Revisiting the horn sharks Heterodontus and Belantsea

The last four days
have been spent reviewing the data on fish in the large reptile tree (LRT, 1578 taxa). Still not finished. Many changes and greater understanding happening now that there seems to be a critical mass of taxa and data present.

Meanwhile, here’s something you might find interesting…

FIgure 1. Ratfish (chimaera) and Heterodontus to scale.

FIgure 1. Ratfish (chimaera) and Heterodontus to scale.

Wikipedia reports, “The horn shark [genus: Heterodontus, Fig. 1] is a sporadic swimmer that prefers to use its flexible, muscular pectoral fins to push itself along the bottom.”

The LRT nests Heterodontus
with the odd chimaerid Belantsea (Fig. 2) at the base of the chimaerids, a sister clade to sharks and their kin. This taxon retains several gill slits, like its sister Heterodontus, distinct from most chimaerids. In prior studies workers counted the gill slits and considered horn sharks to be sharks. Readers know to beware of relying on one or a dozen traits. That is ‘pulling a Larry Martin‘. The LRT uses the ‘last common ancestor’ method for determining interrelationships.

Figure 2. Belantsea has several gill slits, like its sister, Heterodontus.

Figure 2. Belantsea has several gill slits, like its sister, Heterodontus. Compare to figure 3.

Wikipedia also reports, “The bullhead sharks are a small order (Heterodontiformes) of modern sharks (Neoselachii). The nine living species are placed in a single genus, Heterodontus, in the family Heterodontidae. All are relatively small, with the largest species reaching just 1.65 metres (5.5 ft) in maximum length. They are bottom feeders in tropical and subtropical waters.

Figure 3. Heterodontus skull.

Figure 3. Heterodontus skull from and used with permission. Colors added. Compare to figure 2.

The Heterodontiforms appear in the fossil record in the Early Jurassic, well before any of the other Galeomorphii, a group that includes all modern sharks except the dogfish and its relatives. However, they have never been common, and their origin probably lies even further back.

The bullhead sharks are morphologically rather distinctive. The mouth is located entirely anterior to the orbits. Labial cartilages are found in the most anterior part of the mouth. Nasoral grooves are present, connecting the external nares to the mouth. The nasal capsules are trumpet-shaped and well-separated from orbits. Circumnarial skin folds are present, but the rostral process of the neurocranium (braincase) is absent, although a precerebral fossa is present. Finally, the braincase bears a supraorbital crest.

The eyes lack a nictitating membrane. A spiracle is present, but small. The dorsal ends of the fourth and fifth branchial arches are attached, but not fused into a “pickaxe” as in lamniform sharks. Heterodontiforms have two dorsal fins, with fin spines, as well as an anal fin. The dorsal and anal fins also contain basal cartilages, not just fin rays.”

The horn sharks
are basal to chimaerids in the LRT. Belantsea is a horn shark in the LRT. Let me know if these contributions to paleontology were published earlier and I will provide the citation.




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