter, both being transversely broader (∼10 mm) than high (∼6 mm) (Fig. 9), with the first located in a slightly more posteromedial position, resembling the condition in O. avitus (Fig. 9). However, no distinct groove for the ophthalmic artery is preserved in Olympicetus thalassodon, differing from Simocetus rayi, O. avitus and Olympicetus sp. 1 (Fordyce, 2002:fig.13; Figs. 8-9). The foramen rotundum opens ventrolateral to the orbital fissure, with the path for the maxillary nerve (V2) being bound ventrally by the pterygoid and palatine (Fig. 9). Periotic-Only a small portion is visible on the right side. The anterior process contacts the falciform process anteriorly for about half its length. Posterior to this contact, a portion of the anterior process is visible, as is the epitympanic hiatus, which is bounded posteriorly by a prominent ventrolateral tuberosity (Fig. 10C). Tympanic Bulla-Both bullae are still articulated with the cranium and mainly visible in ventral view (Fig. 10) (Fordyce, 2002). Proximally, the pan bone region is transversely thin and likely formed an enlarged mandibular fossa (c.44[1]). Posterodorsally on the right side, the lateral edge of the condyle can be observed, suggesting that its dorsal surface sits at the level of, or below, the alveolar row (c.46[1]; Fig. 8). Anteriorly, the right ramus preserves five double-rooted teeth in-situ, which are interpreted as representing p3-4 and m1-3, whereas the left ramus preserves three teeth that are interpreted as m1-2 and p4 (11)(12). Multiple mental foramina are longitudinally arranged along the rami below the alveolar row; most are oval, ranging in size from 2 to 4 mm in height and up to 10 mm long, with the more posterior ones connected by a fissure as in Olympicetus avitus ( Fig. 8; Vélez-Juarbe, 2017:fig. 7A). Dentition-Taking a conservative approach to the tooth count, this specimen is interpreted as non-polydont as in Simocetus rayi (Fordyce, 2002), although incipient polydonty cannot be entirely ruled out, as it seems to be present on other simocetids from the eastern North Pacific (e.g., LACM 140702; Barnes, Goedert & Furusawa, 2001). Between the teeth and alveoli, the preserved upper and lower dentition is interpreted to represent C, P1-4, M1-2 and p3-4, m1-3 (Figs. 8-9, 11-12). No conspicuous signs of tooth wear are observed in either upper or lower teeth, similar to the condition observed in Olympicetus avitus, and differing from that in Simocetus rayi, which shows signs of apical wear (Fordyce, 2002). In double-rooted teeth, the roots become fused proximally, with broad grooves on both buccal and lingual sides that extend to the base of the crown, giving it an 8-shaped cross section as in Simocetus rayi (Fordyce, 2002). In P4 and M1 the mesial root is cylindrical, tapering distally, whereas the distal root is buccolingually broader and oblong in cross section. In M2 this condition is reversed, with the mesial root being transversely broader; mesial and distal roots of the lower teeth seem to be subequal in size, both being cylindrical and tapering distally.
The anteriormost end of the right maxilla has a single alveolus (diameter = 6 mm) that curves posterodorsally and is interpreted as that of a canine, which is separated by a short interalveolar septum from two adjoining alveoli (each with a diameter ∼7 mm) for a double-rooted P1 (Fig. 8, 11B). The second (P2) and third (P3) upper premolars are missing on the left side and incompletely preserved on the right; they are slightly higher than long, consisting of a main denticle with at least two accessory denticles on the mesial and distal edges, resembling teeth 'ap1' and ap2' of O. avitus ( Fig. S1; Vélez-Juarbe, 2017: fig. 7D-E, Q-R). Three closely associated teeth that became disarticulated from the maxilla are still joined by matrix, and along with three other loose teeth represent left and right P4, M1-2; these have more equilateral crowns, being nearly as long as wide, with stronger lingual and labial cingula and ornamentation along the base of the crowns; the crowns of P4 and M1 consist of a main apical denticle, with four distal and three mesial accessory denticles that diminish in size towards the base (c.328[1], 329[2]; Figs. 11E-11H, 12A-12B, 12E-12F). Their overall morphology resembles that of teeth 'mo1' and 'mo2' of Olympicetus avitus (Fig. S1; Vélez-Juarbe, 2017; fig.7M-N, Z-Aa). The second molar (M2) is the smallest of the series, and the crown is longer than tall. Its crown consists of a main apical denticle, four distal and two mesial accessory denticles, with the apices of all denticles slightly slanted distally (Figs. 11D, 11I, 12C-12D). As in Simocetus rayi and Xenorophus sloanii, the mesial and distal carinae on the upper posterior postcanines trend towards the buccal side of the teeth so that in occlusal view, the apical and accessory denticles are arranged in an arch (Fordyce, 2002;Uhen, 2008). These characteristics and other features discussed below allow for the reassignment of some of the teeth of Olympicetus avitus, with teeth 'mo1' and 'mo2' representing right and left M2, respectively, whereas 'ap1' and 'ap2' represent left upper premolars ( Fig. S1; Vélez-Juarbe, 2017: fig.7). An isolated single-rooted tooth is interpreted as an upper canine or incisor (Figs. 12H, 12I). The crown is conical, with vertical striation along its lingual surface and a buccal cingulum; mesial and distal carinae seem to be present, with larger denticles along the distal carina.
The preserved lower dentition includes p3-4, m1-3, and p4, m1-2 on the right and left mandibles, respectively (Fig. 8, 11A-11C, 12C). As with the upper premolars, p3-4, m1-3 have a triangular outline of the crown in buccal or lingual views; in occlusal view the mesial and distal carinae do not trend buccally as opposed to the upper molars. Furthermore, in p3-4 and m1-2 the mesial carina has two accessory denticles (c.330[2]) that are much smaller than the apical denticle, whereas three to four accessory denticles occur along the distal carina (c.331[4]), with the apical ones being nearly as large as the apical denticle, and then diminishing in size towards the base of the crown (Fig. 8, 11A-11C, 12C). The buccal sides of the lower premolars and molars are unornamented, with only a few inconspicuous vertical striae but no prominent cingulum, while lingually striae are more prevalent, and a cingulum is present (Figs. 11A-11C, 12G). As in the upper toothrow, the last tooth, in this case m3, is the smallest in the series, seemingly lacking accessory denticles on the mesial carina and having three subequal denticles along the distal carina. As with the preceding teeth, ornamentation is nearly absent on the buccal side (Fig. 11A). An isolated tooth adjacent to the posterior end of the left maxilla and mandible may represent the left m3 (Fig. 12J). This tooth resembles the right m3, but its mesial carina is partially damaged, so it is unclear if any accessory denticles were present; its distal carina contains three denticles that diminish in size basally. The lower postcanine dentition of Olympicetus thalassodon appears to be characterized by having less conspicuous ornamentation on the buccal side, and more vertically aligned carinae. Based on these characteristics the lower dentition of Olympicetus avitus is reinterpreted as follows: teeth 'pp1-4' represent left p3-m2, while 'pp5', 'pp7', and 'pp6' represent p3, p4, and m1 from the right side ( Fig. S1; see also Vélez-Juarbe, 2017: fig.7F-G, J, L, S-T, W, Y). Hyoid-Most of the hyoid elements are preserved in LACM 158720, including the basihyal, stylohyals and thyrohyals (Figs. 13A-13C). The basihyal has a rectangular, blocky outline, with both lateral ends expanded, forming broad, quadrangular rugose surfaces for the articulation of the paired elements (stylo-and thyrohyals). The mid portion is subtriangular in cross-section, and the dorsal surface is shallowly concave transversely. The partial left thyrohyal obscures the posteroventral surface of the bone. The partial left and the complete right thyrohyals and stylohyals are preserved (Figs. 13A-13C). The thyrohyals are not fused to the basihyal and are fairly straight, with a transversely oval cross section at mid-length; overall they are shorter but more robust than the stylohyals, and not flattened, wing-like as in extant mysticetes and odontocetes (c.338[0]; Fig. 13). The proximal articular surface has a rectangular outline, and the surface is rugose and shallowly convex. Distally, the shaft is twisted, so that the distal articular surface is nearly perpendicular to the long axis of the proximal surface. The distal articular surface has a more oval outline that is rugose and shallowly convex. The stylohyals are long and slender, and the right stylohyal is nearly in articulation with the paroccipital process (Figs. 13A-13B). Along the long axis they are bowed laterally, with the shaft having a more flattened, oval cross-section along its length, with both, proximal and distal ends expanded, being overall, nearly identical to the stylohyoid of Olympicetus avitus (Vélez-Juarbe, 2017). The proximal end is transversely expanded with a nearly flat, rugose articular surface. Distally, the shaft becomes twisted, so that the distal end is offset at about 45 • from the proximal articular surface. The lack of fusion between the thyrohyal and basihyal, and the cylindrical shape of the thyrohyal resembles the condition observed in basilosaurids (e.g., Dorudon atrox (Andrews, 1906), and Cynthiacetus peruvianus Martínez-Cáceres & de Muizon, 2011;Uhen, 2004;Martínez-Cáceres, Lambert & de Muizon, 2017) and some stem mysticetes (e.g., Mammalodon colliveri Pritchard, 1939, Fucaia buelli Marx, Tsai & Fordyce, 2015, and Mystacodon selenensis Lambert et al., 2017Fitzgerald, 2010;Muizon et al., 2019), whereas in more derived odontocetes (e.g., Brygmophyseter shigensis (Hirota & Barnes, 1995), Kogia breviceps (Blainville, 1838), Albireo whistleri Barnes, 1984, Kentriodon nakajimai Kimura & Hasegawa, 2019, and Tursiops truncatus (Montagu, 1821; Figs. 13D-13G) these bones are partially or completely fused, and the thyrohyals tend to be more flattened and plate-or wing-like (Reidenberg & Laitman, 1994;Hirota & Barnes, 1995;Barnes, 2008;Johnston & Berta, 2011;Kimura & Hasegawa, 2019).  Table 2). The dorsal arch of the atlas has a low, blunt mid-dorsal ridge that extends nearly the whole length of the arch. The vertebral foramen is broken, although it seems to have occupied the same position as that of Olympicetus avitus (Vélez-Juarbe, 2017). The anterior articular facets are obscured because the atlas is still attached to the skull, while the posterior facets have a reniform outline and form a dorsoventrally elongate, smooth, flat surface that extends dorsal to the articulation for the odontoid process (Fig.  14A). On the ventral arch, the hypapophysis that would have articulated with the odontoid process is short as in O. avitus and unlike the longer, more robust process of Simocetidae gen. et sp. A, and Echovenator sandersi (Churchill et al., 2016). The transverse processes are oriented slightly posterolaterally and are divided by a broad, rounded notch into a larger, more robust dorsal process and a smaller, knob-like ventral process (c.278[2]; Fig. 14A). The neural canal has an oval outline.
The axis is missing the dorsal arch. The odontoid process is short and blunt. The anterior articular surface has a subtriangular outline and is flat to shallowly concave, extending anteroventrally and being continuous with the ventral surface of the odontoid process (Fig.  14B). The transverse processes are oriented posterolaterally, with a triangular outline when viewed anteriorly. Their ventral surface is anteroposteriorly broad, forming a flat surface that faces ventrally and slightly posteriorly, with a sharp anterior edge (Figs. 14B-14D). Dorsomedially, the posterior surface of the transverse process forms a relatively deep, concave surface. Cervicals 3-6 are missing their dorsal arches and transverse processes for the most part, while only a small portion of C7 is preserved. The centra are anteroposteriorly flat and slightly wider than high; the epiphyses are unfused (Figs. 14C-14D). The right transverse process of C3 is partially preserved, and its morphology is similar to that of the axis.  (Table 3). Nevertheless, ontogenetic variation can be ruled out to explain this difference because odontocetes show precocial development of the tympanic bullae (Buffrénil, Dabin & Zylberberg, 2004;Lancaster et al., 2015). Other characteristics, such as the number of denticles in the carinae of upper and lower molars, can also be ruled out as resulting from ontogenetic or intraspecific variation. These taxa can further be differentiated from each other by morphological characters of the orbital region, such as the arrangement of the bones that form the dorsolateral edge of the ventral infraorbital foramen, the height of the orbit relative to the lateral edge of the rostrum, and the composition of the posterior wall of the antorbital notch.

Description
The description is based solely on LACM 124105 and will focus on morphological characters that differentiate it from Olympicetus avitus and O. thalassodon. As with the type of Olympicetus avitus, LACM 124105 seems to represent a subadult individual, showing some partially open sutures, such as the basisphenoid-presphenoid suture. Multiple areas of the skulls show evidence of erosion (e.g., rostrum, skull roof), likely as a result of wave action, because specimens from this locality are usually recovered as concretions along the beach. Premaxillae-Only part of the left ascending process of the premaxilla is preserved (Fig.  15). The ascending process borders the external nares as it ascends towards the vertex (c.74[0]); however, its incomplete preservation posterior to the nasals does not permit identification of its posteriormost extent. A relatively deep sulcus extends along its anterior border, which is consistent with the placement and morphology of the posterior extent of the posterolateral sulcus in Olympicetus avitus (c.73[2);Figs. 15 and 17;Vélez-Juarbe, 2017). Maxilla-Only part of the rostral portion of the maxilla is preserved (Figs. 15-18). Ventrally, the palatal surface is incompletely preserved along the midline and along the alveolar rows; however, the parts that are preserved indicate that it was transversely convex, with the alveolar rows slightly more elevated dorsally (Fig. 17). Posteriorly, the contact between the maxillae and palatines seems to have been triangular to anteriorly bowed (c.20[?0], 21[1]; Fig. 16) as in other Olympicetus. The alveolar rows, although incompletely  Abbreviations: a.ps, alveoli for postcanine teeth; as, alisphenoid; bo, basioccipital; boc, basioccipital crest; bs, basisphenoid; ef, ethmoid foramen; ffdv, foramina for frontal diploic veins; insphs, intersphenoidal synchondrosis; j, jugal; la, lacrimal; mx, maxilla; pa, parietal; pf, periotic fossa; pl, palatine; pmx, premaxilla, pop, postorbital process; psf, pterygoid sinus fossa; pt, pterygoid; vo, vomer; zps, zygomatic process of squamosal.
Full-size DOI: 10.7717/peerj.15576/ fig-16 Vomer-The vomer is mostly missing anterior to the antorbital notches and eroded anteroventrally; nevertheless, it is evident that it formed the lateral and ventral surfaces of the mesorostral canal. Ventrally, the vomer likely was exposed through a diamond-shaped window towards the posterior end of the palate as in other simocetids (Fig. 16). Dorsal and posterodorsal to this point the vomer forms the nasal septum, forming the medial walls of the choanae. From the posterior palatal exposure, the vomer gently slopes posterodorsally to form a triangular, horizontal plate extending over the still open, basisphenoid-presphenoid suture, but not reaching as far posterior as the fused basisphenoid/basioccipital contact (c.191[0]; Fig. 16). The horizontal plate of the vomer contacts the dorsal laminae of the pterygoids along its anterolateral ends (Figs. 16-18). Palatine-Only some very small fragments of the right palatine are preserved. Posterodorsally, a fragment of lateral surface of the palatine reaches the frontal, forming part of the infundibulum for the sphenopalatine and infraorbital foramina as well as the posterior border of a round (∼5 mm diameter) sphenopalatine foramen (Fig. 18). The infundibulum has an oval outline, being broader than high (20 mm × 10 mm), and is bounded dorsally by the frontal and lacrimal, and the maxilla ventrally and ventrolaterally (Fig. 18). Nasal-Although incompletely preserved, the nasals seem to have been the highest point of the vertex, were longer than wide and dorsoventrally thin, as in other simocetids (c.114[0], . Along their posterior borders, the nasals are separated by the narrow, narial processes of the frontals (Fig. 15). The anterior edges of the nasals are incompletely preserved, but extended far forward of the anterior edge of the supraorbital processes, whereas posteriorly it seems that they reach a level in line with the anterior edge of the supraorbital processes (c.81[3], 123[0]; Fig. 15). Frontal-As in other Olympicetus, a wedge-shaped exposure of the frontals occurs along the midline, surrounded by the maxillae laterally and nasals anteriorly, although poor preservation of the surrounding bones does not allow precise determination of the size of this exposure relative to the nasals (Fig. 15). Along the midline, the bone is poorly preserved, although it does seem that the frontals are lower than the nasals, preserving the saddle-like profile (in lateral view) seen in other species of Olympicetus. Posteriorly, the frontal-parietal suture seems to have been broadly V-shaped dorsally, and sinusoidal in the temporal region, with no extension of the parietals into the supraorbital processes. Laterally, the supraorbital processes slope very gently ventrolaterally (c.47[?0]; Fig. 17). Dorsally, the maxillae only partially cover the supraorbital processes, leaving the preorbital and postorbital processes broadly exposed dorsally (Fig. 15). Anteroventrally, the preorbital Figure 18 Ventrolateral view of skull of Olympicetus sp. 1 (LACM 124105). Unlabeled (A) and labeled (B) skull in right ventrolateral view focusing on the features of the orbital region. Diagonal lines denote broken surfaces, gray shaded areas are obscured by sediment. Abbreviations: a.ps, alveoli for postcanine teeth; adif, anterior dorsal infraorbital foramina; as, alisphenoid; boc, basioccipital crest; ef, ethmoid foramen; ffdv, foramina for frontal diploic veins; f, frontal; j, jugal; la, lacrimal; mx, maxilla; oa, path for ophthalmic artery; oi, optic infundibulum; pa, parietal; pl, palatine; psf, pterygoid sinus fossa; pt, pterygoid; spf, sphenopalatine foramen; viof, ventral infraorbital foramen; V2, path for maxillary nerve; vo, vomer; zps, zygomatic process of squamosal. Full-size DOI: 10.7717/peerj.15576/ fig-18 process contacts the lacrimal. The postorbital processes are incompletely preserved, but seem to have been relatively short, robust, and oriented posteroventrolaterally (Figs. 15 and 17). In lateral view the dorsal edge of the orbit is highly arched but positioned at a lower position (c.48[1]; Fig. 17) relative to the lateral edge of the rostrum than is observed in Olympicetus avitus or O. thalassodon. A low and sharp temporal crest extends anterolaterally from near the frontal/parietal suture and into the posterodorsal and dorsal surface of the supraorbital process (c.132[2]; Fig. 15), differing from the condition in other Olympicetus.
Ventrally, the frontal contacts the lacrimal anteroventrally and the maxilla and/or palatine more medially, resulting in the frontal forming part of the posterodorsal edge of the infundibulum for the ventral infraorbital and sphenopalatine foramina (Figs. 16 and  18). The optic foramen is partially covered by sediment; its general orientation seems to be anterolateral, with its posterior border being defined by a low, but sharp infratemporal crest (c.63[0]). Similar to other simocetids, a small (∼3 mm diameter) ethmoid foramen is anterolateral to the optic foramen and is accompanied by four to five smaller (1-2 mm) foramina located along the dorsolateral roof of the orbit (Figs. 16 and 18). Lacrimal + Jugal-Only a small portion of the jugal is preserved, but it is evident that it was not fused with the lacrimal (c.54[0], 55[0]; Figs. 17-18). The portion of the jugal that is preserved is stout and cylindrical, tapering medially and wedged between the lacrimal and maxilla, which excludes it from forming part of the ventral infraorbital foramen . The lacrimal is large, and rod-like, broadly visible in dorsal and lateral views, but with a proportionately small ventral exposure (c.51[1], 56[0]). It contacts the preorbital process of the frontal anteroventrally, tapering medially, and seems to have been exposed anteriorly, forming part of the posterior wall of the antorbital notch but not extending dorsally onto the supraorbital process (c.52[0]; Fig. 15, 17-18). Parietal-The parietals are exposed dorsally but badly eroded (c.135[0], 136[?]; Fig. 15). The parietals contact the frontals along a broad, V-shaped suture, but differ from the condition seen in other species of Olympicetus in that they do not extend into the base of the supraorbital processes. In cross section through the intertemporal region, the parietals seem to have an ovoid outline (c.137[?1]), resembling the condition in Olympicetus avitus. Along the temporal surface the parietal becomes more inflated posteriorly towards its contact with the squamosal and alisphenoid (Figs. 17-18). Ventrally, the parietal has an internal projection that contacts the squamosal medial to the periotic fossa, constricting the cranial hiatus as in other simocetids (c.184[2]; Fig. 16). Supraoccipital-The supraoccipital is only partially preserved, with the exception of its dorsolateral borders. The nuchal crests are sharp, directed dorsolaterally, and only slightly overhanging the temporal fossae (c.154[1]; Fig. 15), and curving posteroventrally to join the supramastoid crests of the squamosals. Exoccipital-The exoccipital is poorly preserved. Dorsal to the remaining parts of the right occipital condyle is what seems to be a shallow dorsal condyloid fossa (c.157[?1]). The surface lateral to the condyles is flat to shallowly convex. Basioccipital-As preserved, the basioccipital crests seem to have been relatively thick transversely (c.192[?1]) and oriented posterolaterally, at about an angle of 45 degrees (c.195[3] ; Fig. 16). The rest of the ventral surface is incompletely preserved. Squamosal-The zygomatic processes are incompletely preserved. Posteromedially, the sternomastoid fossa forms a distinct emargination that is overhung dorsally by the supramastoid crest, much more than in Olympicetus avitus (c.145[1]; Fig. 15). The supramastoid crest seems to have been continuous with the nuchal crest (c.150[0]; Fig. 17). The squamosal plate contacts the parietal along an anteroventrally sloping interdigitated suture, meeting the alisphenoid to form part of the subtemporal crest (Fig. 17). Ventrally, the squamosal is heavily eroded and only a small portion of the periotic fossa is preserved, where it contacts the medial extension of the parietal (Fig. 16). Pterygoid-Most of the pterygoid is missing on both sides of the skull. A portion of the dorsal lamina extends posterodorsally towards the parietal and contributes to the posteroventral edge of the optic infundibulum as in Olympicetus avitus (Figs. 17-18). As preserved, the pterygoid sinus fossa is anteroposteriorly longer than wide and is located entirely anterior to the foramen ovale (c.164[2], 169[0]; Figs. 16 and 18). Alisphenoid-As seen in Olympicetus avitus, the alisphenoid forms the posterodorsal surface of the pterygoid sinus fossa (Figs. 16 and 18). The medial and posterior ends of the bone are incompletely preserved or eroded on both sides, making it difficult to determine the position of the alisphenoid-squamosal suture or the path of the mandibular nerve (V3). On the temporal wall, the exposure of the alisphenoid is limited to a small sliver, because it is mostly overlapped by the parietal and the squamosal (c.142[1]; Figs. 17-18). Basisphenoid-Posteriorly the basisphenoid is fused with the basioccipital, and anteriorly its suture to the presphenoid (sphenoidal synchondrosis) is still open, resembling the growth stage of the type of Olympicetus avitus (Vélez-Juarbe, 2017). The ventral surface is flat and covered by the horizontal plate of the vomer (Fig. 16). Optic Infundibulum-The optic infundibulum is a slightly sinusoidal opening bounded by the frontal anteriorly and dorsally, parietal posteriorly, pterygoid ventrally and anteroventrally (Fig. 18). The optic foramen, orbital fissure and foramen rotundum are still partly covered by sediment. The frontal forms most of the borders of the optic foramen anterodorsally, whereas posteroventrally the foramen rotundum was bounded laterally by the parietal and floored by the pterygoid. The anteroventral edge of the parietal that forms part of the infundibulum has a narrow groove that trends anterodorsally and would have carried the ophthalmic artery, resembling the condition in Simocetus rayi and Olympicetus avitus (Fig. 18;Fordyce, 2002;Vélez-Juarbe, 2017). Along the ventral edge of the infundibulum, the pterygoid has a distinct but shallow groove that would have presumably carried the maxillary nerve (V2), extending along its dorsolateral surface and diverging slightly over its lateral surface anteriorly (Fig. 18). Malleus-The left malleus is still attached with the corresponding tympanic (Fig. 19). The head has a semicircular outline, with paired facets for articulation with the incus that are oriented at about 90 degrees to each other; the more anterior facet is about twice as large as the posterior one, as in Olympicetus avitus ( Fig. 19; Vélez-Juarbe, 2017). The tubercule is relatively large, nearly as long as the head (c.199[0]; Fig. 19). The manubrium is prominent, with its apex forming a slightly recurved muscular process (Fig. 19). The anterior process is fused laterally to the tympanic, dorsally forming a continuous surface with the mallear ridge. Meanwhile, the ventral edge of the anterior process is shelf-like and together with the mallear ridge forms a deep, narrow sulcus for the chorda tympani (Figs. 19A,19C,19E). Tympanic Bulla-Only the left tympanic bulla is preserved (Fig. 19) but missing its posterior process. Overall it closely resembles in size and morphology that of Olympicetus avitus (Vélez-Juarbe, 2017). In dorsal or ventral view, the bulla has a heart-shaped outline, being relatively short and wide (c.252[1]), unlike the larger and transversely narrower bulla of Olympicetus thalassodon (Figs. 10, 19). The lateral surface of the tympanic bulla is broadly Abbreviations: ac, anterodorsal crest; ap, anterior process; cp, conical process; ef, elliptical foramen; fi, facet for incus; hm, head of malleus; in, involucrum; ipp, inner posterior prominence; ippe, inner posterior pedicle; lf, lateral furrow; mn, manubrium; mp, muscular process; mr, mallear ridge; ol, outer lip; opp, outer posterior prominence; sc, sigmoid cleft; sct, sulcus for chorda tympani; sp, sigmoid process; tc, tympanic cavity; tr, transverse ridge.
Full-size DOI: 10.7717/peerj.15576/ fig-19 convex, whereas the medial surface is straight; the posterior prominences give the bulla a bilobed outline posteriorly, but anteriorly, the lateral surface converges medially more steeply than the medial surface along a smooth curve. There is no indication of the presence of an anterior spine (c.251[0]). Posteriorly, a broad interprominential notch extends from the level below the elliptical foramen, continuing along the ventral surface of the bulla as a short, shallow median furrow for only about a third of its length (c.267[0]). The   Fig. 19B). Dorsally, the sigmoid process is vertical and perpendicular to the long axis of the bulla (c.259[0]), with its posterior edge curving anteriorly along a smooth curve (c.260[0]). The mallear ridge extends obliquely from the anteromedial base of the sigmoid process towards the dorsalmost extension of the lateral furrow. A narrow, dorsally open sulcus for the chorda tympani extends anteriorly for a length of 17 mm along the dorsomedial edge of the outer lip, originating at the junction between the anterior process of the malleus and the mallear ridge (Figs. 19A,19C,19E). The anterodorsal crest descends steeply towards the anterior edge of the bulla.
In In lateral view, at the base of the anterior process is a shallow, C-shaped sulcus that begins near the anteroventral edge, curves posteroventrally towards the lateral tuberosity, then curves anterodorsally; it is interpreted as a combined anteroexternal+parabullary sulcus (sensu Tanaka & Fordyce, 2014;Figs. 20G-20H). This condition resembles that of other early odontocetes such as Waipatia maerewhenua Fordyce, 1994, andNotocetus vanbenedeni Moreno, 1892, but differs from others like Otekaikea marplesi (Dickson, 1964) where these sulci are separate, and from the much deeper sulcus in Papahu taitapu Aguirre-Fernández & Fordyce, 2014(Tanaka & Fordyce, 2014Viglino et al., 2022). In cross-section, the anterior process is ovoid, being dorsoventrally taller (∼14 mm) than mediolaterally wide (∼9 mm) (c.209[1]). The anterior part of the ventral surface of the anterior process has as well-defined anterior bullar facet (c.210[3];. Posterior to the anterior bullar facet, the fovea epitubaria forms a smooth curve that is interrupted by a prominent lateral (ventrolateral) tuberosity (c.212[1]). The lateral tuberosity has a triangular outline in ventral view but does not extend as far laterally as in other stem odontocetes such as Cotylocara macei (Geisler, Colbert & Carew, 2014), being instead barely visible in dorsal view. A broadly arched epitympanic hiatus lies posterior to the lateral tuberosity and anterior to the base of the posterior process (c.213 [1]). Posteromedial to the epitympanic hiatus, is a small (diameter: ∼2 mm) rounded fossa incudis, while anterior to it and medial to the lateral tuberosity is a broad (diameter: ∼6 mm), circular mallear fossa (c.214[1],215[0];. The lateral surface of the periotic is generally smooth with the exception of the posterior process, whose lateral surface is rugose (c.217[2]; Figs. 20G-20H). Medially, the anterior process is separated from the cochlea by a well-defined groove (anterior incisure, sensu Mead & Fordyce, 2009) that extends anterodorsally, and marks the origin for the tensor tympani muscle (c.218[1]).
In dorsal view, a low crest delimits laterally the dorsal surface of the periotic; it extends from the low pyramidal process towards the anterodorsal spine of the anterior process . Medial to this crest is an elongated depression, the suprameatal fossa, which is about 13.5 mm long by 7 mm wide, and around 1.5 mm deep . The fundus of the internal acoustic meatus is funnel-shaped, with an oval outline, delimited by a low ridge (c.235[0]; 236[0]). The area cribrosa media (sensu Mead & Fordyce, 2009;Orliac et al., 2020;= inferior vestibular area of Ichishima, Kawabe & Sawamura, 2021) and the spiral cribiform tract are separated by a very low ridge, these two are in turn separated from the area cribrosa superior (previously called the foramen singulare, Orliac et al., 2020; = superior vestibular area of Ichishima, Kawabe & Sawamura, 2021) by a low transverse crest that lies about 3 mm below the upraised rim of the internal acoustic meatus, while it is separated from the dorsal opening of the facial canal by a ridge that is slightly lower (∼4 mm from the edge of the rim) (c.237[2]; Figs. 20A-20B). The proximal opening of the facial canal has an oval outline and is located anterolateral to the spiral cribriform tract (c.238[0], 239[1]). Anterodorsally it is bridged, forming a ''second'' foramen, which is smaller and rounded (Figs. 20A-20D), resembling the condition observed in other early odontocetes such as Waipatia maerewhenua, and similarly, is interpreted as the foramen for the greater petrosal nerve (Fordyce, 1994). The aperture for the endolymphatic duct (vestibular aqueduct) is slit-like (∼4 mm long by 1 mm wide) and located posterolateral to the internal acoustic meatus, just below the more vertical posterior surface of the pyramidal process and separated from the fenestra rotunda by a very wide distance (c.230[3]; Figs. 20A-20D). In contrast, the aperture for the perilymphatic duct (cochlear aqueduct) is rounded (diameter = 3 mm) and located posteromedial to the internal acoustic meatus and medial to the aperture for the endolymphatic duct, and broadly separated from the fenestra rotunda (c.228[1], 229[2]). A small, curved depression posteroventral to the aperture for the endolymphatic duct is interpreted as a shallow stylomastoid fossa (c.225[1]). The dorsomedial surface of the cochlear portion has a shallow depression that accentuates the raised medial rim of the internal acoustic meatus. In medial view, the cochlea is dorsoventrally thin (maximum height ∼11 mm), its ventromedial surface is anteroposteriorly convex, and a low, faint ridge extends along its ventrolateral end (c.221[0];. In ventral view, the cochlear portion has a subrectangular outline (c.219[1], 220[1], 222[1]). Posteriorly, the fenestra rotunda is located towards the lower half of the posterior surface, and it is wider than high (4 × 2 mm), with a kidney-shaped outline (c.223[0]). Posterolateral to the fenestra rotunda, the lateral caudal tympanic process projects farther posteriorly than the rest of the posterior surface of the cochlea, although it is not as prominent as that of other simocetids (i.e., CCNHM 1000;Racicot et al., 2019). Its ventral and posterior borders intersect along a curved edge (c.226[1];. Ventrally, the fenestra ovalis is longer than wide (4 × 3 mm) and located towards the posterior half of the cochlea. The ventral opening of the facial canal (∼2 mm in diameter) is lateral to the fenestra ovalis and is separated by a sharp crest.  Fig. 15). These differences are considered to be species-related, and not the result of ontogenetic change as this specimen shows a similar growth stage as the type of Olympicetus avitus (LACM 149156;Vélez-Juarbe, 2017). Nevertheless, because of its incomplete preservation, it is preferably left in open nomenclature until better material belonging to this taxon is identified.  Kasuya, 1973;Racicot et al., 2019).

Maximum length 43
Proximal dorsoventral thickness of anterior process 12 Length of anterior process 16 Transverse width of anterior process at mid-length 9 Dorsoventral height of anterior process at mid-length 13 Maximum width of periotic 22 Least distance between fundus of internal auditory meatus and aperture for endolymphatic foramen 2 Least distance between fundus of internal auditory meatus and aperture for perilymphatic foramen 3 Least distance between fenestra rotunda and endolymphatic foramen 7 Least distance between fenestra rotunda and perilymphatic foramen 3 Length of posterior bullar facet 11 Width of posterior bullar facet 8 Transverse width of cochlear portion 10 Anteroposterior length of cochlear portion 15

DISCUSSION
Although particular attention has been paid to Oligocene mysticetes from the North Pacific over the last few decades (e.g., Barnes et al., 1995;Okazaki, 2012;Marx, Tsai & Fordyce, 2015;Peredo & Pyenson, 2018;Solis-Añorve, Gozález-Barba & Hernández-Rivera, 2019;Hernández Cisneros & Nava-Sánchez, 2022), the same cannot be said with regards to the odontocetes. Oligocene odontocetes from around the North Pacific are not entirely missing from the scientific literature and have been mentioned multiple times, often identified informally as ''non-squalodontid odontocetes'', ''agorophiid'' or ''Agorophius-like'' (see Whitmore Jr & Sanders, 1977;Goedert, Squires & Barnes, 1995;Barnes, 1998;Barnes, Goedert & Furusawa, 2001;Fordyce, 2002;Hernández Cisneros, González Barba & Fordyce, 2017). However, given their importance, most of these have yet to be properly described, and our understanding of species richness and relationships between Oligocene odontocetes from the North Pacific is not fully understood. More importantly, these early odontocetes can potentially advance our understanding of the origins and early diversification of odontocetes, as well as acquisition of some of their distinguishing features, such as echolocation.
The first of these taxa to be described was Simocetus rayi from the early Oligocene (33.7-30.6 Ma) Alsea Fm. of Oregon, which was placed in its own family, Simocetidae, and is currently one of the geologically oldest named odontocetes (Prothero et al., 2001b;Fordyce, 2002). Since then, only two other North Pacific Oligocene odontocetes have been named, specifically, the platanistoid Arktocara yakataga from the Oligocene Poul Creek Fm. in Alaska, which may be amongst the earliest crown odontocetes, and the stem odontocete Olympicetus avitus from the Pysht Fm. in Washington (Boersma & Pyenson, 2016;Vélez-Juarbe, 2017 Herein, the description of three additional specimens from the mid-Oligocene Pysht Formation in Washington have potentially clarified the relationship between stem odontocetes from the North Pacific. The results (Fig. 21, Fig. S2) show a more inclusive Simocetidae, differing from earlier analyses (e.g., Vélez-Juarbe, 2017;Racicot et al., 2019) where Simocetus and Olympicetus occupied different positions within stem odontocetes.  154[2]). Some of these characters, such as the position of the apex of the supraoccipital and the morphology of the nuchal crest are also observed in the neonate skull (LACM 126010) referred to O. avitus, suggesting that these characters change ontogenetically, with neonatal individuals displaying more plesiomorphic conditions. Along these same lines, the presence of a distinct interparietal in CCNHM 1000, most likely another ontogenetic feature, is interpreted in the present phylogenetic analysis as a plesiomorphic character, which when combined with the other ontogenetic characteristics mentioned previously, may account for the more basal position of CCNHM 1000 in the phylogenetic analysis (Fig. 21). Besides this, it seems clear that CCNHM 1000 should be regarded as a neonate of Olympicetus sp. The inclusion of CCNHM 1000 has some interesting implications for Simocetidae. Racicot et al. (2019) described the inner ear morphology of CCNHM 1000, showing that it does not have the capability of ultrasonic hearing, which is suggestive that other taxa within this clade are also non-echolocating odontocetes, at least as neonates. Future studies on the inner ear morphology of the periotics of other simocetids of more advanced ontogenetic stages, such as specimens of Simocetus rayi, Olympicetus thalassodon, Olympicetus sp. (LACM 124105), as well as those of other simocetids that will be described in future works, such as USNM 244226 (Olympicetus sp.), USNM 205491 (Simocetidae gen. et sp. nov.), and LACM 140702 (Simocetidae gen. et sp. nov.), will likely provide more information to this regard.

Stem Odontocetes from the North Pacific
The early odontocete clade Simocetidae now includes six OTUs: Simocetus rayi, Olympicetus avitus, Olympicetus sp. (LACM 124105), O. thalassodon (LACM 158720), Simocetidae gen. et sp. A (LACM 124104) and CCNHM 1000 (Fig. 21). All specimens, with the exception of S. rayi, are from the Pysht Fm., with four of them, LACM 124104, LACM 124105, LACM 158720 and CCNHM 1000, coming from the same general area (LACM Locs. 5123 and 8093). The results of the phylogenetic analysis resemble those of an earlier, preliminary study that also recovered a monophyletic Simocetidae composed of most of the OTUs used here as well as a few others undescribed specimens from the eastern North Pacific, but that also recovered Ashleycetus planicapitis, from the early Oligocene of South Carolina, as part of that clade (Vélez-Juarbe, 2015). In contrast, the results of the present work suggest that Simocetidae represents an endemic radiation of North Pacific stem odontocetes, that parallels that of the Aetiocetidae in the same region (Hernández Cisneros & Vélez-Juarbe, 2021), and the Xenorophidae (here considered to include Ashleycetidae and Mirocetidae; Fig. 21) in the North Atlantic and Paratethys (Marx, Lambert & Uhen, 2016a). Interestingly, simocetids and xenorophids overlap temporally with some platanistoids such as Arktocara yakataga and Waipatia spp. (Fordyce, 1994;Tanaka & Fordyce, 2015;Boersma & Pyenson, 2016;Tanaka & Fordyce, 2017;Gaetan, Buono & Gaetano, 2019;Viglino et al., 2021;but see Viglino et al., 2022 with regards to W. maerewhenua). This suggests that crown odontocetes appeared at least by the late Oligocene, pending a more precise assessment of the age or A. yakataga, and that the initial diversification of odontocetes may have occurred during the latest Eocene to early Oligocene. This is further supported by the early Rupelian (33.7-30.6 Ma;Prothero et al., 2001b) age of the Alsea Fm., where Simocetus rayi was found, which places Simocetidae amongst, if not the earliest, diverging odontocete clade (pending a better age assessment for Mirocetus riabinini; Sanders & Geisler, 2015). The discovery and description of additional odontocetes from the Makah, Pysht, and Lincoln Creek formations in Washington State, and Alsea and Yaquina formations in Oregon, would likely provide new insights with regards to early odontocete diversification. This highlights the importance of the fossil record of the North Pacific towards further understanding the early history and radiation of odontocetes.

Dentition and feeding in simocetids
As in most other groups of stem odontocetes (e.g., xenorophids, agorophiids), simocetids have an heterodont dentition, but do seem to have a more conservative tooth count, closer to that of basilosaurids such as Cynthiacetus peruvianus (Martínez-Cáceres & de Muizon, 2011), which consists of three incisors, one canine, four premolars, two upper and three lower molars, a pattern that is also observed in early mysticetes like Janjucetus hunderi Fitzgerald, 2006, andMystacodon selenensis (Fitzgerald, 2010;Lambert et al., 2017). While the tooth count of some simocetids is hard to interpret (e.g., Olympicetus avitus; Vélez-Juarbe, 2017), others such as Simocetus rayi and Olympicetus thalassodon offer more definite clues with regards to their dentition. In the case of Simocetus rayi, its tooth count seems to be secondarily reduced from the plesiomorphic condition through the loss of the upper incisors, while the lower ones are retained (Fordyce, 2002). Although most are not preserved in the holotype, the teeth of S. rayi were widely separated and small (when compared to those of Olympicetus). In contrast, the teeth of Olympicetus thalassodon are closely spaced, and based on the preserved teeth and alveoli, the dental formula of the latter is tentatively interpreted as ?I3, C, P4, M2/?i3, c, p4, m3. The presence of three incisors is based in part on LACM 140702, although there is also the possibility that O. thalassodon had no incisors, resembling the condition of S. rayi. Nevertheless, if these interpretations are correct, then the dentition of simocetids is the most plesiomorphic amongst odontocetes, paralleling that of early mysticetes. This would contrast with xenorophids, which seem to have a polydont dentition; for example, Xenorophus sloanii and Echovenator sandersi both have a significantly higher count of postcanine teeth (Sanders & Geisler, 2015;Churchill et al., 2016). However, the dentition of many xenorophids is still unknown, including key taxa, such as Archaeodelphis patrius, which may offer additional insight into early odontocete dental evolution.
Although different simocetids seem to share similar conservative tooth counts and generalized features of their teeth, there are some interesting differences between some of the species. One conspicuous difference between the dentition of Olympicetus avitus and O. thalassodon is the presence of a ''carnassial''-like tooth in the former ( Fig. S1; tooth 'mo3' in Vélez-Juarbe, 2017: fig.7O,Bb). This tooth is distinguished from all other postcanine teeth by having a lingual lobe with a secondary carina with accessory denticles that descends lingually from the apex (Fig. 13E), while its root is expanded lingually, giving the impression of the presence of three roots (mesial, distal and lingual), rather than two (mesial and distal) as in the other postcanine teeth. Meanwhile, a third, lingual root seems to be present in the P4 of Simocetus rayi (Fordyce, 2002), in an unnamed Simocetus-like taxon from the Lincoln Creek Fm. (Barnes, Goedert & Furusawa, 2001) and in LACM 124104 (described above), and could be a character that is shared among some simocetids, although better preserved specimens are needed to corroborate this. The presence of a third, lingual root and a lingual lobe is otherwise unknown in other odontocetes, toothed mysticetes, and basilosaurids (Uhen, 2004;Martínez-Cáceres, Lambert & de Muizon, 2017), but present in more basal forms (e.g., protocetids and kekenodontids;Kellogg, 1936;Kassegne et al., 2021;Corrie & Fordyce, 2022). A somewhat similar crown morphology is observed in protocetids such as Indocetus ramani Sahni & Mishra, 1975, Aegyptocetus tarfa Bianucci & Gingerich, 2011, and Togocetus traversei Gingerich & Cappetta, 2014, as well as in Kekenodon onamata Hector, 1881, all of which have a protocone lobe supported by a lingual root in the more posterior upper premolars and molars (Bajpai & Thewissen, 2014;Kassegne et al., 2021;Corrie & Fordyce, 2022). However, the lobe on the lingual side of the teeth of protocetids and K. onomata is located distolingually, differing from the condition observed in O. avitus and LACM 124104, in which the lobe is located mesiolingually, and may thus not be homologous. Interestingly, tooth B7 (sensu Sanders & Geisler, 2015) of Xenorophus sloani seems to present a more inconspicuous version of the ''carnassial'' tooth of simocetids this tooth occupies a position similar to that of P4 in Simocetus rayi, and this character should be explored further as more specimens become available.
Some of the morphological characters observed in described simocetids, such as the arched palate, short and broad rostrum, smaller and widely-spaced teeth, as in Simocetus rayi, were interpreted as features of a bottom suction feeder (Fordyce, 2002;Werth, 2006;Johnston & Berta, 2011). Olympicetus shares some of these features, such as the arched palate. However, O. thalassodon, has closely spaced, larger teeth, as well as a relatively gracile, unfused hyoid apparatus (Figs. 11-13A-13C;Johnston & Berta, 2011;Viglino et al., 2021;Werth & Beatty, 2023), which suggest that this taxon was instead a raptorial or combined feeder (Fig. 22). Taking this into account, it is likely that simocetids employed different methods of prey acquisition, likely akin to the amount of variation observed in other contemporaneous groups, such as xenorophids, which include taxa with long narrow rostra (e.g., Cotylocara macei; Geisler, Colbert & Carew, 2014) that can be interpreted as raptorial feeders, as well as a brevirostrine suction feeding taxon (i.e., Inermorostrum xenops; Boessenecker et al., 2017). Thus it seems that several methods of prey acquisition evolved iteratively across different groups of odontocetes soon after their initial radiation (Hocking et al., 2017;Kienle et al., 2017).

CONCLUSIONS
Three new specimens of odontocetes from the early to late Oligocene Pysht Formation were described herein, further increasing our understanding of richness and diversity of early odontocetes, specially for the North Pacific region. Inclusion of this new material in a phylogenetic analysis showed that Simocetidae is a much more inclusive clade, which besides Simocetus rayi, now includes Olympicetus avitus, O. thalassodon sp. nov., Olympicetus sp. 1, and a large unnamed taxon. Of these, Olympicetus thalassodon is one of the most completely known simocetids, offering new information on the cranial and dental anatomy of early odontocetes, while the inclusion of CCNHM 1000 within this clade suggest that simocetids may not have had the capabilities for echolocation at least during their earlier ontogenetic stages. This shows that some morphological features that have been correlated with the capacity to echolocate, such as an enlarged attachment area for the maxillonasolabialis muscle, and presence of a premaxillary sac fossae (Fordyce, 2002;Geisler, Colbert & Carew, 2014), may have appeared before the acquisition of ultrasonic hearing. Furthermore, the dentition of simocetids, as interpreted here, seems to be the most plesiomorphic amongst odontocetes, while other craniodental features within members of this clade suggests various forms of prey acquisition techniques, including raptorial or combined in Olympicetus spp., and suction feeding in Simocetus (as suggested by Fordyce, 2002). Meanwhile, body size estimates for simocetids show that small to moderately large taxa are present in the group, the largest taxon being represented by LACM 124104, with an estimated body length of 3 m. This length places it amongst the largest Oligocene odontocetes, only surpassed in bizygomatic width (and therefore estimated body length) by Mirocetus riabinini and Ankylorhiza tiedemani (Riabinin, 1938;Boessenecker et al., 2020;Sander et al., 2021). Finally, the new specimens described here add to a growing list of Oligocene marine tetrapods from the North Pacific, further facilitating faunistic comparisons with other contemporaneous and younger assemblages in the region, such as those in Mexico (e.g., El Cien Fm.) and Japan (e.g., Waita Fm.), thus improving our understanding of the evolution of marine faunas in the region.