Page 444 - Introduction to Paleobiology and The Fossil Record

FISHES AND BASAL TETRAPODS 431

Box 16.2 Classifi cation of fi shes

“Fishes” form a paraphyletic grouping, consisting of several distinctive clades of swimming vertebrates.
Ordovician and Silurian records of placoderms, acanthodians, chondrichthyans and osteichthyans
are mainly isolated scales and teeth; these groups are best known from the Devonian onwards.

Subphylum VERTEBRATA
“Class AGNATHA”

• A paraphyletic group of jawless fishes, including armored and unarmored Paleozoic ostraco-
derms, and modern lampreys and hagfi shes
• Late Cambrian to Recent

Class PLACODERMI
• Heavily armored fishes with jaws and a hinged head shield

• Mid Silurian to Late Devonian
Class CHONDRICHTHYES
• Cartilaginous fishes, including modern sharks and rays

• Late Ordovician to Recent
Class ACANTHODII

• Small fishes with many spines and large eyes

• Late Ordovician to Early Permian
Class OSTEICHTHYES

• Bony fishes, with ray fins (Subclass Actinopterygii) or lobe fins (Subclass Sarcopterygii), the latter

including ancestors of the tetrapods
• Late Silurian to Recent

Furnishina are mainly simple cones; and (iii) tapering upward, and sometimes ornamented
euconodonts or true conodonts are more with ridges or costae (Fig. 16.4a, b). Bars or
complex, with cones, bars and blades. The ramiform elements consist of an elongate
protoconodonts are almost certainly unre- blade-like ridge with up to four processes
lated to true conodonts; they may be chaeto- developed posteriorly, anteriorly or laterally
gnaths or arrow worms, a group of basal to the cusp (Fig. 16.4c, d, g). Platforms or
metazoans of uncertain affi nities. pectiniform elements have a wide range of
Euconodonts occur as three broad types of shapes, with denticulate processes extending
element, consisting of laminae of apatite. both anteriorly, posteriorly and/or laterally
These are formed by outer accretion from an from the area of the basal cavity (Fig. 16.4e,
initial growth locus. White matter often occurs f, h–j); some also have primary lateral pro-
between, or crosscuts, lamellae; this material cesses. The cusp is attenuated, whereas the
compares well with the composition and base may be expanded to form a platform
structure of vertebrate bone. The three main with denticles on its upper surface. The basal
morphotypes of conodont element have been cavity is fi lled by the basal body of the element
used in the past as the basis of a crude single in the form of a dentine-like material, although
element or form taxonomy (Fig. 16.4). The this is not always preserved.
cones or coniform elements are the simplest, Conodonts are common in certain marine
with the base surmounted by a cone-like cusp, facies from the Cambrian to the Triassic.