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