Fossil range: Early Devonian - Late Cretaceous
Hybodus fraasi from Eichstätt, Germany.
| Scientific classification
Hybodontiformes are an order of extinct sharks, characterised by possessing 1-2 pairs of hooked protrusions on their heads. Hybodontiformes first appeared in the Lower Triassic, some 320–290 million years ago, but blossomed in the Mesozoic era. They did not survive past the end of the Cretaceous period, which ended some 65 million years ago. Hybodont sharks were originally described in the early 19th century from fossil teeth. Complete or partial skeletons of several hybodont genera have been found, although most of what is known about these sharks is based upon the study of their isolated teeth, fins, and cephalic spines.
Anatomy and PhysiologyEdit
Hybodontiformes were characterized by paired fins that show signs of intrinsic musculature, and greater flexibility, implying that they were used in steering. They had a heterocercal tail, an anal fin and amphistylic jaw suspension. Hybodonts were also the first group to possess a heterodont dentition (different shaped teeth along the length of the jaw). For instance, having sharp cutting teeth in the front part of their jaw and flat crushing teeth in the back part of the jaw allowed them to feed on a variety of food types. The Hybodontiformes also possessed a full set of hemal, as well as neural, arches around the notochord.
It is known that hybodonts had two dorsal fins, each preceded by a fin-spine with a characteristic shape. The leading (front) edge of the fine spine is rounded, and the spine has smooth ridges or rows of small bump-like tubercles running along the sides. The posterior (rear) edge of the spine carries two rows of tooth-like projections.
Such spines, protruding from in front of the dorsal and pectoral fins, are derived from enlarged skin denticles and are made of tooth material, not bone or cartilage. Fin spines are a prominent feature of both ancient and modern sharks. Hybodonts also have cephalic spines on their heads. Spines, like teeth, were apparently derived from single denticles that became greatly enlarged. Spines typically consist of an inner trunk of dentine, the base of which becomes deeply inserted in the body in advanced genera, and an outer mantle of ornamented enameloid material.
The term spiny teleost (bony fish) refers to the fin spines that most members of this group possess. These are particularly conspicuous on the dorsal fin but also occur in the anal and paired [e.g. pectoral] fins. The stiff spines result from fusion of the two halves of primitively paired and jointed dermal fin rays into a median, unjointed structure. The spine loses its flexibility and is effectively moved as a unit by muscles attached to its base.
Male hybodonts also possessed small spines across the top of the head, previously believed to be teeth, that were perhaps used in sexual selection. Examination of well-preserved skeletal material has shown that just behind the orbits (the space in the braincase where the eyes would be positioned), there is an outward projection of cartilage. Hybodonts also had thick, massive jaws that are different among distinct genera, varying in form according to their specific diets; their teeth varied accordingly.
The teeth of hybodonts are characterized by having a central, principal cusp with smaller cusps positioned laterally to it. Also, the tooth base is wide and contains openings for the passage of nutritive canals. Such teeth are quite common in Mesozoic strata from many localities around the world. However, more complete specimens are rare.
The order, Hybodontiformes, includes approximately fifteen genera arranged in five families.
- ^ Welton, Bruce J., Ph.D. The Collector's Guide to Fossil Sharks and Rays from the Cretaceous of Texas. pp. 47.
- Carroll, Robert L. 1987. Vertebrate Paleontology and Evolution. W.H. Freeman & Company.