Pederpes (Clack & Finney, 2005)

Otoo, Benjamin K A, Bolt, John R, Lombard, R Eric, Angielczyk, Kenneth D & Coates, Michael I, 2021, The postcranial anatomy of Whatcheeria deltae and its implications for the family Whatcheeriidae, Zoological Journal of the Linnean Society 193 (2), pp. 700-700 : 700-

Reinterpretation of Pederpes

Pederpes is known from a single, well-preserved and near-articulated specimen ( Clack, 2002). Of relevance to the present discussion, the appendicular skeleton is less ossified than that of adult Whatcheeria , with smaller or absent processes of the humerus, ulna and femur. From the description and figures ( Clack, 2002; Clack & Finney, 2005) it is clear that the limb bone joint surfaces must have been finished with large amounts of cartilage. These observations suggest that Pederpes is subadult, and that this is likely the source of many differences between equivalent structures in Whatcheeria and Pederpes . The degree of ossification in the Pederpes appendicular skeleton resembles Class I Whatcheeria material, consistent with the size of the Pederpes holotype: approximately 50 cm presacral length, including the skull. However, the limbs of Pederpes are already proportionally longer than those of Whatcheeria ( Table 2). Class I Whatcheeria specimens appear to have already established adult proportions of the pectoral, cervical and trunk regions. Thus, it appears that a hypothetical adult Pederpes (assuming a Whatcheeria -like growth trajectory) would have been anatomically distinct from Whatcheeria , with somewhat stouter forequarters.

Status and membership: the ‘what’ in whatcheeriid

The erection of the Whatcheeriidae ( Clack, 2002) was noted as the first new family of Mississippian tetrapods to be named in decades ( Clack & Milner, 2015). Since the publication of Pederpes , Ossinodus has also been linked to the group ( Warren & Turner, 2004; Warren, 2007), as have isolated cranial and postcranial bones from both Devonian ( Daeschler et al., 2009; Olive et al., 2016) and Carboniferous ( Anderson et al., 2015) localities. Here we aim to review such material in light of the improved knowledge of the distinctive characteristics of whatcheeriid morphology.

Synapomorphies associating Ossinodus with the Whatcheeriidae ( Warren & Turner, 2004) include:

• Massive tooth on the maxilla about position 5 or 6. • Broad interclavicle with acutely angled lateral corners.

Whatcheeriid symplesiomorphies also present in Ossinodus ( Warren & Turner, 2004) :

• Supratemporal–postparietal contact.

• (Probable) fangs and row of smaller accessory teeth on vomers, palatines and ectopterygoids.

• Nearly continuous row of coronoid teeth.

• At least some lateral lines in tubes through bone.

• Ilium with postiliac process and dorsal iliac blade.

Characters of uncertain polarity shared with whatcheeriids ( Warren & Turner, 2004):

• Pronounced angle between skull table and cheek in transverse section.

• Trunk ribs with expanded distal flanges.

Aside from Whatcheeria, Pederpes , and Ossinodus , ‘caniniform’ teeth on the maxilla are also present in Ymeria ( Clack et al., 2012) , Eryops ( Sawin, 1941) , and arguably Anthracosaurus ( Panchen, 1977; Clack, 1987a); their presence does not appear to define a natural group or help distinguish between Devonian and post-Devonian tetrapods. However, of these taxa, only Whatcheeria, Pederpes , and Ossinodus have two maxillary caniniform teeth.

The cited features of interclavicle morphology are neither unique to whatcheeriids nor Ossinodus , as the characterization is applicable to colosteids, Crassigyrinus , embolomeres ( Romer, 1957; Holmes, 1984; Clack, 1987b), and others. Although the interclavicles of Pederpes and Ossinodus are very similar, they both differ from that of Whatcheeria , which resembles Ichthyostega . Similarly, distal flanges (uncinate processes) occur in a scattering of disparate early tetrapods (see discussion in Coates (1996)). The uncinate processes of Ossinodus are similar to those of Acanthostega , whereas Whatcheeria resembles Ichthyostega and Eryops . Warren (2007) showed that Ossinodus differs from Pederpes and Whatcheeria in skull shape and proportions. We concur, and offer no additional support for the hypothesis that Ossinodus is a whatcheeriid.

Fragmentary Devonian and Carboniferous material referred to the Whatcheeriidae is listed in Table 3. Given that the whatcheeriid status of Ossinodus is now doubtful, support for the assignment of many of these specimens is now uncertain or absent, because, in turn, these were based, at least in part, on comparison with Ossinodus .

Several ‘whatcheeriid’ specimens have been collected from the Famennian Catskill Formation Red Hill locality in Pennsylvania ( Daeschler et al., 2009; Broussard et al., 2018). ANSP 21873 , a postorbital, has been compared to that of Pederpes in shape. As figured, the resemblance is suggestive and indicates that the animal likely had non-circular orbits. However, noncircular orbits are not unique to whatcheeriids, and both ANSP 21873 and the only visible postorbital of the Pederpes holotype are incomplete . ANSP 21874 , a left lacrimal, has similar ornament and probably belongs to the same taxon as ANSP 21873 . Neither specimen is sufficiently complete to infer the presence of a Red Hill whatcheeriid .

ANSP 21476, a femur, is unlike those of Whatcheeria or Pederpes . The shaft is longer and the ends are narrower. It seems to show the short, distally located adductor crest and large adductor blade present in Acanthostega , and, to a lesser extent, in Ichthyostega and Tulerpeton (cf. Coates, 1996), as well as Ossinodus . Adductor blade size in ANSP 21476 relative to the rest of the femur resembles Ossinodus rather than the Devonian taxa, as does the lack of torsion of the epipodial facets. The distal protrusion of the fibular condyle past the tibial condyle is intermediate between Tulerpeton and Ossinodus . ANSP 23864, an incomplete femur from the slightly older Trout Run North locality within the Catskill Formation, shows similar overall proportions and positions of the adductor crest and adductor blade. We disagree with the identification of a proximally located internal trochanter in ANSP 23864 by Broussard et al. (2018). Given the apparent abrasion of the specimen, we interpret that feature as being an artefact of wear combined with damage to the proximal end and intertrochanteric fossa, with the internal (and, presumably, fourth) trochanters not preserved. Neither ANSP 21476 nor ANSP 23864 is conclusively whatcheeriid.

Additional Famennian ‘whatcheeriid’ fragments, a postorbital and a maxilla, have been described from Strud and Becco in Belgium ( Clement et al., 2004; Olive et al., 2016). The Strud postorbital, IRSNB A.0006, is much more complete than ANSP 21873, and shares the same ornament pattern as well as (inferred) shape. Once again, there is nothing distinctively whatcheeriid about IRSNB A.0006. The Becco maxilla, IRSNB A.0007, has two large teeth with tooth pits starting at positions 3 or 4. This is the deepest part of the maxilla, with depth and tooth size decreasing posteriorly, unlike the Strud ‘ichthyostegid’ maxillae, which show uniformly sized teeth. As previously discussed, maxillary caniniform teeth might be a characteristic of Whatcheeria, Pederpes and Ossinodus , but in the absence of further diagnostic features, these data are insufficient to refer IRSNB A.0007 to Whatcheeriidae .

Specimens from Tournaisian deposits at Burnmouth in Scotland were recently compared to Pederpes and Whatcheeria ( Otoo et al., 2018) . Although more definitive conclusions await further Burnmouth material and a detailed description of the Whatcheeria skull, a few remarks can be made here. As previously assigned, the frontal bones, with at least one partial prefrontal, resemble their Whatcheeria ( Lombard & Bolt, 1995) and Pederpes ( Clack & Finney, 2005) counterparts. Contra Otoo et al. (2018), the cleithrum resembles that of Pederpes in the slight curvature of the stem, the rounded shape of the dorsal expansion and the lack of a posterior notch. The radius is slenderer than that of Pederpes or Whatcheeria , but exhibits no distinctively whatcheeriid features. The phalanges are longer than broad with well-defined flexor attachments (cf. Otoo et al. 2018), thus unlike Whatcheeria . The jaw material with anteroposteriorly compressed teeth is unique. The existence of two disparate size fractions within the collection challenges easy taxonomic assignments, although the presence of maxillae with the unique tooth morphology across both size classes suggests that both fractions might represent, in part, the same taxon. Unfortunately, most currently known skeletal material derives from one size class, but at least some of this likely represents a Pederpes -like whatcheeriid.

Numerous isolated tetrapod bones have been recovered from the Blue Beach locality at Horton Bluff (both names are used interchangeably to refer to this locality) in Nova Scotia ( Anderson et al., 2015). Blue Beach is important because it, and the similarly aged Ballagan Formation in Britain, provide the currently known suite of Tournaisian tetrapods ( Clack & Finney, 2005; Smithson et al., 2012; Clack et al., 2016, 2019a; Chen et al., 2018; Otoo et al., 2018; Smithson & Clack, 2018; Lennie et al., 2020).

Pelvis Type 1 (NSM005GF045.001) has iliac processes similar to those of Whatcheeria and Pederpes : robust, distally expanded and probably short, although the complete length of the posterior process is unknown. As in many early tetrapod ilia, the processes are offset by a notch, but their limited overlap in lateral view is Pederpes -like ( Clack & Finney, 2005). The dorsal process is subrectangular, but it is not clear whether the flat dorsal margin is complete or not. Unlike Pederpes , but in common with Whatcheeria and most (adult) early tetrapods, the pubis is ossified and sutured to the ischium. The acetabulum is too poorly preserved to offer diagnostic features, but the ischium displays a distinct concave dorsal margin, unlike Pederpes or Whatcheeria . On the basis of a general resemblance of the ilia but little else, we agree with Anderson et al. (2015) that NSM005GF045.001 is similar to whatcheeriid pelves.

Femur Type 2 (NSM004GF045.034A, B), contra its original description as a ‘tulerpetonid’ ( Anderson et al., 2015), bears a strong resemblance to the femur of Ossinodus ( Fig. 33 ). The principal difference between the specimen and Ossinodus appears to be the relative sizes of the internal and fourth trochanters: in Femur Type 2, the former is larger, and in Ossinodus , the latter is larger.

Tibia Type 2 (NSM014GF036.003; Anderson et al. 2015) was compared to those of Pederpes and Ossinodus , which differ in their morphology. In fact, it more closely resembles the tibia of Pederpes , having greatly expanded ends, virtually no shaft, an almost straight anterior edge (shin) and a small but distinct concavity for the interepipodial space on the posterior rim. Although the diagnostic value of these similarities is uncertain, in view of the strong resemblance to Pederpes and the other putative whatcheeriid material from Blue Beach, we refer NSM014GF036.003 to Whatcheeriidae .

Humerus Type 1 (NSM005GF045.037, YPM PU 23545) resembles the humerus of Pederpes ( Anderson et al., 2015) . Both have a similarly sized deltopectoral process and a spike-shaped latissimus dorsi process, the latter characteristic also evident in Whatcheeria and Baphetes (Milner & Lindsey 1998) . Like Pederpes ( Clack & Finney, 2005) , the anterior and distal surfaces are unfinished and the distal articular condyles are undifferentiated. Again, this characteristic occurs in other early humeri, such as those of Crassigyrinus ( Panchen, 1985) and Ossinodus ( Bishop, 2014) , but is unlike the far more completely ossified condition of the Whatcheeria humerus. The ~90-degree angle between the shaft and rectangular entepicondyle of Humerus Type 1 closely resembles Pederpes , but, once again, this characteristic occurs further afield, in examples such as Greererpeton ( Godfrey, 1989) . The humerus torsion angle is high for an early tetrapod humerus, ~60 degrees, almost twice that of Pederpes ( Smithson & Clack, 2018) and thrice that of Whatcheeria . In summary, Anderson et al. ’s (2015) comparison with Pederpes is supported, but there remains the possibility that the similarities are more general to early tetrapod humeri and that nothing specific to whatcheeriids has been identified.

NSM.014.GF.036.005 is an incomplete interclavicle that was compared with that of Pederpes (also incomplete: Clack & Finney, 2005) but considered inconclusive as evidence of a whatcheeriid ( Anderson et al., 2015). We find no evidence to extend beyond this conclusion.

Occidens ( Clack & Ahlberg, 2004) , known from a partial jaw of suspected Late Tournaisian–Early Viséan age, has recently been associated with the whatcheeriids in a phylogenetic analysis ( Clack et al., 2016). It shares with Whatcheeria a single row of teeth on each coronoid and near-absence of dermal ornament. The mandible of Pederpes is not known in sufficient detail for comparison. Each of these characters is present in other tetrapods, but their conjunction might be unique to Occidens and Whatcheeria . However, unlike Whatcheeria , the splenial is not sutured to the prearticular posteriorly ( Clack & Ahlberg, 2004; Lombard & Bolt, 2006). It seems likely that the various polytomies and sistergroup combinations of Occidens and Whatcheeria and Pederpes in the analysis of Clack et al. (2016) result from incompleteness rather than genuine similarity. Therefore, its whatcheeriid status is questionable.

Three ilia (CMC VP7261, CMC VP7664 and CMC VP7328) from exposures of the Serpukhovian Buffalo Wallow Formation in Hancock County, Kentucky, have been compared to that of Pederpes ( Garcia et al., 2006; Greb et al., 2016). CMC VP7328 is the largest, the only one figured and resembles the ilium of Whatcheeria ( Garcia et al., 2006: fig. 9). The posterior iliac process is more proximally constricted than in Whatcheeria and expands posteriorly, producing a spoon-like appearance. The dorsal process is incomplete but CMC VP7328 suggests that it was circular or oval. The two processes are separated by a broad groove and probably would have overlapped in lateral view. There is no indication that either process has the fluting seen in Whatcheeria . Notably, CMC VP7328 is very large, approximately 20 cm long ( Garcia et al., 2006: fig. 9). This is comparable to the size that would be expected from the largest Whatcheeria individuals. On the basis of the numerous similarities between CMC VP7328, we refer CMC VP7328 (and by extension CMC VP7261 and CMC VP7664) to Whatcheeriidae . It is notable that the Delta and Hancock County localities are geographically close and the latter is probably only slightly older than the former, with the caveat that there is some uncertainty as to their absolute ages. Given this, there is the question of whether or not the Buffalo Wallow whatcheeriid is taxonomically distinct from Whatcheeria deltae . Further Buffalo Wallow material is needed to resolve this question. In either case, the Buffalo Wallow whatcheeriid represents the geologically youngest occurrence of the family.

Assignments for the fragmentary material are summarized in Table 3. These specimens span the Devonian/Carboniferous boundary and include some of the oldest known fossils of limbed tetrapods. Most material is of uncertain affinity, and only some of it is referable to Whatcheeriidae . The material that is compared to Ossinodus cannot be more precisely identified because it is not yet apparent that the combination of characters seen in the femur of Ossinodus is diagnostic for that taxon among early tetrapods.

Significantly, none of the fragmentary specimens permittheidentificationofwhatcheeriidsintheDevonian, though IRSNB A.0007 (a Strud maxilla) is suggestive. Thus, the fossil record of whatcheeriids remains limited to the Carboniferous, although a Devonian origin has long been mooted based on the Tournaisian age of Pederpes . From the present work, fragment attributions to the family are mostly inconclusive. Given the rarity of morphological innovation (i.e. apomorphies) among whatcheeriids noted earlier, this seems unlikely to change. It follows that whatcheeriid range extension into the Devonian, in the absence of new specimens, is most likely to depend on implications of phylogenetic results (e.g. Ahlberg & Clack, 2020).

Figures

Figure 33. Femora of Ossinodus and other Devonian–Carboniferous femora. QMF 37432 Ossinodus right femur in ventral (A) and dorsal (B) views; QMF 37415 Ossinodus left femur in ventral (C) and dorsal (D) views; ANSP 23864 left femur drawn after (Broussard et al., 2018) in dorsal (E), anterior (F), ventral (G) and posterior (H) views; ANSP 21476 left femur drawn after (Daeschler et al., 2009) in ventral (I), posterior (J), dorsal (K) and anterior (L) views; NSM004GF045.034A left femur drawn after Anderson et al. (2015) in posterior (M), ventral (N), anterior (O), and dorsal (P) views; NSM004GF045.034B left femur drawn after (Anderson et al., 2015) in dorsal (Q), anterior (R), ventral (S) and posterior (T) views. In all specimens, proximal is at the top and distal is at the bottom.