SVP 2021 abstracts – 14: Trawdenia endoskeleton

Caron AM, Tietien K and Coates M 2021 bring us new data
on the Middle Carboniferous fish preserved in 3D, Trawdenia planti (formerly Mesopoma planti; NHM P11656; Fig. 1). The endoskeleton has not been published (that I am aware of).

From the Caron, Tietien and Coates abstract:
“The actinopterygian crown group is thought to have diverged close to the Devonian–Carboniferous boundary, but a persistent failure to find Paleozoic taxa to populate
the stems of extant clades obstructs understanding the origin of modern biodiversity.”

By contrast, the large reptile tree (LRT, 1991 taxa) finds a dual origin for bony fish
deep in the Silurian following moray eels and kin.

Apparently Trawdenia planti is a renamed Mesopoma planti (Figs. 1, 2),
a taxon we looked at earlier here.

“Unexpectedly, the Trawdenia braincase is reminiscent of the Devonian Moythomasia, but sports a paddlefish-like gill skeleton and a host of other morphological characters that beg the question: are these symplesiomorphies or might Trawdenia be a stem chondrostean?”

Several issues here: 1). Trawdenia/Mesopoma nests with Moythomasia in the LRT (Fig. 2), so similarities should be expected.

2) These taxa do not nest with traditional chondrosteans, like paddlefish (= Polydon, a freshwater basking shark with an operculum, according to the LRT).

3. No traditional chondrosteans (e.g. Acipenser, Polypterus, Amia) nest with one another in the LRT, making Chondrostei an invalid clade.

“In pursuit of answers, we built a dataset comprising broadly-applicable characters from the neurocranium, hyoid and gill arches, and parasphenoid. Preliminary analysis recovered a topology with an ‘ancient fish clade’ (in this instance Polypteriformes plus Chondrostei) branching from the base of the tree and Paleozoic taxa branching from the neopterygian stem, thereby conflicting both with accepted molecular phylogenetics as well as current paleontological consensus. To mitigate the influence of convergence, we then excluded characters contingent upon determinate growth, for example regarding the persistence of fissures and fontanelles, and repeated the analysis. This approach generated intriguing results including a populated chondrostean stem – a radical departure from alternative tree topologies that have dominated since the 1980s.”

While it’s always fun to brag about “a radical departure from alternative tree topologies,” these three authors are a year or two late, hundreds of taxa short and less radical than the LRT.

“We recover Trawdenia as a stem-actinopteran, polarizing downstream character state changes and shedding new light on the evolutionary trajectory of endoskeletal systems across Actinopteri.”

Not so fast, kids. The LRT (subset Fig. 2) employs dozens of omitted taxa and nests Trawdenia = Mesopoma far from the origin of Neopterygii + Chondrostei = Actinopteri.
None of these are valid clades in the LRT.

“By employing modern μCT methods on excellently preserved specimens such as Trawdenia and applying discriminate character choice, we might be finally uncovering the deep
Paleozoic roots of the major extant actinopterygian divisions.”

Better to just add more taxa. Use the LRT as your guide. Trawdenia = Mesopoma is closer to tetrapods than to basal bony fish in the LRT. Testing more taxa all at once can sometime be more illuminating than testing one specimen with µCT scans when trying to figure out hypothetical interrelationships.

References
Caron AM, Tietien K and Coates M 2021. Revisiting Trawdenia planti: Endoskeletal data and and a fresh look at Actionopterygian roots. Journal of Vertebrate Paleontology abstracts: 76.
Coates M and Tietjen K 2018. ‘This strange little palaeoniscid’: a new early actinopterygian genus, and commentary on pectoral fin conditions and function. Earth and Environmental Science Transactions of The Royal Society of Edinburgh , Volume 109 , Issue 1-2: Fossils, Function and Phylogeny: Papers on Early Vertebrate Evolution in Honour of Professor Jennifer A. Clack , March 2018 , pp. 15–31.
Traquair RH 1890. Observations on some fossil fishes from the Lower Carboniferous rocks of Eskdale, Dumfriesshire. Annals and Magazine of Natural History 6:493-493.