Cretaceous Aquilolamna nests with Devonian Palaeospondylus in the LRT

Summary for those in a hurry
The authors excluded related taxa that would have helped them identify their strange, new 1.6 m shark with elongate pectoral fins. The authors also failed to identify the correct mouth, eyes, nasal capsules and gill slits.

Vullo et al. 2021 bring us a wonderful new 1.6m Turonian elasmobranch
with graceful, really long, pectoral fins, Aquilolamna milarcae (INAH 2544 P.F.17, Figs. 1, 2). The authors tentatively assigned (without a phylogenetic analysis) their fossil shark to lamniformes, like the mako shark, Isurus, which has a standard underslung mouth and overhanging rostrum. Vullo et al. thought Aquilolamna was a filter-feeder by assuming that it had a wide, ‘near-terminal mouth’ without teeth, as in the manta ray (genus: Manta). That morphology is distinct from lamniformes like Isurus.

This is a difficult fossil to interpret.
More than the fins make Aquilolamna different than most other fossil and extant sharks.

Unfortunately
Vullo et al. put little effort (Fig. 2 diagram) into their attempt to understand the many clues Aquilolamna left us. Those clues are documented here (Fig. 2) by using DGS (= color tracings) and tetrapod homologs for skull bones.

Figure 1. Aquilolamna in situ from Vullo et al. 2021. Colors added here.

For proper identification, it didn’t help that Vullo et al. 

1. imagined the mouth wide and in front, instead of small and below the occiput
2. imagined the eyes on the sides, instead of on top
3. imagined the gill slits on the sides, instead of ventral
4. did not perform a phylogenetic analysis with a wide gamut of taxa
5. did not consider Middle Devonian Palaeospondylus (Figs. 3, 4) as a taxon worthy of their time and consideration
6. did not consider the torpedo ray, Tetronarce (Fig. 5), or the hammerhead, Sphyrna, taxa worth comparing in analysis (as in Fig. 4).

Figure 2. Skull of Aquilolamna and diagram from Vullo et al. 2021. Colors and new labels applied here. The mouth (magenta) appears under the occiput, overlooked by Vullo et al. White lines indicate symmetries. The hyomandibulars are small with fused quadrates at the new jaw corners and link to the intertemporals, as in all other vertebrates.

Despite these issues, Vullo et al. thought there was enough of Aquilolamna
that was strange, new and easy to understand to make it worthy of publication. And it is. And that’s okay. In science it’s okay to leave further details to other workers. Keeps us busy and feeling helpful! It’s okay to make mistakes. Others will fix those. That’s all part of the ongoing process.

From the abstract:
“Aquilolamna, tentatively assigned to Lamniformes, is characterized by hypertrophied, slender pectoral fins. This previously unknown body plan represents an unexpected evolutionary experimentation with underwater flight among sharks, more than 30 million years before the rise of manta and devil rays (Mobulidae), and shows that winglike pectoral fins have evolved independently in two distantly related clades of filter-feeding elasmobranchs.”

By contrast, in the LRT filter-feeding manta rays are more primitive than sharks that bite for a living.

Unfortunately the authors omitted important sister taxa recovered by the LRT from their comparison studies. They looked at other elamobranchs, but not the electric torpedo ray, hammerhead and Palaeospondylus (Figs. 3, 4).

By focusing on just a few traits the authors are trying to “Pull a Larry Martin.” Instead they should have performed a wide-gamut phylogenetic analysis with hundreds of traits.

Figure 3. A specimen of Palaeospondylus in situ with colors added here. This appears to be a ray in the hammerhead shark, Sphyrna family, that also includes the electric torpedo ray, Tetronarce.

Figure 4. Palaeospondylus diagram from Joss and Johanson 2007 who mistakenly considered Palaeospondylus a hatchling lungfish.

From the taphonomy section of the SuppData:
“No teeth can be observed in INAH 2544 P.F.17, possibly due to rapid post-mortem disarticulation and scattering affecting the dentition.”

Turns out the authors were looking for teeth in the wrong place. The real jaws with tiny teeth were partly hidden below the occiput, as in Middle Devonian Palaeospondylus (Fig. 4), not at the anterior skull rim of Aquilolamna.

Figure 4. Subset of the LRT focusing on the elasmobranch clades related to Aquilolamna and Palaeospondylus.

The reported lack of pelvic fins in Aquilolamna
is unexpected in sharks, which otherwise always have pelvic fins. This lack of pelvic fins could turn out to be a synapomporphy of taxa descending from Palaeospondylus. We’ll have to have more taxa for that.

From the Vullo et al. 2021 diagnosis of the ‘family, genus and species’:
“Medium-sized neoselachian shark that differs from all other selachimorphs in having hypertrophied, scythe-shaped plesodic pectoral fins whose span exceeds the total length of the animal. High number (~70) of anteriorly directed pectoral radials. Head short and broad, with wide and near-terminal mouth. Caudal fin markedly heterocercal. Caudal fin skeleton showing a high hypochordal ray angle (i.e., ventrally directed hypochordal rays). Caudal tip slender with no (or strongly reduced?) terminal lobe. Squamation strongly reduced (or completely absent?).”

Figure 5. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m). Note the robust caudal fin. The hyomandibular links the jaw joint to the braincase.

Aquilolamna has more vertebrae than Palaeospondylus,
but the former is much larger, an adult and geologically younger by 280 million years. We looked at Palaeospondylus just three days ago here. Very lucky timing to have Palaeospondylus for comparison prior to studying Aquilolamna.

Figure 6. Ontogenetic growth series of an electric torpedo ray from Madl and Yip 2000. Pectoral fins in green. Pectoral fins enlarge with maturity. Eyes migrate dorsally. Perhaps the same occurred with Aquilolamna and Palaeospondylus.

Taxon exclusion
continues to be the number one problem in paleontology. Phylogenetic analysis with a wide gamut of hundreds of taxa continues to be the number one solution to nesting all new and enigma taxa. Contra the assertions of dozens of PhDs, first-hand examination of the fossil is not required, nor is a degree or doctorate. This is the sort of profession where you learn on the job with every new taxon that comes along. This one was not in any textbooks, so everyone started like a September freshman with Aquilolamna.

And finally, if you can’t find the mouth where you think it should be,
look somewhere else.

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References
Madl P and Yip M 2000. Essay about the electric organ discharge (EOD) in Colloquial meeting of Chondrichthyes head by Goldschmid A, Salzberg, January 2000. Online here.
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
https://science.sciencemag.org/content/371/6535/1253

Online Publicity for Aquilolamna:

1. sciencemag.org/news/2021/03/eagle-shark-once-soared-through-ancient-seas-near-mexico
2. phys.org/news/2021-03-discovery-winged-shark-cretaceous-seas.html
3. nationalgeographic.com/science/article/shark-like-fossil-with-manta-wings-is-unlike-anything-seen-before
4. livescience.com/ancient-shark-flew-through-dinosaur-age-seas.html