Parmastega: not “a sister group to all other tetrapods”

Beznosov, Clack, Lusevics, Ruta and Ahlberg 2019
describe a new Russian basal ‘tetrapod’ Parmastega (Fig. 1), based on a nearly complete skull and pectoral girdle. Dated at 372 million years ago, or 12 million years before Acanthostega (Fig. 1), Ichthyostega and Tulerpeton, this taxon offers insights into the acquisition of basal tetrapod traits.

Parmastega was described as
“a sister group to all other tetrapods.” Of eight published analyses in this paper, most nest Paramastega close to Ventastega (Fig. 1).

Unfortunately,
the Beznosov et al. taxon list includes only the traditional tetrapods typically tested in studies like this and Eusthenopteron. Their taxon list omits many, if not most of the taxa listed in yesterday’s list of taxa transitional between jawless fish and reptiles.

Figure 1. Parmastega compared to scale with Acanthostega and Ventastega.

Figure 1. Parmastega compared to scale with Acanthostega and Ventastega. Both are similar to Parmastega in most regards. The placement of the naris in Ventastega might not have been ventrally, on the jawline, but higher on the snout as shown here.

By contrast,
the large reptile tree (LRT, 1587 taxa) nests Parmastega so close to Acanthostega. As reported yesterday and earlier, a larger taxon list indicates that Acanthostega is not a taxon transitional between fins and feet, but is a derived tetrapod apparently returning to a more aquatic niche. This runs counter to traditional hypotheses put forth by co-authors Ruta, Clack and Ahlberg, the top experts in this niche of paleontology.

A concave rostrum,
elevated orbits and large size mark it as a derived taxon, despite its antiquity. The intertemporal is fused to the supratemporal.

The post-cranial skeleton of Parmastega
was described as “weakly ossified”. That is in direct contrast to Acanthostega and virtually all other taxa in the LRT.

The chronology offered by Parmastega
supports the hypothesis of a radiation of tetrapods much earlier, with the few fossils found in the Late Devonian representing late-surviving radiations of that radiation.


References
Beznosov PA, Clack JA, Lufsevics E, Ruta M and Ahlberg PE 2019. Morphology of the earliest reconstructable tetrapod Parmastega aelidae. Nature 574:527–531.

 

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