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Palaeospondylus: Does the morphology show affinity to tetrapod ancestors?

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Short answer: no.

Hirasawa et al. 2022 re-describe a common Middle Devonian ‘enigma’ taxon
from Scotland, Palaeospondylus (Fig 1). Eight years ago Hirasawa et al. 2016 first described Palaeospondylus as a type of hagfish, a basal chordate. So this team is really interested in being the first to figure out what Palaeospondylus really is, but they keep failing due to taxon exclusion. Others have also tried to nest this traditional Middle Devonian enigma (see citations below) without success.

This tiny swimming basal gnathostome
was earlier (2021) resolved by the large reptile tree (LRT, 2097 taxa, subset Fig 6) when Palaeospondylus nested with the torpedo ray Tetronarce fairchildi (originally Torpedo fairchildi). All other competing candidates were tested and continue to be tested.

Hirasawa et al. 2022 report from their abstract:
“Palaeospondylus gunni, from the Middle Devonian period, is one of the most enigmatic fossil vertebrates, and its phylogenetic position has remained unclear since its discovery in Scotland in 1890. The fossil’s strange set of morphological features has made comparisons with known vertebrate morphotype diversity difficult.

That has always been due to taxon exclusion. The LRT was able to figure it out last year in just a minutes of computer time given the wide gamut taxon list.

Here we use synchrotron radiation X-ray micro-computed tomography to show that Palaeospondylus was a sarcopterygian, and most probably a stem-tetrapod.”

No, they don’t show that. And it doesn’t matter what other tools you use to image specimens if your taxon list excludes pertinent taxa. The authors imagine a tadpole-like juvenile form without limbs only because they do not consider or test the actual closest relatives among the omitted ray-like elasmobranchs.

The skeleton of Palaeospondylus consisted solely of endoskeletal elements in which hypertrophied chondrocyte cell lacunae, osteoids and a small fraction of perichondral bones developed.”

These are traits of elasmobranchs, but no elasmobranchs were tested.

Despite the complete lack of teeth and dermal bones,

The authors are confessing: ‘We’re going to ignore these traits.’ The LRT does not ignore these traits.

the neurocranium of Palaeospondylus resembles those of stem-tetrapod Eusthenopteron and Panderichthys, and phylogenetic analyses place Palaeospondylus in between them.”

Only because closer relatives were excluded from consideration and testing.

Because the unique features of Palaeospondylus, such as the cartilaginous skeleton and the absence of paired appendages, are present in the larva of crown tetrapods, our study highlights an unanticipated heterochronic evolution at the root of tetrapods.”

Be wary when paleontologists use the term ‘unique features’. All features are inherited from relatives. For instance, pterosaurs also seem to have unique features so long as relatives are excluded. When relatives are included then those ‘unique traits’ become synapomorphies.

The authors note a ‘cartilaginous skeleton’, but do not test elasmobranchs, which also have a cartilaginous skeleton. Unfortunately, this sort of omission of taxa is all too common throughout paleontology. It’s the main reason why it took over a century for birds to be widely recognized as theropod dinosaurs and other similar issues.

The authors describe the absence of paired appendages in the larva of crown tetrapods (e.g. frog tadpoles), but this is a derived trait not found in the young of basal tetrapods and their finned ancestors. No basal tetrapods undergo a frog-like metamorphosis.

According to ScienceAlert.com
“A mysterious, extinct creature that has puzzled scientists for more than a century may have finally found its place in the tree of life. The small, fish-like animal is named Palaeospondylus gunni, first discovered in fossils in Scotland in 1890, which lived approximately 390 million years ago during the Middle Devonian. Now, according to a new analysis of well preserved fossils, scientists think that it was one of the earliest ancestors of tetrapods – animals with four limbs, including humans.”

Here is the cladogram from Hirasawa et al.
(Fig 5). Note the lack of elasmobranchs, rays and torpedo rays.

Luckily we all live in an Internet age
where scientific information can be shared worldwide immediately and at no cost. We no longer have to wait for a year or more for a paper critical of invalid published findings. What we do with the original information and the critique that follows is entirely up to each individual reader. At this point there are no umpires or referees, only supporters, critics and the vast majority who ignore and/or forget such studies.

PS
Thank you for your interest in PterosaurHeresies.Wordpress.com.
Recent posts have more than doubled the usual number of readers and page views. Those numbers will likely drop again after the current Prehistoric Planet excitement fades. Confirmation, refutation and correction are always welcome as we work together to model actual evolutionary events.

References
Hirasawa T, Oisi Y and Kuratani S 2016. Palaeospondylus as a primitive hagfish. Zoological Letters. 2 (1): 20.
Hirasawa T, Hu Y, Uesugi, K. et al. 2022. Morphology of Palaeospondylus shows affinity to tetrapod ancestors. Nature https://doi.org/10.1038/s41586-022-04781-3
Hutton FW 1872. Catalogue with diagnoses of the species. Ed. Hutton, FW and Hector J (eds), Fishes of New Zealand, pp. 1-88 pls 1-12, Colonial Museum and Geological Survey Department, Wellington.
Joss J and Johanson Z 2007. Is Palaeospondylus gunni a fossil larval lungfish? Insights from Neoceratodus forsteri development. J Exp Zool B Mol Dev Evol. 2007 Mar 15;308(2):163-71.  https://pubmed.ncbi.nlm.nih.gov/17068776/
Johanson Z et al. 5 co-authors 2017. Questioning hagfish affinities of the enigmatic Devonian vertebrate Palaeospondylus. Royal Society Open Science. 4 (7): 170214.
Smith A 1829. Descriptions of new, or imperfectly known objects of the animal kingdom, found in the south of Africa. South African Commercial Advertiser 3: 2.
Thomson KS 2004. A Palaeontological Puzzle Solved?. American Scientist. 92 (3): 209–211.
Thomson KS, Sutton M and Thomas B 2004. A larval Devonian lungfish. Nature 426(6968):833-834.
Traquair RH 1890. On the fossil fishes at Achanarras Quarry, Caithness. Ann Mag
Nat Hist 6th Ser. 1890;6:479–86.
Vullo R, Frey E, Ifrim C, Gonzalez Gonzalez MA, Stinnesbeck ES and Stinnesbeck W 2021. Manta-like planktivorous sharks in Late Cretaceous oceans. Science 371(6535): 1253-1256. DOI: 10.1126/science.abc1490

wiki/Torpedo_fairchildi
wiki/Palaeospondylus
wiki/Aquilolamna

Publicity
sciencealert.com/this-fish-like-creature-could-be-one-of-your-oldest-known-ancestors

Cretaceous Aquilolamna nests with Devonian Palaeospondylus in the LRT


Source: https://pterosaurheresies.wordpress.com/2022/05/27/palaeospondylus-does-the-morphology-show-affinity-to-tetrapod-ancestors/


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