Microvertebrate faunal assemblages of the Favel Formation (late Cenomanian-middle Turonian) of Manitoba, Canada

Description of indeterminate taxon Vertebrate A

Several unknown chondrichthyan and actinopterygian elements were recovered from sampled Favel Formation horizons, including several of one morphotype in particular, referred to here as Vertebrate A (Figs. 4, S1). Fossil elements assigned to Vertebrate A are small (longest fragment 6.78 mm in length), gently to moderately curved with weakly convex lateral faces and wide (0.19–0.76 mm), serrated edges. Most fragments are between 6.39 and 6.78 mm in length, 1.72 and 1.91 mm in maximum width, and 0.54 and 0.57 mm in thickness, with the smallest fragment 1.95 mm long, 1.03 mm wide, and 0.35 mm thick. The width of the thicker middle portion of the element is between 1.06 to 1.28 mm on most specimens and is 0.45 mm wide on the smallest fragment. Serrated edges do not reach the apices and on the distal edges do not reach the base. Each serration along the edges has a tapered point, a height range of 0.11–0.2 mm, a base width range of 0.06–0.16 mm, and where observed, appear to have two layers (Fig. S1). Approximately nine serrations comprise a 1.0 mm distance along the edges. The apex of one specimen (MM V-3548) bears a weakly and distally curved, rounded tip with bladed edges ending approximately 2 mm away from the apex on one edge with incipient serrations, and 1 mm on the other edge, lacking serrations (Fig. 4D).

Vertebrate A elements were nearly all recovered from the mid-Turonian Marco Calcarenite, Favel Formation with one distal end recovered from the second-topmost, 13 cm-thick limestone bed of the early Turonian Laurier Limestone beds, Keld Member (Fig. 4D). The occurrences of nearly all Vertebrate A specimens from the horizon interpreted as nearest to shore, the Marco Calcarenite, and none from the horizon interpreted as furthest from shore, 1 m above the Keld Member base, suggests this species was likely restricted to nearshore environments.

Faunal assemblage comparisons

Specimens representing previously unknown occurrences recovered from all three bulk-sampled microvertebrate fossil horizons increase the total known apparent diversity of Favel Formation microvertebrate taxa by 14 species, from 26 to 40, and the combined total of macro- and microvertebrate taxa from 42 to 56 species (Tables 1 and 2; Material S1). Total numbers of vertebrate taxa identifiable to order recovered from the three Favel Formation horizons consist of, from most to least: 25 from the Marco Calcarenite, 10 from the Laurier Limestone bed, and 8 from 1 m above Keld base (Table 2). Of the recovered microvertebrate fossil remains from each of the lower, middle, and upper horizons, the most and largest remains were recovered from the upper horizon and the least and smallest from the lower horizon. Preservation of fossil remains is moderate to good from all sites except for remains recovered from the ‘Marco Calcarenite, near Miami’ site, which have poor to moderate preservation because of longer periods of subaerial and subaqueous exposure at a field drainage site.

Laurier Limestone bed assemblage

The Laurier Limestone bed assemblage is mostly composed of actinopterygian remains with at least seven species represented (Figs. 6A–6D, 6G, 7). Furthermore a single occurrence of Vertebrate A (Fig. 4G), four partial teeth possibly belonging to rays or skates, four partial limb elements and two ribs belonging to cf. Ichthyornis sp. (Figs. 8C and 8D), and four partial femora belonging to an indeterminate testudine are also reported (Figs. 8A and 8B; Table 2). Caudal fin elements referred to cf. Apsopelix anglicus (Figs. 7E–7G, S2) are the only species identifiable elements that are more abundant in the Laurier Limestone bed assemblage relative to the lower and upper horizons (Table 2), indicating cf. A. anglicus was a common vertebrate community member along the northeastern WIS margin during early Turonian time.

Marco Calcarenite assemblage

The microvertebrate assemblage sampled from the ‘Marco Calcarenite, near Miami’ site contains the highest abundance of fossil elements, mostly comprised of actinopterygian vertebrae, and greatest diversity of represented taxa with at least 14 actinopterygian, nine chondrichthyan, one avian, one squamate, and one testudine species represented (Figs. 9–14; Table 2).

The ‘Marco Calcarenite, Ochre R.’ microvertebrate assemblage contains actinopterygian, chondrichthyan, avian, and reptilian occurrences that are all also represented from the ‘Marco Calcarenite, near Miami’ assemblage and is comparatively less diverse, most likely because the amount of sorted sediment and fossils is about half (53%) of that sorted from ‘Marco Calcarenite, near Miami’ and not representative of a truly less diverse assemblage. Interestingly, no ray or skate remains were recognized from the ‘Marco Calcarenite, Ochre R.’ assemblage (Table 2) which may indicate that as with the lower apparent diversity, is a result of less sampling coverage through sediment and fossil sorting and the lower probability of recovering less common microvertebrate species.

Preservational biases

Differences in fossil preservation, most represented size classes, and their size distributions preserved within the three sampled horizons are recognized. Strong similarities between the Marco Calcarenite assemblages from both sampled sites and differences between them and the Laurier Limestone bed assemblage are apparent in measurements of actinopterygian vertebrae (Fig. 15; Material S2). The best preserved and smallest fossils were recovered from the lowest and middle horizons, 1 m above the Keld Member base along Vermilion R. and the second topmost Laurier Limestone bed along Edwards Creek, respectively. The Laurier Limestone bed assemblage vertebrae size distribution ranges between 0.36 and 2.82 mm in diameter and contains two peaks with the most vertebrae (17) in the 0.76–0.86 mm size class and the second-most vertebrae (8) in the 1.06–1.16 mm size class (Fig. 15C). The low number (21) and poor preservation of vertebrae recovered from the ‘Near Keld base, Vermilion R.’ site did not allow for reliable assessment of vertebrae size distribution from the lowest horizon. Although not as well preserved, the Marco Calcarenite horizon at the ‘Marco Calcarenite, near Miami’ site yielded the most and the largest microvertebrate remains, with a size distribution range of 0.49–5.14 mm and the 0.99–1.09 and 1.09–1.19 mm size classes containing the most vertebrae (22 and 21, respectively; Fig. 15B). The highest number of vertebrae (350) were measured from the ‘Marco Calcarenite, Ochre R.’ site and the size classes with the most vertebrae are nearly identical to those of the ‘Marco Calcarenite, near Miami’ site, with 41 in the 0.93–1.03 mm size class and 44 in the 1.03–1.13 mm size class, though its 0.33–3.81 mm vertebrae size distribution range is more similar to that of the ‘Laurier Limestone beds, Edwards Creek’ site than the ‘Marco Calcarenite, near Miami’ site (Figs. 15A–15C). Similar size distributions between the two sites with the least and most measured vertebrae, ‘Laurier Limestone beds, Edwards Creek’ and ‘Marco Calcarenite, Ochre R.’, respectively, suggest their microvertebrate assemblages were preserved under similar, low-energy conditions, with the large difference in amounts of recovered vertebrae indicating the sampled horizon at ‘Marco Calcarenite, Ochre R.’ as closer to shore relative to the sampled Laurier Limestone bed where both higher sedimentation rates and greater abundances of small-bodied actinopterygians allowed for preservation of more vertebrae. The relatively low number of recovered vertebrae and noticeable gap of measured vertebrae in the 1.36–2.36 mm size classes of the Laurier Limestone bed assemblage (Fig. 15C) may represent lower fossil preservation potential, particularly of relatively large vertebrae, in a depositional environment with a lower sedimentation rate relative to that of the Marco Calcarenite. The larger fossil elements represented in the ‘Marco Calcarenite, near Miami’ and greater vertebrae size distribution range of 0.49–5.14 mm indicates a depositional environment with higher sedimentation rates capable of rapidly burying and preserving larger fossil elements relative to the those of the ‘Laurier Limestone beds, Edwards Creek’ and ‘Marco Calcarenite, Ochre R.’ sampled horizons. The apparent absence of vertebrae with measured diameters less than 0.49 mm, shape of vertebrae size distribution (Fig. 15B), and poor to moderate preservation of most fossil elements suggests the horizon sampled at ‘Marco Calcarenite, near Miami’ experienced some degree of sediment reworking with preferential preservation bias of fossil elements larger than 0.8–1.0 mm and also supports the interpretation of this horizon as being deposited under high-energy conditions.

Paleoenvironmental interpretations

Clay to fine-sized sediment, the absence of quartz grains, and the low number of recovered vertebrae (21) indicates the ‘Near Keld base, Vermilion R.’ site as furthest offshore from the eastern WIS shoreline relative to the middle and upper Favel Formation horizons. The absence of several taxa associated with nearshore marine habitats, including Ischyrhiza mira, I. texana, and Rhinobatos incertus (Welton & Farish, 1993; Everhart, 2007), also supports ‘Near Keld base, Vermilion R.’ as the site furthest from the WIS shoreline. The exceptional preservation of fossil elements, clay to fine sediment grain sizes, and high abundance of small fossil elements recovered from the Laurier Limestone bed horizon relative to the sampled lower and upper Favel Formation horizons suggests a low energy depositional environment closer to shore than the lower horizon and further from shore than the upper horizon. The relatively poorly sorted sediment and fossil size classes, high abundance of quartz grains and inoceramid clam fragments, and presence of several taxa associated with nearshore environments, including the actinopterygian cf. Albulidae incertae sedis (Cumbaa, Underwood & Schröder-Adams, 2013) and chondrichthyans I. mira, I. texana, and R. incertus (Welton & Farish, 1993; Everhart, 2007), suggests the Marco Calcarenite represents the highest energy and nearest to shore depositional environment relative to the lower and middle Favel Formation horizons, with the ‘Marco Calcarenite, near Miami’ site situated closer to shore than the ‘Marco Calcarenite, Ochre R.’ site.

Favel Formation marine cycle trends

Faunal analyses of microvertebrate assemblages from the Favel Formation of MB has substantially improved understanding of vertebrate communities from the northeastern WIS margin by increasing representation of small-bodied vertebrates. Relative proximity to shore is estimated for all horizons from the four sampled sites with reference to the size range and size class distribution of actinopterygian vertebrae, sediment grain size range, and additionally supported by the presence or absence of vertebrate taxa associated with nearshore habitats. Relative positions from the WIS northeastern shoreline for the four analyzed microvertebrate assemblages are as follows, in order of stratigraphically lowest to highest: ‘Near Keld base, Vermilion R.’ as furthest offshore, ‘Laurier Limestone beds, Edwards Creek’ as offshore, ‘Marco Calcarenite, Ochre R.’ as nearshore, and ‘Marco Calcarenite, near Miami’ as nearest to shore. The overall regression from early to mid-Turonian time indicated by the Favel Formation microvertebrate assemblages of MB is consistent with other interpretations of WIS sea level change during this time interval (e.g., Schröder-Adams, 2014) and represents the overall regressive phase of the Greenhorn WIS third-order marine cycle.

Faunal occurrences

The planktonic foraminiferal species Heterohelix globulosa was observed within the 0.42–0.83 mm size class of sediment sorted from the ‘Near Keld base, Vermilion R.’ horizon (Fig. S3). The presence of H. globulosa further supports the stratigraphic position of the ‘Near Keld base, Vermilion R.’ horizon as within the Keld Member, though does not distinguish whether the horizon is of late Cenomanian or early Turonian age because of its chronostratigraphic range (McNeil & Caldwell, 1981; Dionne, Schröder-Adams & Cumbaa, 2016). Although examination was brief, the presence of H. globulosa and apparent absence of Clavihedbergella simplex and Whiteinella aprica indicates the ‘Near Keld base, Vermilion R.’ horizon is of late Cenomanian age (Dionne, Schröder-Adams & Cumbaa, 2016).

Teeth of benthic chondrichthyan species not previously recognized from the mid-Turonian strata of MB and associated with nearshore habitats (Welton & Farish, 1993) were recovered from the ‘Marco Calcarenite, near Miami’ site, including the sawfishes Ischyrhiza mira and I. texana (Tables 2 and 3), though none were recognized from the more offshore, lowest horizon at the ‘Near Keld base, Vermilion R.’ site. Recognition of the Marco Calcarenite as representing a high-energy, nearshore depositional environment based upon the presence of taxa associated with nearshore habitats, relatively high abundance of quartz grains and inoceramid fragments, and poorly sorted sediment and fossil size classes agrees with the mid-Turonian timing of the Greenhorn marine cycle regressive phases (Schröder-Adams, 2014; Bamburak, Hamilton & Heaman, 2016) and is significant since it provides further insight into the enigmatic position of the northeastern WIS shoreline.

Although fully articulated skulls of Cimolichthys sp. (Fig. S4) and Pachyrhizodus minimus (Fig. S5) have been recovered from the Keld Member, only disarticulated elements corresponding to P. minimus were identified from the Marco Calcarenite and Laurier Limestone bed horizons (Fig. 6A, 10A; Table 2).

Several bony fish skull and hyoid arch elements collected from the four sampled sites were recognized as similar to those of modern bony fish taxa. Actinopterygian material assigned to teleost A–D (Figs. 5, 10–12; Tables 2 and 3) share anatomical similarities with the following modern fish taxa: Teleost A with Family Pleuronectidae, Teleost B with Family Gadidae, Teleost C with Family Scorpaenidae, and Teleost D with Salmonidae (Cannon, 1987). Whether the anatomical similarities between the teleost A-D material and modern fish taxa are due to close phylogenetic relationships or are a result of occupying similar ecological niches has yet to be determined.

The indeterminate element, Vertebrate A, may belong to skull, branchial arch, or fin elements of common Turonian fish, such as A. anglicus. Remains of Vertebrate A are fragile relative to most recovered microvertebrate fossils due to their small size and thin shape and the majority of occurrences were from the Marco Calcarenite horizon deposited under relatively high-energy conditions, which may explain why complete elements were not observed.

Biogeographical implications

New occurrences of actinopterygians cf. Albulidae incertae sedis, Micropycnodon kansasensis, Pachyrhizodus minimus (Fig. S5), Protosphyraena sp., and Thryptodus loomisi (Fig. S6B), chondrichthyans Ischyrhiza mira, I. texana, Ptychodus marginalis (Fig. S6A), P. occidentalis, and P. rhombodus, the avian cf. Ichthyornis sp., and Testudines indet. (Tables 1–3) increases the relatively strong biogeographic affinity between the MB and South Dakota mid-Turonian marine vertebrate assemblages, as well as weaker biogeographic affinities with WIS localities further south in Kansas and Texas (Welton & Farish, 1993; Bice & Shimada, 2016; Kilmury & Brink, 2022). The strengthened biogeographic relationships are demonstrated with the results of a presence-absence analysis (Table S1) shown in Fig. 16 comparing the updated Favel Fm vertebrate faunal assemblage of the MB escarpment (Table 3) with other time-equivalent Western Interior Seaway assemblages in Northwest Territories (Cumbaa et al., 2018; Kilmury & Brink, 2022), west-central Alberta (Fox, 1984; Wilson & Chalifa, 1989; Cook et al., 2013), South Dakota (Kilmury & Brink, 2022), Kansas (Bice & Shimada, 2016; McIntosh, Shimada & Everhart, 2016; Kilmury & Brink, 2022), and Texas (Kilmury & Brink, 2022). The increased biogeographic affinities with WIS localities south of MB provides further support for a relatively large central vertebrate community zone distinct from northern and southern ones during late Cenomanian to early Turonian time (Fig. 16), as well as decreases the gradient of the north-south or central-south community boundary positioned between WIS localities in present-day South Dakota and Kansas during early and mid-Turonian times (Kilmury & Brink, 2022). Since there was only a minor increase in specimens belonging to taxa shared between the MB Keld Member and coeval Northwest Territories Unit E vertebrate assemblages, which include Cimolichthys sp., Enchodus petrosus, and Ichthyodectes ctenodon, nor recognition of taxa specific to the Unit E assemblage (Cumbaa & Murray, 2008; Murray & Cumbaa, 2013, 2015; Murray, 2016; Cumbaa et al., 2018) from the lower and middle Favel Formation (Keld Member) horizons analyzed in this study, a central subprovince distinct from northern one is further supported for late Cenomanian to early Turonian time (Kilmury & Brink, 2022).

Preservational potential

The low apparent diversity of the Laurier Limestone bed assemblage (12 apparent species; Table 2) relative to that of the Marco Calcarenite assemblage (25 apparent species; Table 2) appears to represent an ecologically stressed community and supports the stratigraphic position of the Cenomanian-Turonian OAE2 event near the base of the Laurier Limestone beds of the Keld Member (Kyser et al., 1993). Since diversity sampling coverage tends to be highest in faunal assemblages associated with more calcareous lithologies (Kilmury & Brink, 2022), the excellent fossil preservation conditions of the Laurier Limestone beds should in theory have more complete coverage relative to the poorer preservational conditions of the ‘Near Keld base, Vermilion R.’ horizon associated with calcareous mudstone, though its apparent diversity is nearly identical with only nine apparent species observed from ‘Near Keld base, Vermilion R.’ and significantly lower than that of the Marco Calcarenite assemblage (Table 2). Together the higher preservation potential and low apparent diversity of the Laurier Limestone bed assemblage relative to the ‘Near Keld base, Vermilion R.’ and Marco Calcarenite microvertebrate assemblages provides further support for the stratigraphic position of the OAE2 event near the base of the Laurier Limestone beds in addition to the organic δ¹³C excursion observed by Kyser et al. (1993). The OAE2 stratigraphic position within the upper Keld Member may also explain why there appears to be significantly more macrovertebrate occurrences and higher apparent diversity in the Keld Member compared to the Assiniboine Member (Table 1).