Redescription of Phymolepis cuifengshanensis (Antiarcha: Yunnanolepididae) using high-resolution computed tomography and new insights into anatomical details of the endocranium in antiarchs

Background Yunnanolepidoids constitute either the most basal consecutive segments or the most primitive clade of antiarchs, a highly diversified jawed vertebrate group from the Silurian and Early Devonian periods. Although the general morphology of yunnanolepidoids is well established, their endocranial features remain largely unclear, thus hindering our further understanding of antiarch evolution, and early gnathostome evolution. Phymolepis cuifengshanensis, a yunnanolepidoid from the Early Devonian of southwestern China, is re-described in detail to reveal the information on endocranial anatomy and additional morphological data of head and trunk shields. Methods We scanned the material of P. cuifengshanensis using high-resolution computed tomography and generated virtual restorations to show the internal morphology of its dermal shield. The dorsal aspect of endocranium in P. cuifengshanensis was therefore inferred. The phylogenetic analysis of antiarchs was conducted based on a revised and expanded dataset that incorporates 10 new cranial characters. Results The lateroventral fossa of trunk shield and Chang’s apparatus are three-dimensionally restored in P. cuifengshanensis. The canal that is positioned just anterior to the internal cavity of Chang’s apparatus probably corresponds to the rostrocaudal canal of euantiarchs. The endocranial morphology of P. cuifengshanensis corroborates a general pattern for yunnanolepidoids with additional characters distinguishing them from sinolepids and euantiarchs, such as a developed cranio-spinal process, an elongated endolymphatic duct, and a long occipital portion. Discussion In light of new data from Phymolepis and Yunnanolepis, we summarized the morphology on the visceral surface of head shield in antiarchs, and formulated an additional 10 characters for the phylogenetic analysis. These cranial characters exhibit a high degree of morphological disparity between major subgroups of antiarchs, and highlight the endocranial character evolution in antiarchs.

166 Systematic paleontology 167 Placodermi McCoy, 1848 168 Antiarcha Cope, 1885 169 Yunnanolepidoidei Gross, 1965 170 Yunnanolepididae Miles, 1968 171 Phymolepis Chang, 1978 172 Type species. Phymolepis cuifengshanensis Chang, 1978 173 Included species. Phymolepis guoruii Zhu, 1996 174 Emended diagnosis. Yunnanolepididae in which the posterior median dorsal plate 175 bears a strong posterior process; the anterior median dorsal plate with anterior division 176 longer than posterior division; anterior ventral process and pit situated just below a 177 conspicuous tergal angle and at the same level of the lateral corners of the anterior 178 median dorsal plate; sharp median dorsal ridge between the tergal and posterior dorsal 179 angles. 180 Remarks. The diagnosis of this genus follows Zhu (1996) with a minor revision. While 181 examining V4425.2 based on high-resolution CT, we noticed that the anterior ventral 182 process and pit are situated at the level of the lateral corners of the anterior median 183 dorsal plate rather than behind it. Phymolepis cuifengshanensis -Zhu, p. 257, figs. 11-12, pls. I (8-10), IV (1-9) 239 The postpineal plate (PP, Fig. 7C) is wider than long. Its anterior margin is concave, 240 unlike the straight margin in Parayunnanolepis. The postpineal thickening (pp.th, Fig. 7A) 241 is extremely developed as a prominent tuberculate elevation, which totally encompasses 242 the posterior border of the orbital fenestra and occupies about half of the postpineal plate 243 length. On the visceral surface, paired postorbital cristae (cr.po, Fig. 7D) run somewhat 244 obliquely along the anterior margin of the plate. Two cristae on either side of the 245 postpineal plate are separated far away by a faint median ridge (mr, Fig. 7B), which lies 246 at anterior margin of the plate and does not extend backwards as in many euantiarchs. 247 The nuchal plate (Nu, Fig. 7A) is broadest across the anterolateral angle. The 248 postpineal notch is broad and deep. The posterolateral margin is about twice as long as 249 the anterolateral one (Table 1). The robust anterolateral ridge (alr, Fig. 7A) sits mainly in 250 the anterior division of the nuchal plate. The transverse nuchal crista (cr.tv, Fig. 7B) on 251 the visceral surface is well developed and thickened laterally.  Like other antiarchs, only the dorsal aspect of the endocranium can be inferred in P. 260 cuifengshanensis from the impressions on the visceral surface of the head shield, which 261 is digitally visible with a high level of details.

262
The otico-occipital depression of P. cuifengshanensis is deeper posteriorly, along with 263 the gradually thickened paramarginal crista (cr. pm, Figs. 7B and 7D). This depression is 264 laterally extended at the suture between the lateral and paranuchal plates, where the 265 paramarginal crista lies underneath the infraorbital sensory groove. As such, the 266 paramarginal crista in P. cuifengshanensis with the convex median part, differs from the 267 straight one in asterolepidoids (Hemmings, 1978;Young. 1983) and laterally concave 268 one in bothriolepidoids (Young, 1988). 269 The anterolateral corner of the otico-occipital depression (p.apo, Fig. 7B), which 270 represents the imprint for the anterior postobital process of endocranium (Young, 1984), 271 is weakly developed and apically rounded. Significantly, it is located at the same level 272 with the posterior border of the orbital notch.

277
The semicircular depressions (dsc, Fig. 7D) sit just in front of the level of cranio-spinal 278 processes. The anterior and posterior semicircular depressions are relatively short, and 279 meet in a confluence that is located midway between the posterior border of the orbital 280 notch and the transverse nuchal crista. As the lateral extension of the otico-occipital 281 depression roughly levels with the confluence, this lateral extension appears to relate 282 with the labyrinth cavity (d.sac?, Fig. 7D) as seen in Arenipiscis westolli (Young, 1981b: 283 fig. 6). In view of the otic region, which can be estimated by the position of semicircular 284 depressions, lies mainly in the anterior half of the otico-occipital depression, so the 285 occipital region of P. cuifengshanensis is fairly long compared with euantiarchs.

286
Median to the posterior end of the semicircular depression, the internal pore for the 287 endolymphatic duct (d.end, Figs. 7B and 7D) is rounded and situated far ahead of the 288 transverse nuchal crista, while the external pore (d.end, Fig. 7A) is situated far 289 posteriorly at the anterior margin of obtected nuchal area. The distance between the 290 internal pores of both sides is 2.5 times longer than the distance between the external 291 ones. The digital visualization reveals that the endolymphatic duct of P. 292 cuifengshanensis is a long and roughly straight tube. It runs posterodorsally within the 293 nuchal plate, swings laterally while close to the midline of the plate and opens to the 294 exterior (Figs. 8A-C).

295
Posteriorly, a pair of supraoccipital pits (sop, Figs, 7B and 7D) is positioned just in front 296 of the transverse nuchal crista. This pit is easily distinguished from the internal pore of 297 the endolymphatic duct by its large size and ellipsoidal shape. The pit is dorsomedially 298 oriented within the nuchal plate, and gradually tapers off just beneath the ornamented 299 surface (Figs. 8A-C). The supraoccipital pit also occurs in Vukhuclepis (Racheboeuf et 300 al. 2006: fig. 4) and Yunnanolepis at the same position. It is noteworthy that Liu (1963: fig.  301 1) misidentified the supraoccipital pit in Yunnanolepis as the internal pore for the 302 endolymphatic duct.

303
Just anterior to the cavity for the cranio-spinal process, a corner (c.vg, Fig. 8F) in a 304 nearly right angle is set on approximately at the posterior end of a semicircular 305 depression level, and thus the hindmost level of the otic region. This corner is also 306 positioned between the anterior postorbital process and cranio-spinal process of 307 endocranium. Therefore, we tentatively interpret this corner as the depression of the 308 vagal process as it shares the same topological relationships to that of arthrodires and 309 petalichthyids.  The dorsal wall has a convex anterior margin. The median dorsal, dorsal diagonal and 318 dorsal transverse ridges (dmr, ddr, dtr, Figs. 1-3A) on the dorsal wall radiate from the 319 tergal angle as in Mizia longhuaensis, Yunnanolepis porifera and Chuchinolepis 320 qujingensis (Zhu, 1996: figs. 4C, 5C-5D, 21A). The lateral wall carries the lateral and 321 oblique ridges (lr, or, Figs. 1C, 3C and 9D), which are widely developed in 322 yunnanolepidoids.   (Fig. 10B). The canal passes ventrally along the lateral wall of the trunk shield. 363 With a relatively large diameter, it probably carries both vessels and nerves and 364 corresponds to the rostrocaudal canal in Chuchinolepis , 365 sinolepids and euantiarchs, which is similarly positioned to supply the fin muscles 366 (Young, 2008).

367
The posterior dorsolateral plate (PDL, Figs. 2C and 3C) consists of the dorsal and 368 lateral laminae. The dorsal lamina is slightly less than twice as long as it is broad (Table  369 2).  On the visceral surface of the trunk shield, a fossa (f.lv, Figs. 11B and 11C) is located 377 at the thickened junction of the AVL, PVL and PL plates, as in Yunnanolepis and 378 Zhanjilepis (Zhu, 1996). The fossa was termed the 'lateroventral fossa', and regarded as 379 a synapomorphy of yunnanolepids by Zhu (1996). 380 Posteriorly, a deeply grooved internal structure (cg, Fig. 11E) is developed along the 381 caudal opening of trunk shield. The groove has a smooth internal surface, delimited 382 anteriorly and posteriorly by the developed crista transversalis interna posterior and 383 posterior margins of trunk plates (PVL, PL and PDL) respectively. It consists of upper 384 and lower halves divided by a thin septum (ms, Figs. 11D and 11F). The similar structure 385 in Yunnanolepis porifera, as well as in Pterichthyodes milleri (Hemmings, 1978: fig. 15D), 386 was assumed to be related to internal fertilization .

387
The semilunar plate (SL, Fig. 9B) is triangular in shape, and approximately twice as 388 broad as long. It is overlapped posteriorly by the AVL. Internally, the postbranchial 389 lamina extends anteromesially from the AVL onto the semilunar plate, and meets the 390 lamina from the opposite.

391
The median ventral plate (MV, Figs. 1B, 2B and 9B) is rhombic. The exposed surface 392 accounts for two fifths of the ventral wall of trunk shield in length and a half of the ventral 393 wall in breadth. The plate is thinner than the surrounding plates. The restoration of the endocranium in antiarchs was mainly based on the imprints of 398 its dorsal aspect on the visceral surface of the head shield (Stensiö, 1948;Stensiö, 1969;399 Miles, 1971;Denison, 1978;Young, 1984). The exception was Minicrania, which 400 preserved the internal cast of the endocranial canals and part of the cranial cavity, thus 401 providing information on its deeper endocranial structures (Zhu & Janvier, 1996).

402
In yunnanolepidoids, the visceral surface of head shield was known in Yunnanolepis 403 (Liu, 1963: fig. 1) and Chuchinolepis (Tông-Dzuy & Janvier, 1990: fig. 17). The digital 404 visualization of Phymolepis shows not only its visceral surface of head shield but also 405 some internal architecture within the dermal plates, such as the trajectory of the 406 endolymphatic duct and the cavity for the cranio-spinal process. We also re-examine 407 the holotype (V2690.1, Fig. 12A) and one referred specimen (V4423.3, Fig. 12B) of Y. 408 chii from the Early Devonian of Qujing, and provide more details for the visceral 409 surface of head shield in Yunnanolepis. Based on these new data, we make 410 comparisons in antiarchs, and show a high degree of morphological disparity with 411 respect to the endocranium. . Accordingly, the process 424 behind the anterior border of the orbital notch is referred to a plesiomorphy of 425 antiarchs and this state has been simply summarized as "anterior postorbital process 426 short" in previous phylogenetic analyses .

427
When examining the short anterior postorbital process in antiarchs, we recognized 428 that this state can be subdivided into two conditions: the anterior postorbital process at 429 the same level with the posterior border of the orbital notch in yunnanolepidoids, 430 Minicrania (Zhu & Janvier, 1996), and probably Sinolepis (Liu & P'an, 1958;Long, 1983); 431 the process anteriorly beyond the posterior border of the orbital notch, but behind its 432 anterior border in euantiarchs excluding Grossilepis and Bothriolepis. In this case, the

485
The cranio-spinal process in yunnanolepidoids is strongly developed, as indicated 486 by the large cavity for the process. The process and the corresponding cavity in 487 euantiarchs were either reduced or absent (Young, 1984).  (Young, 1988).

497
In non-antiarch placoderms, the supraoccipital pit has been observed in 498 petalichthyids (Liu, 1991 Manuscript to be reviewed 513 shared by antiarchs, Brindabellaspis and petalichthyids. Our study herein shows the 514 condition in antiarchs is more complicated than previously thought.

515
In antiarchs, the distance between the internal pores is usually greater than that of 516 external ones (Stensiö, 1948;Karatajūte-Talimaa, 1966;Long, 1983), and the 517 endolymphatic duct extends dorsomesially. As the external pore of endolymphatic duct 518 is always positioned close to the posterior edge of the nuchal plate in antiarchs, the 519 relative position of the internal pore along the antero-posterior axis reflects the relative 520 length and orientation of the endolymphatic duct.

521
In yunnanolepidoids, the internal pore of endolymphatic duct is located far in front of 522 the transverse nuchal crista, and thus far from the external pore. As such, the 523 endolymphatic duct is elongated through the nuchal plate and obliquely oriented. 524 Sinolepids Janvier, 1996) and euantiarchs differ in having a short, 525 slight oblique endolymphatic duct as the internal pore is positioned just anterior to the 526 external one.

527
In non-antiarch placoderms, the elongated endolymphatic duct is also present in 528 arthrodires (Young, 2010;. However, the endolymphatic duct of 529 arthrodires is directed dorsolaterally, not dorsomesially as in antiarchs. 530 Occipital portion of endocranium. The internal pore for the endolymphatic duct in 531 antiarchs, is located roughly at the posterior boundary of the semicircular depression on 532 the visceral surface of head shield as that in arthrodires (Zhu, Zhu & Wang, 2016), hence 533 we use this pore as a proxy to denote the otic-occipital boundary. Taking the length of 534 the otic-occipital depression as the constant variable in antiarchs, the length between the 535 internal pore of endolymphatic duct and the posterior border of otic-occipital depression 536 represents the occipital proportion in endocranium.

537
In yunnanolepidoids, the internal pore on the visceral surface of the head shield is far 538 from the transverse nuchal crista as described above, implying the occipital portion of the 539 endocranium is elongated as in arthrodires (Zhu, Zhu & Wang, 2016). By comparison, 540 the occipital portion is short in other antiarchs (Stensiö,1948;Ritchie et al., 1992).  In yunnanolepids and Minicrania (Zhu, 1996), the confluence is halfway between 548 the posterior border of the orbital notch and the transverse nuchal crista. By 549 comparison, the confluence is closer to the transverse nuchal crista than to the 550 posterior border of orbital notch in sinolepids and euantiarchs. The maximum parsimony analysis yields 726 MPTs of 179 steps each (consistency 554 index= 0.4693; retention index= 0.8045). All the MPTs are summarized as a strict 555 consensus tree (Fig. 13A) and a 50% majority-rule consensus tree (Fig. 13B). One MPT 556 is selected to illustrate the character transformations at nodes (Fig. 14A), and the list of 557 synapomorphies defining various nodes is shown in Supplementary Information. 558 Antiarchs (Fig. 14A: node 1) are characterized by up to 10 synapomorphies including 559 two newly proposed cranial features (Character 27 0 , absence of posterior process of 560 head shield; Character 38 1 , presence of supraoccipital pit). Character 27 shows a 561 reversal in euantiarchs (Fig. 14A: node 15). Character 38 is a highly homoplastic 562 character, and shows a reversal in euantiarchs and a parallelism in Bothriolepis. 563 Yunnanolepidoids (Fig. 14A: node 2) form the basal members of antiarchs, consistent 564 with the position as they were first phylogenetically analysed (Zhu, 1996). However in 565 new scenario, Chuchinolepis, Vanchienolepis and a clade formed by yunnanolepids, 566 Zhanjilepis and Heteroyunnanolepis fall into a polytomy with remaining antiarchs. In 567 yunnanolepids, Yunnanolepis is the sister group of a polytomic clade comprising 568 Phymolepis, Mizia and Parayunnanolepis.

592
At the node comprising sinolepids and euantiarchs (Fig. 14A: node 10), there are two 593 derived endocranial character states: short endolymphatic duct (Character 40 1 ) and 594 short occipital region (Character 41 1 ). Euantiarchs differ from sinolepids in possessing 595 the following derived states: the anterior postorbital process lying in front of the posterior 596 level of orbital notch (Character 36 1 ), and the absence of the supraoccipital process 597 (Character 38 0 ). In short, there exists a large morphological disparity relating to the 598 dorsal aspect of endocranium between yunnanolepidoids, sinolepids and euantiarchs. The re-investigation of Phymolepis cuifengshanensis with assistance of high-602 resolution CT scanning, offers comprehensive information for this taxon and new insights 603 into the morphology and phylogeny of antiarchs.

604
The exoskeleton of Phymolepis cuifengshanensis shows typical yunnanolepid 605 characters, such as the small orbital fenestra, presence of both developed postbranchial 606 lamina and crista transversalis interna anterior on the trunk shield. The endocranium of P. 607 cuifengshanensis also resembles that of other yunnanolepidoids in the presence of 608 developed cranio-spinal process and supraoccipital process, the anterior postorbital 609 process lying at the same level with the posterior border of the orbital notch, elongated 610 endolymphatic duct and long occipital region. 611 We compare cranial characters among subgroups of antiarchs, and formulate ten 612 additional characters that deemed to be of phylogenetic significance. Phylogenetic 613 analysis of a revised and expanded dataset draws new perspectives on the 614 interrelationships of antiarchs, and corroborates the monophyly of yunnanolepidoids by 615 the presence of cavity for cranio-spinal process.

616
The character transformations relating to the dorsal aspect of endocranium in 617 antiarchs are inferred under the new phylogenetic scenario. By comparison to 618 yunnanolepidoids and Minicrania, which retain several primitive endocranial traits, 619 sinolepids and euantiarchs evolved two apomorphic features (short endolymphatic duct 620 and short occipital portion). Euantiarchs are more derived in the anterior postorbital 621 process lying in front of the posterior level of orbital notch, and the absence of the 622 supraoccipital process.       Manuscript to be reviewed Breadth ratio between orbital fenestra and head shield 0.24 Length ratio between posterolateral and anterolatereal margins of nuchal 2.45 Length ratio between obtected nuchal area and nuchal 0.