Subclass Elasmobranchii

Five to seven separate gill openings on each side; dorsal fin(s) and spines, if present, are rigid; males without clasper organ on head; dermal placoid scales usually present; palatoquadrate (upperjaw) not fused to cranium (suspension amphistylic or hyostylic); branchial basket mostly behind the neurocranium; tooth replacement relatively rapid; teeth numerous; some ribs usually present; spiracle opening (remains of hyoidean gill slit) usually present. As noted in Maisey (2001b), in modern elasmobranchs the anterior and posterior semicircular canals are separated dorsally (they are variously united dorsally in chimaeroids, sarcopterygians, and actinopterygians). Silurian to present.

Elasmobranchs are typically predaceous fishes that use both smell and sight for obtaining their food.

This subclass is recognized with three lineages ranked as infraclasses, only one of which has extant members. The first two superorders may be the most primitive chondrichthyans. Some Paleozoic taxa, not otherwise mentioned, that are too poorly known to properly classify include the following taxa:

fPlesioselachus. A Late Devonian stem-group elasmobranch with amphistylic jaw suspension and thought to have a single dorsal fin and no anal fin (Anderson et al., 1999).

fSquatinactiformes. One family, Squatinactidae, with the Mississippian Squatinactis from Montana which resembles the extant Squatina in some body form features (e.g., Zangerl, 1981). Placed in the Cladodontiformes in Lund (1990).

fProtacrodontiformes. Includes the Tamiobatidae and shows some similarity to the Orodontidae and Ctenacanthiformes (e.g., Zangerl, 1981).

0 0

Post a comment