Box Classification of fishes

"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 hagfishes

• 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) euconodonts or true conodonts are more complex, with cones, bars and blades. The protoconodonts are almost certainly unrelated to true conodonts; they may be chaeto-gnaths or arrow worms, a group of basal metazoans of uncertain affinities.

Euconodonts occur as three broad types of element, consisting of laminae of apatite. These are formed by outer accretion from an initial growth locus. White matter often occurs between, or crosscuts, lamellae; this material compares well with the composition and structure of vertebrate bone. The three main morphotypes of conodont element have been used in the past as the basis of a crude single element or form taxonomy (Fig. 16.4). The cones or coniform elements are the simplest, with the base surmounted by a cone-like cusp, tapering upward, and sometimes ornamented with ridges or costae (Fig. 16.4a, b). Bars or ramiform elements consist of an elongate blade-like ridge with up to four processes developed posteriorly, anteriorly or laterally to the cusp (Fig. 16.4c, d, g). Platforms or pectiniform elements have a wide range of shapes, with denticulate processes extending both anteriorly, posteriorly and/or laterally from the area of the basal cavity (Fig. 16.4e, f, h-j); some also have primary lateral processes. The cusp is attenuated, whereas the base may be expanded to form a platform with denticles on its upper surface. The basal cavity is filled by the basal body of the element in the form of a dentine-like material, although this is not always preserved.

Conodonts are common in certain marine facies from the Cambrian to the Triassic.

basal cavity Hertzina

(a) anterior anterk ' |T maln cusp blade denticles basal cavity Hertzina

(a) anterior

posterior blade basal cavity

Ozarkodina posterior blade basal cavity

Ozarkodina

basal cavity Furnishina

basal cavity Furnishina posterior i- main cusp anterior denticles posterior b cavity

Prioniodina b cavity

Prioniodina posterior /i

anterior

Polygnathus

posterior /i

S tfiH—platform-

if « basal cavity lateral process inner side outer side anterior

Polygnathus

posterior — platform posterior — platform lateral process

secondary platforms

blade anterior

Amorphognathus

Figure 16.3 Descriptive morphology of the main types of conodont elements: (a) protoconodont Herzina (x40); (b) paraconodont Furnishina (x40); and (c) euconodonts Ozarkodina (x40), Prionodina (x20), Polygnathus (x40) and Amorphognathus (x40). (Based on Armstrong & Brasier 2004.)

Paraconodonts are reported from the Mid Cambrian; older records are doubtful. During the Late Cambrian, simple conical eucon-odonts appeared. In the Early Ordovician, apparatuses with coniforms, and some with coniform and ramiform element types, appeared. Conodont diversity peaked during the Mid Ordovician, with a global maximum of over 60 genera. During this interval of experimentation, there was a huge diversity of apparatus patterns never again matched; later apparatuses are relatively uniform, perhaps indicating stabilization of feeding modes. Pectiniform elements were common from the Early Ordovician, together with a wide variety of blades and platforms in the Mid to Late Ordovician. This great diversity of forms was wiped out by the Late Ordovi-cian mass extinction (see p. 169). Silurian faunas are less variable, mainly apparatuses with ramiform and pectiniform elements. The conodonts again radiated during the Late Devonian, with specialized ramiform and pec-tiniform elements; over 1000 conodont taxa have been named from the Upper Devonian. Carboniferous conodonts (Fig. 16.5a) were characterized by a lack of coniform elements, together with pectiniform elements in the P apparatus position, whereas ramiform elements occupied the M and S positions (see

Figure 16.4 Conodont elements: (a, b) coniform, lateral view; (c, d) ramiform, lateral view; (e) straight blade, upper view; (f) arched blade, lateral view; (g) ramiform, posterior view; and (h-j) platform, upper view. Magnification x20-35 for all. (Courtesy of Dick Aldridge.)

next paragraph). Conodonts became rarer during the Early Permian, and most Late Permian and Triassic species had small apparatuses. They became extinct at the end of the Triassic.

The first piece of evidence about the identity of the conodont animal is that the elements sometimes occur in a cluster or apparatus consisting typically of 15 elements, 14 of them arranged bilaterally and one symmetric element positioned on the midline. The elements are arranged in a particular way in the apparatus: pectiniform (P elements) at the back, makelliform (M elements) at the front, and symmetry transition series (S elements) in between. Generally bars and platforms occupy P positions, whereas bars and cones are found in M and S positions. The P, M and S posi-

Conodont Animal

Figure 16.5 Homing in on the conodont animal: (a) natural assemblage of conodonts from the Carboniferous of Illinois (x24); and (b) the conodont animal from the Carboniferous Granton Shrimp Bed, Edinburgh, Scotland, with the head at left-hand end (x1.5). (Courtesy of Dick Aldridge.)

Figure 16.5 Homing in on the conodont animal: (a) natural assemblage of conodonts from the Carboniferous of Illinois (x24); and (b) the conodont animal from the Carboniferous Granton Shrimp Bed, Edinburgh, Scotland, with the head at left-hand end (x1.5). (Courtesy of Dick Aldridge.)

tions may be defined more precisely with subscripts, for example Pa and Pb elements.

The first conodont apparatus was found in 1879, and this gave some idea about the function of conodonts, perhaps as some sort of teeth, and provided some clues about the whole animal. Several supposed conodont animals were identified in the 1960s, but most of these turned out to be predatory critters that had just eaten a conodont animal, and so had lots of conodont elements inside them!

Despite the mystery of their identity, con-odonts became key tools in biostratigraphy (Box 16.3). In addition, because color changes of the elements can be related to changing temperature, conodonts are important indicators of thermal maturation. Now paleontologists believe they know what conodont animals looked like, but it took 150 years to work this out.

The solution came in 1983, when the first complete conodont animal was found in the Granton Shrimp Bed, a dark Carboniferous mudstone on the seacoast near Edinburgh, Scotland (Fig. 16.5b). This was an eel-like animal with a conodont apparatus at its front end. Detailed examination showed that the elements were in place and, this time, had not been merely eaten by the animal. Ten con-odont animals have now been found, as well as examples from other localities (Aldridge et al. 1993a). The Scottish conodont animal is up to 55 mm long, and has a short, lobed head with large goggling eyes that are fossilized black, perhaps a stain produced by the visual pigments. Below and behind the eyes is the conodont apparatus, clearly located where the mouth should be, showing that conodont elements really did function as teeth. The

Box 16.3 Conodonts and biostratigraphy

Detailed biostratigraphic schemes based on conodonts have been established for many parts of the Paleozoic and Triassic. For example, over 20 conodont zones have been determined for the Ordovi-cian System, while the Upper Devonian is the most congested interval, with over 30 biozones, each less than 500,000 years long. In northwest Europe the Carboniferous is routinely correlated on the basis of conodont zones.

Remarkable precision is now available in some zonal schemes. This has permitted the development of models for global environmental change during the Early Silurian (Fig. 16.6) tied to a tight conodont zonation (Aldridge et al. 1993b). Two oceanic states are recognized: those with oxygenated cool oceans that had a good vertical circulation and adequate supplies of nutrients (termed "primo"), and those with warm stratified oceans that had deep saline levels and poor nutrient supplies (termed "secundo"). Sudden changes between ocean states altered the vertical circulation and nutrient supply dramatically, perhaps causing extinction events.

These kinds of stratigraphic schemes may depend on geographic zonations. Cambrian conodont faunas were divided into equatorial (low latitude) warm-water associations and polar (high latitude) cool-water associations. During the Early Ordovician, these low- and high-latitutde assemblages further divided into six discrete provinces. Conodonts evolved independently at high latitudes, and there were only a few incursions from lower-latitude faunas. Towards the end of the Ordovician high-latitude, cold-water faunas migrated into lower latitudes. Thus Late Ordovician equatorial mid-continent assemblages originated in polar and subpolar regions and themselves formed the foundation for the Silurian fauna. During the Mid and Late Paleozoic, conodonts were mainly restricted to tropical latitudes. Devonian and Carboniferous faunas show some biogeographic differentiation among shelf associations. These differences among the geographic provinces can affect the stratigraphic schemes and the possibility of correlation from area to area.

Conodonts occur in a wide range of marine and marine-marginal environments, although the group is most common in nearshore carbonate facies, commonly in the tropics. Distinct environment-related conodont paleocommunities have been identified in many parts of the Paleozoic, and statistical analysis may discriminate, for example, deeper-water from shallow-water assemblages. It is important to be aware of the influence of depth and other factors on the distribution of communities before they are used in establishing biostratigraphic zones. It would clearly be a mistake to identify distinctive depth-determined conodont assemblages and then to interpret them as indicators of different time intervals.

Web links are available through http://www.blackwellpublishing.com/paleobiology/.

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Malm0ykalven Secundo Episode

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