Middle Devonian Paleospondylus nests with extant torpedo rays

Summary for those in a hurry:
a traditional enigma fish taxon, Paleospondylus (Figs. 1, 2) nests in the large reptile tree (LRT, 1815+ taxa) with the electric torpedo ray (Fig. 3) genus = Tetronarce), a taxon overlooked by all prior studies.

Before the addition of Paleospondylus,
the closest relative of the torpedo ray in the LRT was the hammerhead shark, Sphyraena. Both contributed to understanding the taxonomy and anatomy of Paleospondylus, a tiny juvenile ray with a relatively big, shark-like tail (Fig. 1). The LRT is the first wide gamut phylogenetic analysis attempt for Paleospondylus. Earlier studies compared only a few traits and few taxa, thereby “Pulling a Larry Martin” in the process.

Figure 1. A specimen of Palaeospondylus in situ with colors added here. This appears to be a ray in the hammerhead shark, Sphyraena family.

Figure 1. A specimen of Palaeospondylus in situ with colors added here. This appears to be a juvenile ray in the hammerhead shark (Sphyraena) family. The torpedo ray, Torpedo, is also a member of this clade. Shown twice life size.

According to Wikipedia:
“Palaeospondylus gunni (Gunn’s ancient vertebrae, Traquair 1890) is a mysterious, fish-like fossil vertebrate. The fossil as preserved is carbonized, and indicates an eel-shaped animal up to 6 centimetres (2 in) in length. The skull, which must have consisted of hardened cartilage, exhibits pairs of nasal and auditory capsules, with a gill apparatus below its hinder part, and ambiguous indications of ordinary jaws.”

The phylogeny of this bizarre fossil has puzzled scientists since its discovery in 1890, and many taxonomies have been suggested. In 2004, researchers proposed that Palaeospondylus was a larval lungfish. Previously, it had been classified as a larval tetrapod, unarmored placoderm, an agnathan, an early stem hagfish, and a chimeraThe most recent suggestion is that it is a stem chondrichthyan.”

Palaespondylus diagram from Joss and Johanson 2007, colorized here.

Figure 2. Palaespondylus diagram from Joss and Johanson 2007, colorized here with tetrapod homologs. The authors considered Paleospondylus a larval lungfish. Late Johanson et al. 2017 no longer supported this hypothesis of interrelationships. The lungfish, Dipterus, occurs in the same fossil beds.

From Wikipedia continued,
“Most recently, Palaeospondylus has been identified as a stem-group hagfish (Myxinoidea). However, one character questioning this assignment is the presence of three semicircular canals in the otic region of the cartilaginous skull, a feature of jawed vertebrates.”

Figure 1. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m)

Figure 3. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m)

From Wikipedia continued,
“According to Johnson et al. 2017, “Previously, Palaeospondylus has been assigned to almost every major jawless and jawed vertebrate group and identified as both larval and adult. Most recently, Palaeospondylus has been identified as a stem-group hagfish (Myxinoidea). However, one character questioning this assignment is the presence of three semicircular canals in the otic region of the cartilaginous skull, a feature of jawed vertebrates.”

“Additionally, new tomographic data reveal that the following characters of crown-group gnathostomes (chondrichthyans + osteichthyans) are present in Palaeospondylus: a longer telencephalic region of the braincase, separation of otic and occipital regions by the otico-occipital fissure, and vertebral centra. As well, a precerebral fontanelle and postorbital articulation of the palatoquadrate are characteristic of certain chondrichthyans.”

Johnson et al. 2017 conclude, “the absence/non-preservation of teeth, scales and fins continues to be problematic in determination of Palaeospondylus as a jawed vertebrate. Also problematic with regards to a chondrichthyan association is the composition of the Palaeospondylus cartilaginous skeleton that includes hypertrophied chondrocyte lacunae surrounded by mineralized matrix, previously interpreted as representing an early stage in endochondral bone development, a type of bone found in bony fishes (Osteichthyes).”

Figure 2. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

Figure 4. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

When Palaeospondylus was added to the LRT,
it nested with the torpedo ray while retaining many traits (like a precerebral fontanelle) found in hammerhead sharks,  Palaeospondylus lived in the Middle Devonian, so transitional and primitive precursors that look like a ray with a shark tail are to be expected. Lack of fusion in the skull elements, the overall small size and the appearance of several specimens in a small area suggesting a nursery, combine to indicates a juvenile status.

Figure 1. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

Figure 5. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

In the most recent paper on Palaeospondylus
(Johnson et al. 2017) the following taxa were not found in the text, but at times appear in the citations: 1) shark; 2) ray; 3) torpedo. The authors reported, “The presence of
centra within the synarcual of Palaeospondylus is reminiscent of the synarcual in batoid chondrichthyans.” They did not follow up on that clue. Contra tradition, in the LRT members of the traditional batoid clade are split apart and distributed among other chondrichthyans and basal gnathostomes.

In their conclusion Johnson et al. 2017 reported,
“Palaeospondylus gunni has been a perplexing vertebrate fossil since Traquair first described it in 1890; here X-ray tomography provides new data and morphological characters demonstrating that Palaeospondylus is a jawed vertebrate. Characters that associate Palaeospondylus with chondrichthyans are a precerebral fontanelle, foramina for lateral dorsal aorta in the chondrocranium, and the articulation of the palatoquadrate to the ventral postorbital process. Palaeospondylus also lacks bone and instead manifests an entirely mineralized cartilage in the endoskeleton.”

Taxon exclusion is the number one problem affecting paleontology today
and for several prior decades. The LRT minimizes taxon exclusion by testing a wide gamut of extant and extinct taxa in a trait-based phylogenetic analysis. If only prior workers had included hammerheads and torpedos in their own phylogenetic analysis, Paleospondylus would not have been the enigma it remained until today.


References
Hirasawa T, Oisi Y and Kuratani S 2016. Palaeospondylus as a primitive hagfish. Zoological Letters. 2 (1): 20.
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.
Thomson KS 2004. A Palaeontological Puzzle Solved?. American Scientist. 92 (3): 209–211.
Traquair RH 1890. On the fossil fishes at Achanarras Quarry, Caithness. Ann Mag
Nat Hist 6th Ser. 1890;6:479–86.

wiki/Palaeospondylus

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