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Primer on Fishes

Introduction

A diverse and very informative assortment of fishes have been excavated from the Bear Gulch Limestone. This primer contains systematic, morphologic and other information on the groups (Agnatha, Acanthodii, Chondrichthyes and Osteichthyes) represented in the fauna.

Agnatha (fishes without jaws)

Prior to the evolution of jaws, the Agnatha had diversified into many different groups. Only two agnathans survive to modern times, the lampreys and the hagfish.  These two are representatives of two different ancient agnathan groups. The Bear Gulch beds have produced the earliest known fossil lamprey.

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Acanthodii

(Class of jawed fish with spines supporting all fins except the caudal fin, very fine scales, and few bones).

There is only one genus of acanthodian found in the Mississippian, the toothless Acanthodes, and one species is known from fresh/brackish shales of the Heath Formation, Acanthodes lundi. Specimens from the Bear Gulch beds appear to be a different species, but they very rarely preserve in an attractive condition and usually look like well-chewed hors d’oeuvres with many embedded toothpicks. The specimen illustrated here is one of the better ones.

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Chondrichthyes

(Class of jawed fish with primarily cartilaginous endoskeletons mineralized by small polygonal prisms as adults, and males with endoskeletal claspers extending from pelvic fin axis that transmit sperm for internal fertilization with copulation). For more information on the anatomy of this group, check out the primer on fish anatomy.

The chondrichthyans consist of two living subgroups: the elasmobranchs or sharks, skates, and rays; and the chimaeroids. These two groups are very different in appearance, anatomy, life histories, and habits. One of the oldest questions in the study of fish concerns the possible relationships of the two, whether they indeed are related through a common ancestral chondrichthyan form or not, and if they are related what might that form have been. The Bear Gulch fishes are now providing answers to this question. The two groups are related, and the more plesiomorphous morphologies can be found in a newly named group (Euchondrocephali) that is intermediate between them.

Teeth, and fin spines, are the most durable body parts of sharks. Furthermore, they are frequently shed and replaced during the lifetime of any individual, and commonly are found in rows with 5 or 6 replacement teeth for each functional tooth position (tooth families). They are thus rather common and may be preserved in many sedimentary environments even if the shark has swum away, been eaten, or decomposed.

Many isolated Carboniferous shark teeth have been given scientific "tooth-species" names during the last 200 years. However, in most cases, the bearers of these isolated teeth are still unknown. In fact we can not determine the possible shapes of other teeth in the same jaw or mouth from the shape of one isolated tooth, nor can we determine anything of their relationships except by guessing. Only by finding whole jaws full of teeth, in whole fish, as we do in the Bear Gulch, can we associate teeth into dentitions and determine the relationships and paleobiology of these mysterious fish. Teeth of Bear Gulch sharks can be matched against isolated teeth from Iowa, Indiana, Kentucky, Illinois, Alabama, England, Ireland and Belgium, all of about the same age, to give the tooth species names some meaning.

The teeth and dentitions illustrated in this website are all drawn as they appear in the particular specimens, complete (or incomplete) with cracks and chips. Where possible we have illustrated several views of the same specimens. Definitions of a few terms may help:

For what we theorize about the relationships of these fish, see a relatively recent cladistic diagram of the Chondrichthyes. Please understand that any cladistic diagram is simply a summary of work in progress, and will change with new material that provides additions to our knowledge.

The following are synopses of selected higher taxa of Chondrichthyes:

Iniopterygii These strange fish are currently under study. A series of species from the mid-continental Pennsylvanian are collectively characterized by large pectoral fins mounted at the nape of the neck, and many denticulated bony plates on the head and jaws.

Euchondrocephali (Subclass with autodiastylic jaw suspension, elongate, solidly roofed ethmoid region of the braincase and gill arches nested below the otic region)

Gregoriidae (Family with primitive-looking teeth and relatively plesiomorphous braincases and jaw suspensions, but quite variable in cranial proportions, fin and fin spine arrangements, and body proportions).

The fish included within this group are very varied in sizes, fin morphologies, scale coverings, and cranial proportions. They each display a different variant of the position of their autodiastylic suspensorial articulations.

Debeeriidae (Family with heterodont teeth far forward on the jaws.)

Orodontiformes are known from many isolated teeth from across the world. Whole-body specimens from Bear Gulch are under study.

Petalodontiformes (An order known mainly from isolated teeth, each tooth shaped and curved like an artichoke petal).

The most common of the Upper Mississippian petalodont teeth come from Polyrhizodus digitatus, so called from the finger-like tooth roots. Unfortunately, only one denture of Polyrhizodus has been found at Bear Gulch. Aside from Belantsea, the only other petalodont known that comes with a body is the Permian Janassa bituminosa. Unlike the Bear Gulch Limestone petalodonts, J. bituminosa is flattened like a skate and has greatly flattened teeth, as has Janassa clarki from the Bear Gulch

Holocephalimorpha (This subclass includes fish with holostylic suspensoria (Holocephali) and those clearly related taxa that have autodiastylic suspensoria.

Holocephalimorpha of uncertain affinity (This assortment contains fish that have autodiastylic suspensoria but are related to the holostylic holocephalans.) Harpacanthus frimbriatus is an example from Bear Gulch.

Holocephali (Subclass or Superorder principally noted for the early embryonic fusion of the jaws to the braincase, the reduction of the dentition to few tooth positions, and usually the of presence of tooth plates that are not shed, rather than separate independently replaced teeth.)

Chondrenchelyiformes (Order containing the Scottish Chondrenchelys problematica and the Bear Gulch Harpagofututor volsellorhinus, both shaped like modern knife-fish.)

Cochliodontimorpha (Superorder containing a large variety of holocephalans with one or more pairs of tooth plates and a variety of head spines or plates.)

The Cochliodontimorpha includes two fairly well understood orders, the Chimaeriformes and the Chondrenchelyiformes, plus a poorly known group referred to as cochliodonts. Few cochliodonts are known from anything but fragmentary remains (such as isolated tooth plates), but the Bear Gulch Limestone contains quite a variety of excellently preserved whole fish. The Bear Gulch cochliodonts are currently under study. They are provisionally divided into the Order Menaspiformes, which lacks first dorsal fin spines, and the cochliodont-1 group, which has first dorsal fin spines. All known cochliodonts seem to be sexually dimorphic: this usually is expressed with enlarged scales, spines, spikes, and plates of bone and accessory hard tissues, in addition to claspers, pre-pelvic tenaculae, and a great variety of cephalic tenaculae. In accord with modern sexually dimorphic animals that depend upon visual clues for species recognition and determination of sexual status, it is tempting to suggest that male cochliodonts may have also been brightly colored for further advertising value.

Chimaeriformes (Order containing the modern chimaeras: ratfish, rabbitfish, elephantfish, spookfish, as well as a few other esoteric fish and names.) Chimaeriformes are known for their sexual dimorphism; males are equipped with denticulated, small median ethmoid tenaculae as well as muscularized, denticulated pre-pelvic tenaculae attached to the pelvic girdles. They characteristically lack all but a few specialized cranial and mid-dorsal scales, and bear 2 paired upper tooth plates and one paired lower tooth plate.

Echinochimaeridae (Family containing the fossil genus Echinochimaera). While whole specimens are currently known only from the Bear Gulch limestone, jaws of various species of Echinochimaera have also been found in the Mississippian midcontinental United States.

Menaspiformes (Order of cochliodonts lacking first dorsal spines, with tooth plates growing anteroposteriorly along jaws that are just about transverse to the long axis of the head, paired small anterior tooth whorls followed by a pair of larger posterior tooth plates of Sandalodus-shape (uppers) or Deltodus-shape (lowers). Calcified cartilage appears rough, as if prismatic.) This order was erected for what was formerly the only known whole-animal Paleozoic cochliodont, Menaspis armata from the Permian of Germany. The head of Menaspis is adorned with a few pairs of fish-hook shaped spines very much like those seen in one of the Bear Gulch cochliodonts, Traquairius agkistrocephalus.

            Menaspidae (Family containing the Permian Menaspis armata).

Traquairiidae (Family with various cranial plate and spine combinations, and small anterior paired tooth whorls lacking any crown tritoral tissues.)

Named for the pioneering Scottish student of Paleozoic sharks, Ramsey Traquair. Three closely related species are discussed below. These species also illustrate the problems of understanding cochliodonts from the limited information presently available. They differ from each other subtly, almost imperceptibly, in dentitions, but differ strongly in their patterns of cranial spines and plates as well as what is known about their body scales, spines, or plates.

Cochliodontiformes (Order of cochliodonts with many members, most based on isolated tooth plates: few upper and lower tooth plates; two dorsal fins, the first with a strong fin spine. This group is currently under study). The type species of the typical genus is Cochliodus contortus, that consists of a lower jaw with tooth plates. A jaw conforming to C. contortus is found in the Bear Gulch Limestone.

Elasmobranchii (Subclass noted principally for amphistylic or hyostylic jaw suspensions; gills extending behind the head and opening to the outside via separate gill clefts; shoulder girdle distant from neurocranium).

All modern sharks, skates and rays, as well as a large assortment of fossil chondrichthyans belong to the Elasmobranchii. The subclass Elasmobranchii contains the dominant apex predators of Paleozoic waters.

Cladodontida (Superorder noted principally for its multicusped "cladodont" teeth.)

Cladodont sharks are united into one group because they all share, among other unique features, a similar tooth type. A cladodont tooth has five or more tall, sharp cusps, the central one being the largest, mounted on the labial edge of a broad bony platform. Articular buttons and grooves brace each platform under the next youngest tooth in the tooth family.

Cladodont sharks also are characterized by an amphistylic suspensorium (a hatchet-shaped palatoquadrate [upper jaw] that articulates against the postorbital process of the braincase), and a pectoral fin endoskeleton with several anterior basals, a large metapterygial plate, and an extended, free, usually whip-like axis behind the main part of the fin.

Stethacanthidae (Family of cladodont sharks with amphistylic suspensoriums, pectoral fins with a long trailing whip-like appendage behind the body of the fin, and very characteristic morphological and developmental differences between males and females of each species).

In all known Stethacanthidae the females lack a first dorsal fin and spine. Males develop the first dorsal fin and spine at sexual maturity, while other chondrichthyans develop these structures at or before birth, if at all. The spines have broad basal shoulders, and vary among the species from broad upright triangles in side view to forwardly curved, sickle-shaped objects. The spines themselves were not movable; they do not have enameloid/dentine ridging, and they articulate with the highly modified fins in several different ways. The fins vary from mobile, inverted triangles to fixed, compact rods, and are invariably surmounted with highly modified, usually enlarged denticles that face downward. The tops of the heads of the males (and sometimes their "necks"), are also invariably coated with a crown of enlarged denticles. Snout shapes and scale dispositions also vary greatly between males and females. The spine-fin complex and the other differences thus helped to visually identify a male of a species, his sexual maturity, and presumably his condition, to other males as well as to females.
Stethacanthid dorsal fins
The primitive chondrichthyan dorsal fin condition is to have a sharp, slim, distally exposed first dorsal spine followed by a roughly triangular fin (A), in both sexes; these primarily defensive structures being present from birth. We see a morphocline among the Stethacanthidae to progressively modify the first dorsal spine into an upright, then a forward curvature, and a progressive narrowing and hardening of the fin into a spine-like and then a rod-like structure accompanied by a crust of enlarged and specialized scales; and progressive immobilization of the fin upon the spine. As we do not have whole mature specimens of the largest and most primitive of the spine species, S. productus (B), the spines of male S. altonensis (C) are our first-order derived condition for this morphocline, and the more specialized representatives are Orestiacanthus fergusi (D), Falcatus falcatus (E), and Damocles serratus (F).

The most fascinating and important aspects of stethacanthid sexual dimorphism are that: a) it characterizes every known species in the family; b) that the male characteristics involve a highly modified first dorsal fin and spine plus specialized scale coverings on the top of the heads; females lack the first dorsal fin and spine as well as the specialized scales on the head; and c) that these features develop, as is physiologically understandable with secondary sexual characters, at and after sexual maturity (puberty) rather than at birth. Thus these features, so vitally important to identification by the females of sexually mature, healthy, strong males of their particular species, presage the same sorts of evolutionary developments so many millions of years later in certain ornithischian dinosaurs, birds such as peacocks, and many mammals such as deer and elk, to name but a few. For the primary reference to this phenomenon see Charles Darwin’s On Selection in Relation to Sex, and a voluminous later literature.

Stethacanthid sharks ranged in size from the diminutive Falcatus falcatus (about 150 mm total length) upward to Stethacanthus productus, estimated to be around 3 or more meters in length. A great variety of stethacanthid spines were named in the 19th century, but many have not yet been found attached to whole animals. The Stethacanthids thus were dominant predatory sharks in Lower Carboniferous oceans and shallow seas.

Squatinactidae (Family recognized for its convergent resemblance to the modern Angel shark Squatina.)

Selachii (Superorder containing modern sharks, skates, and rays and those few earlier sharks that had similar gill structure, jaw suspension, and paired fin supporting skeletons.)

Osteichthyes

The Osteichthyes (bony fish) is a class known for endoskeletal and membrane-based bones that occur in particular patterns. For more information on the general anatomy of this group, check out the primer on fish anatomy.

Actinopterygii (Subclass, known for bony, jointed fin rays, and primitively with only one dorsal fin.

Paleozoic Actinopterygii were for the most part small and are known predominantly from poorly preserved remains. Actinopterygians represent about 33% of fish species in the Bear Gulch beds, but today's ray-finned fish constitute 97% of all living fish species.

An annotated diagram of skull bone terminology for a generalized lower actinopterygian is available here.

For a study of the relationships of these fish, see a relatively recent cladistic diagram of the lower Actinopterygii. Please understand that any cladistic diagram is simply a summary of work in progress, and will change with new material that provides additions to our knowledge.

Tarrasiiformes (The Tarrasiiformes are knifefish-like, and are known from the Scottish Tarrasius problematicus and the slightly younger Bear Gulch form.)

Cladistia (Group containing the modern Polypterus and Erpetoichthys, plus those Bear Gulch fish that appear to be related, such as the Guildayichthyiformes

Platysomiformes (Group of deep-headed and deep-bodied lower actinopterygians).

Paleonisciformes (An unorganized collection of fusiform lower actinopterygians that is currently under study.)

Sarcopterygii (Subclass, noted for the fleshy lobed fin bases resembling arms.)

Coelacanthiformes (Order, noted for its particular caudal and dorsal fin structures and arrangements, and for its extreme conservatism in body adaptation over the last 400 million years). At least one species survives today off the Comoro Islands and Indonesia, Latimeria chalumnae.  Six species of coelacanths swam in the Bear Gulch Bay; and the two most different in body form and mouth gape dimensions were the most common. Caridosuctor populosum is the most abundant fish in the Bear Gulch fauna, at one out of every nine fish that we find.

Rhizodontiformes (Order of Carboniferous lobe-fined fishes). We have only found large isolated scales and large conical teeth of a fish that can be attributed to Strepsodus, but we keep hoping for a whole fish. They are large elongate fish that reputedly reached the size of about 5 feet in length. We hypothesize at this time that the two types of scales shown here came from different parts of the body of the same species of fish.

2/1/2006

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