SVP abstracts – Silurian gnathostome

Zhu et a. 2019 bring us
a new Silurian fish they claim is close to the origin of jawed vertebrates (= Gnathostomata).

From the abstract:
“Modern jawed vertebrates or crown-group gnathostome include the last common ancestor of living bony and cartilaginous fishes and all its descendants. The gross morphology of the earliest modern jawed vertebrates, and how they arose from stem gnathostomes, were previously unknown due to a lack of articulated fossils.”

These taxa are not unknown in the large reptile tree (LRT, 1592 taxa). Put enough taxa in an analysis and one will end up close to the origin of gnathostomes. There will be a last common ancestor. In the LRT Thelodus, a ?jawless (phylogenetic bracketing indicates some sort of transverse jaws are present) Silurian fish is the current proximal outgroup to all tested taxa with jaws. In LRT the extant whale shark (Rhincodon), angel shark (Squatina) and horn shark (Heterodontus) are basal members of the Gnathostomata and the first taxa with primitive tooth carpets.

“The recent discovery of the Xiaoxiang Fauna from the Silurian of South China revolutionarily adds to the diversity of Silurian jawed vertebrates. However, considerable morphological gap is still present between stem- and crown-group gnathostomes.”

Not so, when appropriate taxa are included.

“Here, we report a new bony fish very close to the crown-group gnathostome node, also from the Xiaoxiang Fauna. The attributed specimens include a head, jaws and an articulated postcranial skeleton.”

“The new fish displays a unique suite of characters: the dermal pectoral girdle condition transitional between Entelognathus and osteichthyans, the braincase profile recalling the condition in Janusiscus and early chondrichthyans, and the premaxillae and lower jaw largely showing osteichthyan features. This mosaic character combination suggests the tentative phylogenetic position of this new taxa in the most basal segment of the osteichthyan stem, possibly forming a quintessential component of the evolutionary transition between placoderms and osteichthyans.”

In the LRT taxa between placoderms and osteichthyans are either acanthodians (spiny sharks) on one branch, or catfish (also with spiny fins) on the other branch. Catfish are whales-shark mimics with regard to their jaws and teeth, likely representing some sort of reversal to that basal condition.

“For the first time, we are able to look into a near-complete bony fish close to the last common ancestor of all the living jawed vertebrates, and reconstruct the acquisition sequence of osteichthyan characters based on a series of fossils in morphological proximity. The fact that most of these fossils are from the Silurian Xiaoxiang Fauna, suggests that this fauna is unprecedentedly close to the initial radiation of jawed vertebrates.” 

Figure 2. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus. This tree does not agree with previous fish tree topologies.

Figure 1. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus. This tree does not agree with previous fish tree topologies. Check out the LRT for a slightly updated version of this cladogram.

This is all very interesting, and welcome, but let them look at the structure of Rhincodon as it relates to Thelodus at least once before settling down with the Zhu et al. hypothesis.

Figure 4. Manta compared to Thelodus and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes.

Figure 4. Manta compared to Thelodus and Rhincodon. All three have a terminal mouth essentially straight across, between the lateral eyes.


References
Zhu et al. 2019. A new Silurian bony fish close to the common ancestor of crown gnathostomes.

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