SVP 2021 abstracts – 01: Basal gnathostomes and their precursors

Saturday, November 6, 2021 8:02

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The Society of Vertebrate Paleontology
just held their annual conference last week. This year it was virtual again. The embargo for the abstracts is over. All the presentations have been made. Over the next few weeks we’ll take a look at 35 of the most interesting and the most problematic, several a day to get through them ASAP.

Whalen 2021 looked at vertebrate and tetrapod origins
From the abstract: “Vertebrates, especially jawed vertebrates (gnathostomes) are among the most disparate and diverse animal groups. As such, their origins and early evolution are a central paleobiological question, before considering the connection to our own evolutionary heritage. Despite this, no integrated framework exists to understand the key evolutionary transitions on the gnathostome stem.”

Not true. The the large reptile tree (LRT, 1987 taxa) resolved all these issues over the last ten years with an ‘intergrated framework’.

“Instead, early vertebrates are primarily understood through three overlapping but disconnected phylogenetic lenses: gnathostomes (jawed vertebrates), cyclostomes (extant jawless vertebrates), and the ostracoderm-grade.”

The LRT integrates all three. Sounds like Whalen is setting up a ‘straw dog‘.

“Gnathostome phylogenies have focused on the crown group (chondrichthyans, acanthodians, and osteichthyans) with placoderms primarily serving as an extended outgroup.”

This is incorrect according to the LRT. Dr. Whalen needs to create his own cladogram from scratch and not borrow those of others. This is a major shortcoming of the current university lecture and textbook system.

“This has prompted debate on placoderm monophyly/paraphyly, but phylogenies explicitly
designed to test the placoderm question have not polarized them with an extended “ostracoderm” outgroup – instead, representation is skewed towards crown gnathostomes.”

Catfish and kin are close relatives of placoderms in the LRT. Catfish are crown gnathostomes. Ostracoderms are good outgroups to sturgeons and then toothless Chondrosteus (Fig. 1), but Whalen is not aware of this. It’s not currently in Benton’s textbook.

“Cyclostome phylogenetics has focused on questions of monophyly/paraphyly; the broader context among vertebrates is discussed, but for the most part only anaspid “ostracoderms” have been encoded as individual genera in these studies.”

Check out the LRT. It’s an online resource. All these problems have be resolved.

Figure 3. Chondrosteus animation (2 frames) in situ and reconstructed in lateral view. This is the transitional taxon linking sturgeons to bony fish + sharks.

The Whalen abstract continues:
“Conodonts are barely considered despite potentially being the most successful cyclostome
group. “Ostracoderm” phylogenies compare class-level taxa with assumed monophyly and ancestral conditions; an oversimplification that extends to their consideration of gnathostomes, despite the considerable uncertainty on the early gnathostome condition.”

Whalen is still setting up the problem. He paints the problem as a dire uncertainty. There is no uncertainty in the LRT. The first toothless jaws (= gnathostomes) appeared in Chondrosteus (Fig. 1), a late survivor of early Silurian radiations.

“I present the first phylogeny of early vertebrate evolution to consider individual genera spanning the vertebrate, cyclostome, and gnathostome stem groups in a common phylogenetic framework. “

Dr. Whalen presents the second such phylogeny. The LRT was first.

“This is the first study to explicitly test the monophyly of each “ostracoderm” class. Likewise, it is the first study to reconstruct interrelationships across the entire gnathostome stem, allowing the full range of “ostracoderms” to inform placoderm interrelationships and vice versa.”

Ostracoderms and placoderms? Good luck there. Those two clades are not related, as tests would have told Whalen if taxon exclusion is minimized. My guess is Whalen is following the Vertebrate Paleontology textbook written by Michael Benton, which we looked at earlier here. I saved both time and tuition creating the LRT without being sent down detours and dead-ends by that textbook and outdated paleo lectures at the university level.

“The Bayesian tip-dated analysis incorporates over 1200 morphological characters and
nearly 200 genera, including a new exceptionally preserved early vertebrate from the Early Ordovician Fezouata Lagerstätte.”

Sadly it sounds like Whalen is excluding taxa that did not appear in the Early Ordovician. This is short-sighted due to the late appearance of many fossil taxa in the fossil record and extant taxa that had their genesis in the Cambrian, Ordovician and Silurian. I wonder if Whalen includes Ordovician cephalopods? We discussed that earlier here.

“The phylogenetic results are discussed in the broader context of Paleozoic marine metazoan evolution in the water column.”

Figure 2. Subset of the LRT. Placoderms and spiny sharks both nest as derived taxa within the bony fish subse of the LRT. Hemicyclaspis is an ostracoderm, a little off the lineage of gnathostome development.

This abstract sounds like it’s going to suffer from taxon exclusion
compared to the wider gamut of the LRT. Whalen does not mention Chondrosteus.
Let me know how this goes.

References
Whalen CD 2021. A Bayesian tip-dated phylogeny of stem vertebrates, total-group cyclostomes, and stem gnathostomes. SVP abstracts p. 267.

wiki/Ostracoderm

Source: https://pterosaurheresies.wordpress.com/2021/11/06/svp-2021-abstracts-01-basal-gnathostomes-and-their-precursors/