Calligenethlon watsoni ( Steen, 1934 )

Adams, Gabrielle R., Otoo, Benjamin K. A., Bohus, Caleb P. W., Micucci, Logan M. & Maddin, Hillary C., 2025, Anatomy and revised diagnosis of the embolomere Calligenethlon watsoni from Joggins, Nova Scotia, based on micro-computed tomography, Zoological Journal of the Linnean Society (Statistics in Society) 203 (2), pp. 109-109 : 109-

Calligenethlon watsoni ( Steen, 1934)

Type and only species: Calligenethlon watsoni .

Holotype: R.M. 2.1122 .

Type horizon and locality: Found in a slab on the beach adjacent to the Joggins Formation, near the town of Joggins, Nova Scotia, Canada. Exact horizon of origin cannot be determined.

Referred specimens: RM 2.1193.a, RM 12115, NMC 10096, NHMUK R 442, NHMUK R 4553, NSM 988GF70.1, RM 20.4984, and NSM 994GF1.1.

Revised diagnosis: Embolomere characterized by: narrow skull table with parallel lateral margins and lacking lateral lines; large tabular with elongate single horn extending posteriorly well beyond the occiput; tabular horns parallel to each other; surangular with horizontal crest; conical, non-recurved marginal dentition that is not closely packed; interclavicle with strong fimbriation on anterior edge; atlas pleurocentrum completely ossified dorsally but incomplete ventrally; postaxial pleurocentra robust, well-ossified, and always larger than corresponding intercentra; intercentra wedge-shaped in lateral view; intercentra open dorsally but with increasing dorsal closure and eventual full fusion moving posteriorly along vertebral column in larger individuals; humerus ectepicondyle well-developed with sharp distal hook; anocleithrum present; posterior iliac process extremely elongate and gracile; tail dorsoventrally short.

Reconstruction of Calligenethlon watsoni

Full-body reconstructions of Calligenethlon watsoni are presented in Figure 25 . Tese draw on NSM 994GF1.1 and the specimens referred to the genus above. Unpreserved appendicular bones are based on Proterogyrinus . Te first reconstruction ( Fig.25A ) is the most conservative, with a short, Proterogyrinus / Palaeoherpeton - like skull, ‘standard-length’ neck region, and short trunk. A longirostrine skull reconstruction ( Fig. 25B ) was created by moving the maxilla/jugal contact anterodorsally. Two further reconstructions ( Fig. 25C, D ) atempt to achieve a more standard presacral count by adding six vertebrae to the posterior end of the NSM 994GF1.1 presacral column.

In each reconstruction, Calligenethlon emerges as an unusual embolomere with a large head and short, shallow trunk. Te brevirostrine versions especially ( Fig. 25A, C ) bear a superficial resemblance to Gephyrostegus ( Carroll 1970) in the proportions of the appendicular limbs and morphology of the axial skeleton.

Further questions regarding the morphology of Calligenethlon Te confirmation of consistency in the anatomy observed among specimens of Calligenethlon leads to the question of the great disparity in sizes from the smallest specimen, NHMUK R 4553, to the largest, RM 20.4984. Te former pertains to ‘ Dendryozousa ’, now known as a junior synonym of Calligenethlon ( Holmes and Carroll 2010) , and shows more poorly ossified centra and a large perforation for the notochord, both of which are considered juvenile characteristics ( Holmes 1984). Te other Calligenethlon specimens vary slightly in size, most pertaining to individuals about half the size of Proterogyrinus , but two represent a larger, more Proterogyrinus -sized individual ( Godfrey et al. 1991). Such a dramatic difference in size makes a simple explanation of individual variation unlikely, although Godfrey et al. (1991) point out that the differing body sizes may be growth stages of Calligenethlon watsoni . However, most specimens atributed to Calligenethlon show evidence of some maturity, such as tight suturing of the cranial bones, well-developed processes of limb bones, and overall advanced ossification ( Carroll 1967, Godfrey et al. 1991, Holmes and Carroll 2010). It is especially telling that the braincase of NSM 994GF1.1 is well-ossified. Tese features all indicate that Calligenethlon must have atained some level of maturity while still at a small size ( Godfrey et al. 1991, Holmes and Carroll 2010) but much larger sizes were possible. Te common, small specimens might represent individuals near the onset of sexual maturity, and the rare, large specimens might represent older individuals that survived that period of elevated mortality. Morphological and skeletochronological evidence supports this interpretation of Whatcheeria specimens from Delta ( Otoo et al. 2021, Whitney et al. 2022). Alternatively, RM 20.4984 and NSM 988GF70.1 could represent a second, larger Calligenethlon species than C. watsoni .

Embolomere diversity at Joggins

Te potential presence of a large species of Calligenethlon calls for the re-examination of several other large-bodied tetrapod remains from Joggins. An incomplete lower jaw was named ‘ Baphetes ’ minor by Dawson (1870) and figured by Romer (1963). Te classification of this specimen as a baphetid has been challenged and there has been some speculation as to whether this animal should instead be considered an embolomere ( Romer 1963, Holmes and Carroll 2010). Te strongly convex curvature of the jaw margin is reminiscent of Archeria , but the dentition is more similar to that of NSM 994GF1.1.Te jaw, about 15 cm in length, could belong to a large individual of Calligenethlon watsoni or a new, large-bodied Calligenethlon species. More material is required for a definitive taxonomic conclusion.

Another specimen, FGM 998GF7.1, is a large pelvis atributed to the Hebert beds’ horizon of the Joggins Formation ( Hebert and Calder 2004). Tis pelvis was tentatively assigned to a baphetid, but has recently been re-assigned to an embolomere (Adams et al. in press). Te pelvis preserved in FGM 998GF7.1 is distinct in morphology from all other known embolomeres, including Calligenethlon , suggesting the presence of yet another large-bodied embolomere at the locality. Several other specimens, such as FGM 000GF15––a block preserving scatered cranial and postcranial bones, including possible skull material, ribs, neural spines, and vertebrae––may add even more embolomere diversity to the locality. Further investigation is required to determine whether these specimens pertain to the larger Calligenethlon species or morph, a distinct taxon encompassing the ‘ Baphetes ’ minor material, or represent a new large embolomere at Joggins. Te presentation of the new Calligenethlon anatomical data here provides a more complete knowledge of embolomere morphology and will aid both the revision of existing embolomere material and the identification of new embolomere material from Joggins.

Figures

Figure 25. Reconstructions of Calligenethlon watsoni. Short-trunk versions, with brevirostrine, Proterogyrinus-esque skull (A) and longirostrine, Archeria-esque skull (B); long-trunk versions, with brevirostrine (C) and longirostrine (D) skulls.Portions restored afer other taxa are coloured grey.