New basal thalattosaur: Gunakadeit joseeae

Druckenmiller et al. 2020 bring us news of
a new sharp-snouted thalattosaur, Gunakadeit (Figs. 1, 2; Norian,Late Triassic; UAMES 23258) represented by a beautiful, nearly complete and almost fully articulated skeleton.

From the abstract:
“The holotype is the most complete North American thalattosaurian yet described and one of the youngest occurrences of the clade worldwide.”

Figure 1. Gunakadeit in situ with colors added for bone identification.

Figure 1. Gunakadeit in situ with colors added for bone identification.

The premaxilla
was extremly sharp, as were the recurved dentary teeth. Only tiny teeth are present on the upper jaws. The tail may have been quite short, distinct from other thalattosaurs.

A few small bones posterior to the occiput
marked with question marks (Fig. 2) are a string of anterior cervicals (Fig. 1). Overlooked and not mentioned in the text, but common to all thalattosaurs is the septomaxilla forming the lower rim of the naris. Taphonomically, the skull is split right down the middle, giving a rare view of both the interior and exterior.

Figure 2. Gunakadeit skull in situ and reconstructed using DGS methods.

Figure 2. Gunakadeit skull in situ and reconstructed using DGS methods. Note the lack of a posterior process on the jugal, a traditional hallmark of higher thalattosaurs.

With regard to outgroups, the authors state,
“Given the uncertainty regarding placement of Thalattosauria within Diapisda, three basal diapsids, Petrolacosaurus kansensis (Carboniferous), Youngina capensis (Permian) and Claudiosaurus germaini (Permian) were selected as outgroups.”

Figure 3. Cladogram of the Mesosauria + Thalattosauria with Gunakadeit now included.

Figure 3. Subset of the LRT cladogram focusing on the Mesosauria + Thalattosauria with Gunakadeit now included.

That is unfortunate
as the list of extended outgroups for the clade Thalattosauria were recovered several years ago at www.ReptileEvolution.com/reptile-tree.htm (subset Fig. 3). Excluded by the authors, Serpianosaurus is basal to both clades of thalattosaurs. This clade is a sister to the Mesosauria. That clade is a sister to Wumengosaurus and the Ichthyosauria. That clade arises from basalmost Sauropterygia (pachypleurosaurs). Together those clades comprise the traditionally outdated, but here resurrected clade, Enaliosauria.

Figure 2. The Thalattosauria and outgroups (Wumengosaurus and Stereosternum) to scale.

Figure 4. From 2013, the Thalattosauria and outgroups (Wumengosaurus and Stereosternum) to scale. Several taxa have been added since then.

Gunakabeit nests basal to the major dichotomy in Thalattosauria,
(Fig. 3) between Serpianosaurus and that split in the clade, despite its relatively late appearance in the fossil record. Other than coeval sharp-snouted Endennasaurus, most thalattosaurs are known from the Middle to Early Late Triassic (Carnian).

Also excluded by Druckenmiller et al. 2020
are the derived thalattosaurs, Eusaurosphargis, Helveticosaurus and Vancleavea.

Adding taxa gives you these solutions.
Adding characters does not. Hopefully this phylogenetic fact is starting to become clear.


References
Druckenmiller PS, Kelley NP, Metz ET and Baichtal J 2020.
 An articulated Late Triassic (Norian) thalattosauroid from Alaska and ecomorphology and extinction of Thalattosauria.
Scientific Reports 10, Article number: 1746
doi: https://doi.org/10.1038/s41598-020-57939-2
https://www.nature.com/articles/s41598-020-57939-2

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