Revisiting the Early Carboniferous tadpole-mimic Tarrasius

Earlier I trusted the reconstructions
of Sallan 2012 when adding Tarrasius problematicus to the large reptile tree (LRT, 1655+ taxa). I intend to rectify that mistake here with DGS tracings of the in situ fossil skull (Fig. 1).

Figure 1. From Sallan 2012 the NHM-P18062 skull of Tarrasius, the tadpole mimic. DGS colors added and used to create the reconstruction shown here. Note the complete lack of utility offered by the Sallan 2012 tracing.

Figure 1. From Sallan 2012 the NHM-P18062 skull of Tarrasius, the tadpole mimic. DGS colors added and used to create the reconstruction shown here. Note the complete lack of utility offered by the Sallan 2012 tracing.

Tarranius problematicus (Traquair 1881; Sallan 2012; Viséan, Early Carboniferous, 340mya; 10cm) was considered similar to the bichir, Polypterus, but phylogenetically close to Eusthenopteron and Phanerosteon. Here it nests with at the base of the clade that includes Pholidophorus (Fig. 3).

Distinct from other fish, the vertebral column of this tadpole-mimic is divided into cervical, dorsal, lumbar, sacral and caudal regions, despite lacking pelvic fins. Those divisions are not apparent in some of Sallan’s figures (Fig. 2).

Figure x. Subset of the LRT focusing on basal vertebrates. This represents the latest hypothesis of interrelationships and includes several changes from prior versions of this section.

Figure x. Subset of the LRT focusing on basal vertebrates. This represents the latest hypothesis of interrelationships and includes several changes from prior versions of this section.

The closest living representatives of Tarrasius
in the LRT are the salamander fish, Lepidogalaxias salamandroides, and the lizardfish, Trachinocephalus myops. The Pholidophorus clade includes the long nose gar, Lepidosteus osseous. Despite two dozen scoring changes, Tarranius only shifted two nodes from where it nested before.

Figure 2. Diagram of Tarrasius reconstructions from Sallan 2012, colorized here with the addition of the DGS tracing at lower right.

Figure 2. Diagram of Tarrasius reconstructions from Sallan 2012, colorized here with the addition of the DGS tracing at lower right. No wonder this taxon has been difficult to nest taxonomically.

Funny thing…
way back when I added Pholidophorus to the LRT, I felt I had to add pelvic and anal fins with a yellow overlay  (Fig. 3), because they were not apparent. On second thought, perhaps those fins should not have been added, given the morphology of Tarrasius (Fig. 2).

Figure 3. Pholidophorus in situ and two skulls attributed to this genus. Compare the one on the left to figure 2. No tested fish in the LRT is closer to Robustichthys than Pholidophorus.

Figure 3. Pholidophorus in situ and two skulls attributed to this genus. Compare the one on the left to figure 2. No tested fish in the LRT is closer to Robustichthys than Pholidophorus.

DGS (Digital Graphic Segregation) continues to be
a valuable graphic tool for sorting through the chaos of crushed fossils (Fig. 1). It was first employed to pick out the details of the pterosaur, Jeholopterus in 2003 (see blog masthead above). I’m seeing colorized bones more and more often in online publications where color does not cost extra. That works so much better than line art despite the misgivings of naysayers.


References
Sallan LC 2012. Tetrapod-like axial regionalization in an early ray-finned fish. Proceedings of the Royal Society B 279:3264–3271.
Traquair RH 1881. Report on the fossil fishes selected by the Geological Survey of Scotland in Eskdale and Liddesdale. I. Ganoidei. Trans. R. Soc. Edin. 30,
14–71.

 

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