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.

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.

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

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.

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 shark clades related to Aquilolamna and Palaeospondylus.

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

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. Pectoral fins in green.

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.


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

Goodbye Batoidea, another traditional clade invalidated by the LRT

According to Wikipedia,
“Batoideais a superorder of cartilaginous fishes commonly known as rays. They and their close relatives, the sharks, comprise the subclass Elasmobranchii. Rays are the largest group of cartilaginous fishes, with well over 600 species in 26 families. Rays are distinguished by their flattened bodies, enlarged pectoral fins that are fused to the head, and gill slits that are placed on their ventral surfaces.”

Figure 1. Spotted eagle ray skull shows the anterior portions of the pectoral fins jointed medially to create a digging snout.

Figure 1. Spotted eagle ray skull shows the anterior portions of the pectoral fins conjoined  medially to create a digging snout.

Aeobatus narinari (Figs. 1–3 originally Raja narinari Euphrasén 1790; 5m in length, 3m wingspan) is the extant spotted eagle ray and the subject of today’s post.

The distinctive flat muscular snout
is created by the anterior processes of the pectoral fins conjoining anteriorly, as in other stingrays that also have detachable venom spines at the base of their tail.

Figure 2. Subset of the LRT focusing on basal vertebrates. Purple taxa are traditional rays, here shown to be convergent in their morphology.

Figure 2. Subset of the LRT focusing on basal vertebrates. Purple taxa are traditional rays, here shown to be convergent in their morphology.

Traditionally
Aeobatus was considered a ray that should have nested with the guitarfish, Rhinobatos and even closer to Manta, the manta ray. Everyone considered that clade, Batoidea, monophyletic prior to today’s post.

When you expand your taxon list, as in the large reptile tree (LRT, 1586 taxa; Fig. 2), Aeobatus nests with Squatina, the angel shark, not with Manta or Rhinobatos. That means the three tested rays are convergent.

So say goodbye
to the Myliobatiformes. Say goodbye to the Rajiformes. And say goodbye to the Batoidea. These clades are not monophyletic in the LRT, but evolved a ray-like appearance by convergence. This hypothesis of interrelationships was apparently overlooked by prior workers. Please let me know if otherwise and I will promote that citation. Meanwhile, following the scientific method, independent testing using a similar taxon list should take place to confirm or refute this hypothesis.

While free swimming (rather than bottom dwelling)
and capable of leaping clear of the water, the spotted eagle ray feeds on shelled invertebrates hiding beneath sea sands. Distinct from Squatina, the marginal jaws of Aetobatus are nearly toothless. The vomer and a medial plate between the dentaries include a series of flat plates acting as crushing palatal teeth distinct from other tested rays.

Figure 2. The spotted eagle ray, Aetobatus in vivo.

Figure 3. The spotted eagle ray, Aetobatus in vivo.

Compare Aetobatus to its LRT sister,
Squatina oculata
 (Bonaparte 1840; Figs. 4, 5), the extant smooth back angelshark. In this  basal fish some of the gill bones are transformed to jaws with teeth, as in typical sharks. In general morphology Squatina is little changed from the Early Silurian jawless thelodonts that preceded it.

Figure 5. Squatina skull. Note the gill bars framing the mouth. These are modified in Aetobatus into a digging snout.

Figure 4. Squatina skull. Note the gill bars framing the mouth. These are modified in Aetobatus into a digging snout.

Distinct from rays,
the gill slits appear anterior to the expanded anterior processes of the pectoral fins in Squatina (Fig. 6), demonstrating how the gill slits shift ventrally in rays. These same anterior processes form the rostrum in Aetobatus (Fig. 1).

Figure 6. Squatina in vivo, lateral view. The large pectoral and pelvic fins give Squatina a broad, ray-like appearance in dorsal view.

Figure 5. Squatina in vivo, lateral view. The large pectoral and pelvic fins give Squatina a broad, ray-like appearance in dorsal view.

I’m only guessing,
but based on the present results, long-nosed stingless skates are going to nest with Rhinobatos, the guitarfish. Stingray, including cow nose rays, will nest with Aetobatus. And Manta will continue to nest alone among rays, as no other is a plankton feeder with an anterior gaping mouth without teeth. It’s closest relative is Rhincodon, the whale shark, the most primitive gnathostome (vertebrate with jaws) in the LRT.

FIgure 7. Squatina in ventral view showing the anterior processes of the pectoral fin that develop into a rostrum in Aetobatus and shift the gill slits ventrally.

Figure 6. Squatina in ventral view showing the anterior processes of the pectoral fin that develop into a rostrum in Aetobatus and shift the gill slits ventrally.

We’ve seen convergence before
in pterodactyloid-grade pterosaurs, turtles, whales, and dozens of other taxa. Convergence can produce false positive results if you omit key taxa. So far the LRT has been able to sort it all out by including overlooked taxa and avoiding genomic data.

When I started
ReptileEvolution.com eight years ago, I thought many of these issues were resolved long ago. While discoveries like this keep me digging for more, academic workers should have resolved these issues decades ago. Traditions persist for a reason.


References
Euphrasén BA 1790. Raja (Narinari). Kongl. Vetenskaps Academiens Nya Handlingar, 11:217-219.

wiki/Spotted_eagle_ray