Australia: The Land Where Time Began
At the time of writing the arthrodires comprise nearly half of all known placoderms with nearly 200 genera known, and new taxa are still being found.
The arthrodires ("euarthrodira") are undoubtedly the most successful group of placoderms, making up about 60 % of the fossils found. They are found in Devonian deposits throughout the world. They are the only placoderms to have 2 pairs of tooth plates (superognathals) in the upper jaw. The bones of the skull were in a regular pattern, the eyes being to the sides of the head. The cheek unit was separate, being hinged along the side of the skull roof. In all advanced arthrodires, a ball-and-socket type of joint connected the head and trunk shields. They lacked a true ball-and-socket neck joint, instead having a sliding neck joint, the head shield sitting in a bone platform that protruded from the trunk shield, in the most primitive forms, as was found in such forms as the primitive actinolepidoids (grade group Actinolepidoidei; Dupret et al., 2009), in which the head shield sat on a bone platform protruding from the trunk shield.
In appearance, their bodies were sharklike, with broad, fleshy paired pectoral fins, pelvic fins and an anal fin, as well as a single dorsal fin. In many advanced arthrodires, the tail was naked, but in more primitive forms the tail was covered with scales that didn’t overlap, that were small polygonal plates of bone with an ornamentation of tubercules.
The many arthrodire families have generally been divided into those of the, the primitive grade groups and the advanced groups. The primitive grade groups had large spinal plates on their long trunk shields, antarctaspidids, wuttagoonaspidids, actinolepids, and phyllolepids. The phylctaenids, the advanced group, with a trunk shield that was shorter and lacked a spinal plate (aspinothoracids), some having pectoral fins that were not completely enclosed by the trunk shield. Arthrodires had a worldwide distribution throughout the Devonian, in both freshwater and marine deposits.
The antarctaspidids and wuttagoonaspidids, both of which had long shields, were the most primitive of the known arthrodires. Some step group arthrodires are included, such as Yujiangolepis, from the Early Devonian of China, described by Vincent Dupret et al. It is most closely related to Antarctiaspis from the Middle Devonian of Antarctica. Nuchal plates that are long and narrow that contact the pineal plate in the skull are present in both species. Yiminaspis, a wuttagoonaspidid from Australia and China is a representative of the next group up, as they lack contact between the nuchal and pineal plates, as do all other arthrodires.
In Coccosteus (Miles & Westoll, 1968) from the Mid-Devonian, only part of the dorsal surface is covered by the trunk shield, extending back as far as the shoulder region below. It had pectoral and pelvic fins that were both paired, with limb girdle support, though much smaller than is found in sharks. It has a long dorsal fin on its back and the tail is heterocercal. Small scales cover the posterior portion of the body, though they are not often preserved. The author (Benton, 2005) suggests Coccosteus was probably a powerful swimmer, using lateral swings of its tail and posterior body, and is suggested by its shape to have probably lived close to the bottom of seas or lakes. Several plates make up the head and trunk shields, with a gap, the nuchal gap, between the 2 shields at the hinging line. The join of the lower jaw plates in the middle is weak, and their margins are worn to a sharp edge against a series of 8 small plates in the upper jaw.
Wear on the plates results in sharp beak-like plates that, though they are not teeth, would have been equally effective in cutting, puncturing and crushing. Some have claimed true teeth are present in arthrodires (Smith & Johanson, 2003), and as a result of an unexpected discovery it has been suggested that teeth may have had a separate origin in this clade, as well as in other gnathostomes. This has been disputed (Young, 2003). In Arthrodires the teeth are located inside the mouth, behind the main shearing bone plates at the edges of the jaws.
An upward swing of the skull, combined with a dropping of the lower jaw, opens the jaws. The width of the nuchal gap determines the height of the gape, the skull being hinged about the ball and socket joints within the lateral margins of the dorsal portion of the head shield. It has also been suggested that when feeding on the bottom of the lake or sea placoderms lifted their head, as it would have been easier to drive the lower jaw into the mud then lifting the head than by dropping the lower jaw.
Arthrodires that arose later than Coccosteus had armour that was more reduced, often consisting of a very limited trunk shield. Dinichtyidae and Titanichthyidae from North America and northern Africa reached sizes of at least 6-7 m. The largest predator in the Devonian oceans was Dunkleosteus that had jaws that could have crushed any other animal of its time.
The arthrodires had retained the full armour of the head and trunk that was found in the earliest placoderms. They had a well-developed neck joint between the head and trunk shields. Most arthrodires remained bottom or near-bottom feeders, so they raised their head to open the mouth. Of the few that moved to open-water feeding, they seem to have become formidable hunters. Eastmanosteus from the Australian Late Devonian, was a formidable predator.
The ball-and-socket joint first appears in the phlyctaeniid arthrodires, enabling the head shield more vertical movement, resulting in less restricted opening of the mouth for catching prey. Some of these forms, such as Lehmanosteus, Sigaspis and Heintzosteus, from the Early Devonian of Spitsbergen (Daniel Goujet), looked very streamlined, with widely-splayed spinal plates. They had many small teeth, leading to the suggestion that they ate small, soft-bodied prey, such as worms.
The Phyllolepids, “leaf scale”, were a group of armoured fish with a flattened body that were thought to be jawless until 1936 when it was shown that they were actually placoderms. The first complete specimens, from Mt Howitt in southeast Australia, of the group were studied in 1984 by John Long. It was then shown that they had jawbones with teeth. These fish have flat armour with a single large plate on the dorsal surface of the head and trunk, a regular series of smaller plates surrounding the large plates. There were fine ridges and tubercules forming a radiating pattern, the feature that was most characteristic of them.
Prior to more recent findings in Australia and America only a single genus, Phyllolepis was known, from the Famennian (Late Devonian) of Europe and America. Austrophyllolepis, “southern leaf scale” and Placolepis, “plate scale” were from an earlier time, the Givetian-Frasnain, that appear much more primitive than Phyllolepis. In 2005 another specimen, Cowralepis, from Canowindra in New South Wales, was described by Alex Ritchie. It had a median ventral plate, as did Austrophyllolepis, but it had a different shape of the arrangement of its head-shield plate. Cowralepis gill arches and mechanisms of feeding have been studied more recently (Carr et al., 2010). It was shown by their study to be an ambush predator with a powerful buccal pump mechanism that allowed it to swallow prey whole, as is present in angel sharks (Squatina) at the present.
Mount Howitt deposits in eastern Victoria have produced 2 species of Austrophyllolepis. Other fragmentary remains of the genus have been found in Antarctica. As these fossils display the outline of the whole fish, as well as impressions of jaws, pelvic girdles, parasphenoid (palate bone) and bones from parts of the cheek, it has been possible to learn more about the phyllolepids. The picture this has presented of Phyllolepids is of flat predators that lie on the bottom mud of lakes waiting for prey to pass above it. A function could then be demonstrated for its tail, that was unusually long, that is believed to have been used to thrust the fish up rapidly to grasp the prey. It is suggested by Long that phyllolepids were probably blind, as they have no bones circling the eyes, and unlike most other placoderms, the head shield is not embayed for eyes. The may have been assisted in prey detection in the absence of eyes by the radiating patterns of sensory-line canals that were especially well-developed in this group. They grew to about 50-60 cm long.
Long has suggested that phyllolepids could have arisen from forms such as Gavinaspis, that has been found in Ynnan, China, in deposits dating from the Early Devonian. There is a suggestion that the phyllolepids originated in China, then spread to East Gondwana, that was made up of Australia and Antarctica in the Middle Devonian, and reaching the Northern Hemisphere in the Late Devonian (Dupret & Zhu, 2008). It has also been found that in male phyllolepids the pelvic fins may have been modified, long lobes developing for copulation.
Among the arthrodires the most advanced group is the Brachythoraci, a characteristic of which is a ball-and-socket joint on the head shield. Holonema westolli, the most basal known member of the group, was found in the form of 3-D fossils in the Gogo Formation, Western Australia and has been studied (Miles, 1971). The trunk shield of Holonema was long and barrel-shaped. The tooth plates were unusual, being concave tooth plates in the lower jaw on which there were parallel ridges of raised dentine. Based on the presence of many small stones in its gut suggests that it was possibly an oncolite feeder that scooped up algal balls that were probably present on the fore-reef at Gogo (Long, 2006).
Other well-known brachythoracids include forms such as groenlandaspids, named after Groenlandaspis, a common genus, the group having median dorsal plates that were high-crested. They appeared first in the Early Devonian, Tiaraspis, becoming abundant in eastern Gondwana in the Early Devonian, gaining a world-wide distribution by the end of the Devonian.
The fossil record of Eubrachythoracids, in Australia is said by Long to be excellent. This group were the first advanced arthrodires.
Arthrodires are one of the groups found in the Taemas-Wee Jasper, New South Wales and Buchan region of Victoria, Australia. Well-preserved 3-D skulls and plates from the Early Devonian have been found in these deposits, many species being present, with a size range from 20 cm body length to 40 cm skull length, probably with a body length of 3 m. Buchanosteus confertituberculatus (rounded tubercules) was the most common species found in these deposits. Buchanosteid arthrodires, the most primitive family of advanced arthrodires (with a shortened trunk shield), have been found in China and Russia.
Also found in the same deposits as Buchanosteus, were Errolosteus, that had a skulk roof that was slender and on which the ornamentation was delicate sandgrain-like, hence its name “sand fish”
The largest arthrodire skull found in the Taemas-Wee Jasper deposits, Dhanguura, is 40 cm-long, at the front of which is a small T-shaped rostral plate. This feature links the primitive arthrodires from the Early Devonian to the more advanced forms from the Middle Devonian. This large arthrodire is believed to possibly be related to homosteids from Europe. Their weak, toothless jaws have led to the suggestion they may have been filter feeders, as are large whale sharks.
The Buchanosteus and Taemasosteus, both broad-headed types, followed the trend present in this group of reduced body armour. This adaptation seems to have been an adaptation directed to lightening the armour and opening it more to allow for greater efficiency and control in swimming, and of course increased speed, important for an open water predator.
The many families of Arthrodire are divided into 2 groups, the primitive groups with a long trunk shield and large spinal plates. A shortened trunk shield, that in some didn't completely enclose the pectorals and lacked a spinal plate, and reduced spinal plates, characterised the more advanced groups.
The arthrodires have been further divided into a number of groups, according to the amount of armour that has changed over time, the most primitive, the closest to the ancestral condition, having the most armour. The most primitive of the arthrodires is the Actinolepids, such as Dicksonosteus from Spitsbergen, from Early Devonian shallow marine and river deposits, especially in Euramerica. Very well preserved specimens of Dicksonosteus from this area displayed details of the braincase and its internal anatomy, the details of which have been described (Goujet, 1984). These had the simpler sliding neck joint mentioned above.
Aethaspis is an actinolepid from the Early Devonian deposits in the Bear Tooth Butte locality of Wyoming that was described by Robert Denison in the 1950s. Kujdanowiaspis from Podolia, of Early Devonian age, is suggested by Long to be probably the best known arthrodire of its era. Beginning with the appearance in the fossil record of Russia and Arctic Canada in the Early Devonian, the shorter, broader nuchal plate was becoming more common, a characteristic of most of the advanced arthrodires.
The least primitive, pachyosteomorph arthrodires, were the first group to have full armour, with no joint between the head and thoracic shield, and the last group to have a well-developed crano-thoracic joint, and great reduction of thoracic armour. This trend has been called dolicothoracids and brachythoracids. The most primitive arthrodires, having a long, solid trunk shield with a small opening for the pectoral fin are the dolicothoracids, the brachythoracids were the more advanced forms with reduced thoracic armour and larger holes for the pectoral fin.
Northern hemisphere sites, such as those in Morocco and Germany, have produced forms similar to the Australian genera, so the marine forms could migrate over great distances in the Early Devonian.
Williamsaspis, Wuttagoonaspis, and Burrinjucosteus, were among arthrodires that are unique to Australia, but their relationships are unclear. Wuttagoonaspis has now been found in Early Devonian deposits in China (Dupret & Zhu, 2008). These groups have been given their own families, the Williamsaspidae, Wuttagoonaspidae and Burrinjucosteidae. Wuttagoonaspis is the most primitive of the 3, based on surface area reduction, and other characteristics.
Most consider Wuttagoonaspis, from the Middle Devonian, to be at the actinolepid level of organisation. It had a long skull in which the eye holes were small and distinct. The skull roof plates, that are anomalously small, are in a pattern that has been described as bizarre, that is not like patterns seen in other known arthrodires. Many would place this fish in an arthrodire group of its own. Some have suggested it indicates an early link between Wuttagoonaspis and the phyllolepids that were flattened. A deposit near the Toowoomba Range in south-western Queensland has produced large plates of Wuttagoonaspis indicating the some species could reach up to 1 m in length. The very distinctive pattern of ornamentation on the skull bones is similar to the radiating pattern of ridges and tubercules on the skull of phyllolepids. It may have been a bottom-feeder in shallow marine and estuarine environments, as it had weak jaws suggest it may have fed of small prey such as worms.
Williamsaspis and Burrinjucosteus are considered to be at the phlyctaenaspid organisation level. Both forms have reduced armour and a well-developed ball and socket joint between the head and trunk armour, the socket being on the back of the head armour and the ball on the front of the trunk shield, the opposite of the situation in the antiarchs, so its movement and feeding would be expected to be more efficient.
The size of arthrodires ranged from less than 100 mm to more than 6 m, Buchanosteus was one of the larger forms. The largest found so far in Australia is Westralichthyes at about 2.5 m. Many of them were predators on the tropical Devonian reefs, as are preserved in the Gogo Formation.
Another fish found in the Canowindra deposit was the arthrodire Groenlandaspis, about 50 cm long. It has been found around the world, the first specimen being found in Greenland. The body shield of Groenlandaspis rose steeply to form a dorsal ridge that had a fin-like appearance, and the pectoral fins were protected by bony projections from the side of the shield.
More than 20 types of arthrodire were found in the Gogo Local Fauna from the Late Devonian of Western Australia, some of which are so unique they have been allocated their own family. There were 2 small forms of arthrodire common on the reef of the Gogo deposits, the camuropiscids and incisoscutids. Characteristics of these forms include spindle-shaped, elongate armour, with large eye holes and large crushing tooth plates. Forms such as Rolfosteus and Tubonasus developed extreme elongation of the armour and tubular snouts, possibly to improve their streamlining (Long, 1995). It has been suggested that these fish, about 30 cm long, may have been surface water feeders, possibly feeding on shrimp-like crustaceans.
In Incisoscutum, one of the commonest placoderms of the Gogo Formation, the trunk shield was incised, allowing the pectoral fins to be unenclosed by bone, probably to improve the mobility of the fins. As in the camuropiscids, it had strong crushing tooth plates.
The small predatory plourdosteids (named after Plourdosteus, Late Devonian forms from Canada and Russia) were the most diverse of the Gogo arthrodires. The unique skull roof bone patterns among the many species of plourdosteid, along with the dentition, distinguish the different species, such as Torosteus, Harrytombsia, Macnamaraspis and Kimberleyichthys. There are a number of well-developed cusps and tooth areas, and where the jaws meet, there are many teeth along the midline. The cartilaginous braincase was braced by well-developed bony buttresses that allowed a powerful biting action. It has been suggested that, based on the broad, robust shape of their armour, these small fish, from 30 to 50 cm long, may have hunted near the seafloor in cavities in the reef, possibly lunging at passing prey (Long, 1995).
Also hunting on the Gogo reef were large predators, such as Eastmanosteus calliaspis, up to 3 m long, that had large dagger-like cusps on their jaws. This species is regarded as displaying the basic pattern of Dinichthyids, (terrible teeth) the largest of the placoderms, large skulls and bony plates of which have been found the Cleveland Shales in New York and in the Sahara desert, limestones of Morocco. At more than 1 m long, these skulls suggest the fish could have been 6-8 m long. As in Dunkleosteus and Gorgonichthys, there were pointed cusps on the tooth plates of the lower jaw.
A species from Morocco, Titanichthys, that was at least 7 m long, had weak jaws with no sharp cusps as in the dinichthyids, that Long suggests might have been a filter feeder similar to the whale sharks of the present, a suggestion previously made for homosteids, an earlier group of large placoderms (Elga Mark-Uurik). A diet of other arthrodires, or possibly carrion, has been suggested for giant placoderms from the Cleveland Shale. Long has suggested they may have preyed on the cladoselachian sharks of the time, but he doubts they would have been fast enough to catch them as these placoderms were heavily armoured.
For some unknown reason the placoderms went extinct at the end of the Devonian, in spite of being one of the most successful vertebrate groups, dominating aquatic vertebrate life for almost 60 million years.
There were also many smaller predatory fish with tooth-like structures on the jaws.
|Author: M.H.Monroe Email: email@example.com Sources & Further reading|