Jawless Fish - Agnathans

Australia: The Land Where Time Began

Agnathans have a fossil record spanning almost 500 million years. The earliest recognisable bony shield from an agnath came from a 450 million years old Early Ordovician shallow marine sandstone in Central Australia. The oldest almost complete specimen of a related genus is from Bolivia, South America, another part of Gondwana. The name agnathan of the jawless fish is now seen as the basal level of fish evolution instead of referring to a specific group of fish, they are thought of as a paraphyletic group not being characterised by any features that are unique.

Among these fish can be found the break-throughs in design that became standard characteristics for vertebrate design on to the present, the development of cellular bone, paired limbs, sensory-line systems, dentine-like tissue, complex eye muscle patterns, the inner ear with 2 semicircular canals.

The primitive Ordovician forms, including the Australian forms, Arandaspis and related species, share the primitive feature of a number of paired openings for the gills. The number of paired openings for the gills was reduced in all known subsequent agnathans except some anaspids.

The high point of jawless vertebrates was during the Ordovician to Early Silurian. There were 2 major groups of jawless fish that existed during the Devonian, the Pteraspidomorphi (or Diplorhina) and the Cephalaspidomorphi (or Monorhina). There is still controversy about how the placoderms should be divided between the 2 groups. It seems only the Pteraspidomorphi had much of a presence in Australia, and possibly in Gondwana, including the heterostracans and thelodonts. The Cephalaspidomorphi were essentially restricted to the continents in the Northern Hemisphere, included the osteostracans and the anaspids.

A number of characteristics differentiate the pteraspidomorphs from the Cephalaspidomorphs. An important distinguishing characteristic is the position and number of the nasal sacs. The pteraspidomorphs had paired nasal sacs, 1 on either side of the midline (called diplorhinans). The Cephalaspidomorphi had a centrally located single nasal sac. In this feature the pteraspidomorphs are similar to that in the Arandaspis and Sacambaspis, so are thought to be very primitive fish.

Heterostracan Pteraspidomorphs had a fairly solid cover over their head and throat (pharynx), with a single outlet for the water passing over their gills, they lacked any internal ossification of their internal Skelton and their bony external shield was acellular, and none developed paired fins.

There is a macrofossil and microfossil record of the pteraspidomorphs. The heterostracans have left a reasonable macrofossil record, the thelodonts, first appearing in the earliest Silurian, are mostly known as microfossils. The bodies of the thelodonts seem to have been covered by thousands of tiny, rhombic, non-overlapping scales. They are useful for dating rocks of Silurian-Devonian age because of these microscopic remains.

The Agnatha, including both groups, diversified greatly in the Late Silurian to Early to Middle Devonian, throughout the world. It was a time of explosive adaptive radiation for the placoderms and for fish in general.

There is a break in the known fossil record of the Agnathans until the Devonian. When the agnathan record resumes it is mostly thelodonts. The Australian forms seem to follow the trends in diversity of the rest of the world. By the Middle Devonian they are almost totally eclipsed by the extremely successful placoderms. They, in turn, are replaced by the end of the Devonian, by the evolution of more advanced fish. The agnathans were dominant for almost 100 million years, a few even surviving until the present in the form of the lampreys and hagfish.

The placoderms coexisted with the agnathans for 10-20 million years and when the more advanced fish evolved they persisted for at least another 15 million years, competing with "spiny sharks" and the early bony fish.

The agnathans never developed both paired pectoral fins and pelvic fins, so their speed and manoeuvrability was limited, which is probably why they were unable to survive competition from the bony fishes in the long run.

A variety of body shapes developed, flattened, fusiform, or cylindrical, but they were mostly restricted to filter feeding by the lack of jaws, with the exception of the lampreys, successfully evolving as parasites, and the similar hag fish that specialised as scavengers.

When Arandaspis and Porophoraspis lived in the warm shallow seas of the Ordovician, Australia was still part of Gondwana and was situated near the equator. It was on the north-west sector of Gondwana and was divided into 2 large sections. Part of present-day Central Australia formed one block and to the south another island. The 2 parts of Australia were separated by the Larapinta Sea, a large tropical sea that formed when the water invaded much of the continent. To the south was the ocean.

Most of the agnathan families were extinct by the Late Devonian. Among the few survivors were 4 species osteostracans and lamprey-like anaspids, Euphanerops, that have been found in the Escuminac Formation in Canada, 1 possible, though indeterminate, species of galeaspid from Ningxia, northern China, 1 thelodont genus, Australolepis, from Australia, and from Europe a few species of psammosteid heterostracans that were large and flattened. Long suggests that the steep decline in agnathan diversity at this time may be the result of the rapid increase in the diversity of the gnathostomes that also occurred at this time that may have either displaced them or preyed upon the agnathans, their larvae or eggs.

Both in diversity and biomass the agnathans were the dominant fish type in the Silurian, as the gnathostomes were comparatively rare at this time. All major groups of gnathostomes were present in the oceans by the start of the Devonian, some, such as the stem gnathostomes, acanthodians, reaching high levels of diversity in the latest part of the Early Devonian. Many of the gnathostome groups, such as placoderms, porolepiforms, tetrapodomorph fish and dipnoans reached a peak of diversity by the Middle and Late Devonian.

Long suggests that some gnathostomes could possibly have taken over the niches of agnathans that had been eliminated by increasing predation pressure because they had similar body shapes. The phyllolepid placoderms appeared immediately after the flattened psammosteid agnathans, the transition being seen in the same succession in the east Baltic region. It is suggested that the early placoderms with long shields outcompeted the heterostracans with long shields, possibly because they had better defences or because they were faster swimmers. A suggestion is that the many new forms of bottom-feeding placoderms, such as antiarchs, may have displaced the detrital bottom-feeding agnathans. The lampreys and hagfishes, forms of agnathans that lacked armour, were the only surviving agnathans by 355 Ma at the close of the Devonian.

Sources & Further reading