A tadpole-like fish with a tetrapod-like backbone: Tarrasius problematicus

Updated March 1, 2020
with a DGS tracing of an in situ skull. 

Completely by convergence
this fish, Tarrasius problematicus (Traquair 1881; Viséan, Early Carboniferous 340mya; 10cm) developed morphologically distinct and diverse vertebrae (Fig. 1), like those of tetrapods, according to Sallan 2012.

Figure 1. From Sallan 2012, reconstruction of Tarrasius with colors added. Note the five vertebral regions.

Figure 1. From Sallan 2012, reconstruction of Tarrasius with colors added. Note the five vertebral regions. See Figure 1 updated below.

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

Figure 1 updated. Diagram of Tarrasius reconstructions from Sallan 2012, colorized here with the addition of the DGS tracing at lower right.

From the Sallan abstract:
“Here, I show that, Tarrasius problematicus, a marine ray-finned fish from the Mississippian (Early Carboniferous; 359–318 Ma) of Scotland, is the first non-tetrapod known to possess tetrapod-like axial regionalization. Tarrasius exhibits five vertebral regions, including a seven-vertebrae ‘cervical’ series and a reinforced ‘sacrum’ over the pelvic area. Most vertebrae possess processes for intervertebral contact similar to tetrapod zygapophyses. The fully aquatic Tarrasius evolved these morphologies alongside other traits convergent with early tetrapods, including a naked trunk, and a single median continuous fin. Regional modifications in Tarrasius probably facilitated pelagic swimming, rather than a terrestrial lifestyle or walking gait, presenting an alternative scenario for the evolution of such traits in tetrapods. Axial regionalization in Tarrasius could indicate tetrapod-like Hox expression patterns, possibly representing the primitive state for jawed vertebrates. Alternately, it could signal a weaker relationship, or even a complete disconnect, between Hox expression domains and vertebrate axial plans.”

Sallan reports,
“Tarrasius problematic us (Traquair 1881) is a fossil ray-finned fish (Actinopterygii) found in the Mississippian marine sediments of Scotland, phylogenetically branching off either the actinopterygian or actinopteran stem.”

By contrast
in the large reptile tree (LRT, 1655+ taxa) Tarrasius (Figs. 1, 2) nests with  Pholidophorus (Fig. 3), far from the origin of tetrapods, but deep within the lineage.

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 2 updated. 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.

Odd that a taxon like this has five distinct vertebral types.
It doesn’t even have a pelvis and hind fins,

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 at the base of the Pholidophorus clade.

Figure x. Subset of the LRT focusing on basal vertebrates (= fish).

Figure x updated. Subset of the LRT focusing on basal vertebrates (= fish).

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.

https://en.wikipedia.org/wiki/Tarrasiiformes

doi:10.1098/rspb.2012.0784

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