Platysomus sheds new light on placoderms

Short one today.
One more fish enters the LRT. Some changes (like a prefrontal) are added to previously nested taxa.

Adding the Carboniferous fish,
Platysomus (Fig. 1) , to the large reptile tree (LRT, 1713+ taxa; Fig. 2) to no one’s surprise nests it with Cheirodus (= Chirodus, Amphicentrum; Fig. 1), a less stretched-out version.

The heresy is
these two taxa nest with catfish and placoderms (Fig. 2) when allowed to do so by taxon inclusion, as we’ve seen previously. Placoderms evolve from ordinary fish.

Figure 1. Platysomus and Cheirodus are both platysomids, related to catfish and placoderms. All these taxa lack maxillae.

Figure 1. Platysomus and Cheirodus are both platysomids, related to catfish and placoderms. All these taxa lack maxillae. Note the relabeling on Platysomus.

None of these taxa
have a maxilla and they share a long list of other synapomorphic traits.

Figure 3. Subset of the LRT focusing on fish and updated here.

Figure 3. Subset of the LRT focusing on fish and updated here. Catfish and placoderms are located in the center of this diagram.

Another traditional platysomid, 
Eurynotus (Fig. 4), is even closer to the placoderms Coccosteus (open sea predators) and Entelognathus (bottom dwellers).

Figure 2. Eurynotus is another platysomid, basal to the placoderms Coccosteus and Entelognathus.

Figure 2. Eurynotus is another platysomid, basal to the placoderms Coccosteus and Entelognathus. Sharp-eyed readers will notice several skull identity changes in placoderms based on what was learned from this taxon.

Platysomus parvulus (Agassiz 1843, Carboniferous to Permian; 18cm long) is a taller, more disc-like fish related to Cheirodus. Note the reduction of the mandible. Considered a plankton eater.

Apologies for the bone ID changes.
I’m learning as I go and revising the naming system so homologies with tetrapods can be more readily understood. Someone had to do it. Why wait until 2021 or thereafter?


References
Agassiz L 1833, 1837 in Agassiz L 1833-1843. Recherches sur les Poissons fossiles-I, I, III, Neuchatel, pp 1420.

 

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