This one came as a surprise
as I scored Tetronarce (= the New Zealand torpedo, an electric ray, Fig. 1), I thought:
- this taxon is breaking some rules, and
- I’ve seen that bizarre nasal before… but where?
Figure 1. Tetronarce fairchildi (originally Torpedo fairchildi Hutton 1872, 1m). The long red elements are tabular homologs, separated from the rest of the skull.
Tetronarce fairchildi
(originally Torpedo fairchildi Hutton 1872, 1m) is the extant New Zealand torpedo, an electric ‘ray’ on the outside. Here it nests with Sphyrna, the hammerhead shark, based on its skeleton. So this ‘ray’ is convergent with other rays. Note the broad nasals with open medial architecture, underslung jaws with tiny, single-cusp teeth and shark-like tail. Here the eyeball stalks are preserved, distinct from most other tetrapods tested in the LRT, probably due to careful dissection to get at its cartilaginous skeleton. Two dorsal fins are preserved.
Figure 1. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.
Skates,
like the guitarfish, Rhinobatos, and the sawfish, Pristis, have an elongate narrow rostrum and nasal. Angel sharks and eagle rays have other distinguishing traits that nest them with each other and not with the aforementioned. So do manta rays. When more rays and skates are added to the LRT that may change. Or not.
Every possibility must always be left open,
as Torpedo gently, but firmly reminds us. Do not be tempted into “Pulling a Larry Martin” here. A short list of traits don’t make a taxon. Only a nesting in a wide gamut phylogenetic analysis can do that.
Yes, outward appearances are very different.
But when you look at the skeletons and test them in phylogenetic analysis no other taxon shares as many traits with hammerheads as torpedoes. Evolutiion leaves clues. It’s up to us to find them. You won’t find a similar laterally extended nasal with a perforated medial architecture in any other tested sharks or rays. Though many skeletal traits are indeed different, taken as a suite of characters no other tested taxon comes closer.
Figure 2. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.
Sphyrna tudes
(orignally Zygaena tudes Valenciennes 1822; 1.3m in length) is the extant smalleye hammerhead shark. It prefers muddy habitats with poor visibility. Sphyrna has a tendency to inhabit coastal waters along the intertidal zone rather than the open ocean, as their prey item, invertebrates, fish, rays, small crustaceans and other benthic organisms hide in the sands and sediment along these zones. Gestation is 10 months. Females produce 19 pups each year. The eyes and nares are further separated by the lateral expansion of nasals, prefrontals and postfrontals creating the cephalofoil. Compare to Torpedo (above).
The largely overlooked value of the LRT
comes from testing together taxa that have never been tested together before with a generic character list not designed specifically for sharks and rays, other fish, birds and mammals. Convergence runs rampant in the Vertebrata. Scientists need a wide gamut cladogram that minimizes taxon exclusion and character selection bias.
By the present evidence
the former clade Batoidea has now been divided into quarters. This appears to be a novel hypothesis of interrelationships. If there is a prior publication, let me know so I can promote it.
References
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.
The Pterosaur Heresies 