Australia: The Land Where Time Began

A biography of the Australian continent 

Thelodonti (nipple tooth)

Thelodonts are a type of agnathan, 1 of 3 groups of agnathans found in Australia. They are known mostly from their scales, that are of characteristic form, with a distinctive crown of bone base with a covering of dentine, with a large pulp cavity on the lower side. The tissues of the many varieties of scales are variable. They had a flattened body shape, with wide wing-like pectoral fins, and a large head and slanting rows of gill openings, e.g., Turinia. They were mostly small fish, less than 15 cm long, though some grew to almost 1 m. Thelodonts had thousands of scales that ranged from 0.5-2.0 mm in size. The scales even lined the inside of the mouth and the gill slits. Each individual fish had many scale shapes, head scales that were short and squat, elongated scales on the trunk, many types of fin scales and pharyngeal scales, and the tissue of the scales was also variable, making the determination of the species based only on the scales, as is often the case, very difficult, requiring a thelodont specialist. The scales are from 0.1 to a bit more than 0.3 mm wide. The scales are very much like mammalian teeth. The shape of the scales can vary over the body of an individual fish. As a result of the study of the rarely found complete fish shows that accurate species identification can be made by examining isolated scales.

There are denticles on the pharynx of some thelodonts, such as Loganellia, like gnathostomes.

Based on the rare finds of complete fossils of whole thelodonts indicate that more were flattened and had broad, wing-like fin folds that lacked radials as are found in anaspids. They had long heads, the ventral surface of which had rows of gill openings, as seen in Turinia.

According to one of these specialists, Dr Sue Turner, one of the foremost thelodont experts in the world, thelodonts probably evolved to have a number of different lifestyles, and that the larger, flat forms, such are Turinia may have been bottom feeders that were probably slow moving. They were probably similar to angel sharks, squatina, at the present, either digging in the sediment for invertebrates or possibly as ambush predators, catching passing prey.

The fossils of complete fish found in the New Territories of Canada show that many different forms of thelodont evolved, some with deep bodies and large forked tails. Modern jawless fish, lampreys and hagfish lack a stomach, but some fossils from Canada have a large stomach.

Since the finds from Canada, it has been suggested that the thelodonts may be much more advanced than previously believed, and possibly closer to the link between jawed and jawless fish,  based on primitive scales of early thelodonts that show close similarity to scales of early sharks, and they had well-formed stomachs, or possibly close relatives of the heterostracans, based on the structure of the unique fork-tailed caudal fin. Such a feature is also present in the family of irregulariaspidid heterostracans, also found in northwest Canada (Pelleren & Wilson, 1995). Long suggests that thelodonts are possibly not naturally monophyletic, with some appearing to be closer to heterostracans and others appearing closer to the gnathostomes.

The oldest known thelodonts, found in Siberia, are of Late Ordovician age. They went extinct by the Late Devonian. In Euramerica, most thelodonts were gone by the early part of the Devonian, whereas in Gondwana forms such as Turinia and Australolepis survived later, going extinct by the end of the Frasnian in the Late Devonian.

All major groups of fish are represented by the microfossils that have been found in Australian sites. These microfossils are aiding in the dating of the Australian rocks. One of the most commonly found fish in Australian rocks are the thelodonts (nipple tooth). These thelodonts were small agnathans of an average of about 150 mm, but they could reach up to about 1 m. They lived in marine and freshwater, and they were covered with small, thick dentine-covered scales, but no body armour.

The deep-bodied species found in Canada are believed to have been active swimmers that fed by either filter feeding or catching free-floating live prey. There were body spines that were strongly pointed in such forms as Lanarkia, that have been suggested to have possibly been moveable, lying flat along the side but being raised to point outward if the fish was threatened, possibly in the manner of a puffer fish (Turner, 1992).

The only group known from Australia are the hybodontids, that range from the Late Silurian to the Late Devonian, and includes such genera as Turinia and Nikolivia. Australolepis, from the Late Devonian (Early Frasnian) Gneudna Formation in the Carnarvon Basin of Western Australia, is thought to be possibly the youngest known thelodont in the world.  By the end of the Early Devonian, thelodonts had almost completely disappeared from the rocks of Europe and America, though they still flourished around the shores of Gondwana, in Australia, as well as Iran and Thailand. 

Sources & Further reading

  1. John A Long The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 1995
  2. Long, John A., 2011, The Rise of Fishes - 500 Million years of Evolution, University of New South Wales Press, 2011
Author: M. H. Monroe
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Last Updated 01/11/2011

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading