Trimerorhachis and kin to scale

Updated April 23 with a revision to the tabulars of Panderichthys. Thanks DM! My bad. 

Yesterday we took a revisionary look at Trimerorhachis insignis (Cope 1878, Case 1935, Schoch 2013; Early Permian; 1m in length; Fig. 1). Today we take a quick peek at the taxa that surround it in the large reptile tree (LRT, 980 taxa, Fig. 1) all presented to scale. Several of these interrelationships have gone previously unrecognized. Hopefully seeing related taxa together will help one focus on their similarities and differences.

Figure 1. Trimerorhachis and kin to scale. Here are Panderichthys, Tiktaalik, Ossinodus, Dvinosaurus, Acanthostega, Batrachosuchus and Gerrothorax. Maybe those tabular horns on Acanthostega are really supratemporal horns, based on comparisons to related taxa.

Figure 1. Trimerorhachis and kin to scale. Here are Panderichthys, Tiktaalik, Ossinodus, Dvinosaurus, Acanthostega, Batrachosuchus and Gerrothorax. Maybe those tabular horns on Acanthostega are really supratemporal horns, based on comparisons to related taxa.

And once again
phylogenetic miniaturization appears at the base of a tetrapod clade. Note: the small size of Trimerorhachis (Fig. 1) may be due to the tens of millions of years that separate it in the Early Permian from its initial radiation in the Late Devonian, at which time similar specimens might have been larger. Provisionallly, we have to go with available evidence.

We start with…

Panderichthys rhombolepis (Gross 1941; Frasnian, Late Devonian, 380 mya; 90-130cm long; Fig. 1). Distinct from basal taxa, like Osteolepis, Pandericthys had a wide low skull, a wide low torso, a short tail and five digits (or metacarpals). No interfrontal was present. The orbits were further back and higher on the skull. Dorsal ribs, a pelvis and large bones within the four limbs were present.

Tiktaalik roseae (Daeschler, Shubin and Jenkins 2006; Late Devonian, 375mya: Fig. 1) nests between Pandericthys and Trimerorhachis in the LRT. Distinct from Panderichthys the opercular bones were absent and the orbits were even further back on the skull.

Ossinodus pueri (Warren and Turner 2004; Viséan, Lower Carboniferous; Fig. 1) was orignally considered close to Whatcheeria. Here it nests between Trimerorhachis and Acanthostega. The presence of an intertemporal appears likely. Distinct from Acanthostega, the skull is flatter, the naris is larger. Distinct from sister taxa, the maxilla is deep and houses twin canine fangs. A third fang arises from the palatine.

Acanthostega gunnari (Jarvik 1952; Clack 2006; Famennian, Late Devonian, 365mya; 60cm in length; Fig. 1) was an early tetrapod documenting the transition from fins to fingers and toes. Based on its size and placement, the nearly circular bone surrounding the otic notch is here identified as a supratemporal, not a tabular, which appears to be lost or a vestige fused to the supratemporal. This taxon is derived from a sister to Ossinodus and appears to have been an evolutionary dead end.

Trimerorhachis insignis (Cope 1878, Case 1935, Schoch 2013; Early Permian; 1m in length; Fig. 1) was considered a temnospondyl close to Dvinosaurus, but here nests as a late surviving basal tetrapod from the Late Devonian fin to finger transition. It is close to Ossinodus and still basal to Dvinosaurus (Fig. 1) and the plagiosaurs. As a late survivor, Trimerorhachis evolved certain traits found in other more derived tetrapods by convergence, like a longer femur and open palate. The presence of a branchial apparatus indicates that Trimerorhachis had gills in life. Dorsally Trimerorhachis was covered with elongated scales, similar to fish scales.

Dvinosaurus primus (Amalitzky 1921; Late Permian; PIN2005/35; Fig. 1) Dvinosauria traditionally include Neldasaurus among tested taxa. Here Dvinosaurus nests basal to plagiosaurs like Batrachosuchus and Gerrothorax and was derived from a sister to Trimerorhachis.

Batrachosuchus browni (Broom 1903; Early Triassic, 250 mya; Fig. 1) nests with Gerrothorax, but does not have quite so wide a skull.

Gerrothorax pulcherrimus (Nilsson 1934, Jenkins et al. 2008; Late Triassic; Fig. 1) was originally considered a plagiosaurine temnospondyl. Here it nests with the Trimerorhachis clade some of which  share a lack of a supratemporal-tabular rim, straight lateral ribs and other traits.

This clade of flathead basal tetrapods
is convergent with the flat-headed Spathicephalus and Metoposaurus clades and several others.

References
Berman DS and Reisz RR 1980. A new species of Trimerorhachis (Amphibia, Temnospondyli) from the Lower Permian Abo Formation of New Mexico, with discussion of Permian faunal distributions in that state. Annals of the Carnegie Museum. 49: 455–485.
Broom R 1903. On a new Stegocephalian (Batrachosuchus browni) from the Karroo Beds of Aliwal North, South Africa. Geological Magazine, New Series, Decade IV 10(11):499-501
Case EC 1935. 
Description of a collection of associated skeletons of Trimerorhachis. University of Michigan Contributions from the Museum of Paleontology. 4 (13): 227–274.
Clack JA 2006. The emergence of early tetrapods. Palaeogeography Palaeoclimatology Palaeoecology. 232: 167–189.
Clack JA 2009. The fin to limb transition: new data, interpretations, and hypotheses from paleontology and developmental biology. Annual Review of Earth and Planetary Sciences. 37: 163–179.
Coates MI 2014. The Devonian tetrapod Acanthostega gunnari Jarvik: Postcranial anatomy, basal tetrapod interrelationships and patterns of skeletal evolution. Earth and Environmental Science Transactions of the Royal Society of Edinburgh.
Coates MI and Clack JA 1990. Polydactly in the earliest known tetrapod limbs. Nature 347: 66-69.
Colbert EH 1955. Scales in the Permian amphibian. American Museum Novitates. 1740: 1–17.
Daeschler EB, Shubin NH and Jenkins FA, Jr 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature. 440 (7085): 757–763.
Gross W 1941. Über den Unterkiefer einiger devonischer Crossopterygier (About the lower jaw of some Devonian crossopterygians), Abhandlungen der preußischen Akademie der Wissenschaften Jahrgang.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.
Jenkins FA Jr, Shubin NH, Gates SM and Warren A 2008. Gerrothorax pulcherrimus from the Upper Triassic Fleming Fjord Formation of East Greenland and a reassessment of head lifting in temnospondyl feeding. Journal of Vertebrate Paleontology. 28 (4): 935–950.
Nilsson T 1934. Vorläufige mitteilung über einen Stegocephalenfund aus dem Rhät Schonens. Geologiska Föreningens I Stockholm Förehandlingar 56:428-442.
Olson EC 1979. Aspects of the biology of Trimerorhachis (Amphibia: Temnospondyli). Journal of Paleontology. 53 (1): 1–17.
Pawley K 2007. The postcranial skeleton of Trimerorhachis insignis Cope, 1878 (Temnospondyli: Trimerorhachidae): a plesiomorphic temnospondyl from the Lower Permian of North America. Journal of Paleontology. 81 (5):
Warren A and Turner S 2004. The first stem tetrapod from the Lower Carboniferous of Gondwana. Palaeontology 47(1):151-184.
Williston SW 1915. 
Trimerorhachis, a Permian temnospondyl amphibian. The Journal of Geology. 23 (3): 246–255.
Williston SW 1916. The skeleton of Trimerorhachis. The Journal of Geology. 24 (3): 291–297.

 

wiki/Ossinodus
wiki/Acanthostega
wiki/Tiktaalik
wiki/Panderichthys
wiki/Trimerorhachis
wiki/Gerrothorax
wiki/Batrachosuchus

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3 thoughts on “Trimerorhachis and kin to scale

  1. Oh, I overlooked this post.

    Note the twin supratemporals on Panderichthys determined by comparison to related taxa.

    What… oh. You didn’t know what an extrascapular is. That transverse row of three oval bones at the back of the skull of Panderichthys are the median extrascapular and the lateral extrascapulars (not tabulars). This row is part of the connection between skull and shoulder girdle and is missing in Tiktaalik and in tetrapods. Just caudal to the lateral extrascapular, you can find the posttemporal, followed caudoventrally (caudal to the opercular) by the supracleithrum, the anocleithrum and the cleithrum.

    And maybe those tabular horns on Acanthostega are really supratemporal horns, based on comparisons to related taxa.

    I don’t understand. Please explain.

    he presence of a branchial apparatus indicates that Trimerorhachis had external gills in life.

    No, it just indicates that Trimerorhachis had any gills at all. Judging from Dvinosaurus and Thabanchuia, they were internal gills, not external gills. External gills are outgrowths of the septa between the internal gills; the arteries that supply them don’t leave grooves on the ceratobranchials because they’re not in the same place in the septum.

    I’ll send you the necessary papers (Witzmann & Schoch, 2010 “2011”; Witzmann, 2013).

    Dorsally Trimerorhachis was covered with elongated scales, similar to fish scales.

    Uh, yes, that’s normal for tetrapods – lost in amniotes, a few temnospondyls, and hardly anywhere else.

    I’ll send you the necessary papers (Witzmann, 2007, 2010 “2011”).

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