Non-reptilian life in Mesozoic Australia
Although the Mesozoic has been dubbed "The Age of Reptiles", not every ecological niche was dominated by them. There were a whole host of other life forms that were just as interesting, perhaps even more so, than the dinosaurs and their reptilian kin. For a more extensive list see the Early Cretaceous Non-Dinosaur Fossils page (for Victoria only).
Mesozoic mammal fossils are rare in any part of the world. In Australia, which would have had a polar or sub-polar climate during the Mesozoic, they are even rarer. Their scarcity is probably due to the tiny nature of these ancient mammals. Competition from the highly successful dinosaurs prevented mammals from reaching sizes much larger than a domestic cat. Most were much smaller.
The first Mesozoic mammal found in Australia was a monotreme (egg-laying mammal) dubbed Steropodon galmani, meaning "flash of lightning tooth". It was found in the opal fields at Lightning Ridge in New South Wales, and has been beautifully opalised. It was probably about the size of a domestic cat, making it one of the largest mammals known from the Mesozoic. Like all of the Australian Mesozoic mammal fossils, it dates to the Early Cretaceous. The fossil consists of a jaw fragment with three teeth, that somewhat resemble the milk teeth of baby platypus.
Also from Lightning Ridge, and also opalised, are the teeth of Kollikodon ritchieri ("bun-like tooth"). It was also a monotreme, which had teeth shaped like hot cross buns. In fact one of the early suggestions for the name of this beast was "hotcrossbunodon". It was perhaps slightly larger than Steropodon. Its teeth seem adapted for crushing, and it may have fed on snails, shellfish or small crustaceans.
Other mammal material is known from the more southerly sites of Victoria. The species Kryoryctes cadburyi is based on an echidna-like humerus from Dinosaur Cove at Cape Otway, dating to around 106 million years ago. The mammal material from the Strzelecki Group of sites is much better preserved, and dates to about 115 million years ago. These include Teinolophos trusleri (another monotreme), the possible placental mammals Ausktribosphenos nyktos and Bishops whitmorei, and a possible multituberculate tooth found during the 2004 season.
When first prepared, Teinolophos ("extended ridge") was thought to be a eupantothere, a group of mammals that is thought to be ancestral to both marsupials and placental mammals. Once the single tooth of the specimen (a lower jaw) had been fully prepared, it turned out to be more similar to Steropodon than to the primitive eupantotheres, indicating that it was another monotreme. The lower jaw is only about one fifth the size of that of Steropodon, indicating a very small monotreme indeed (probably less than 10 cm, or 4 inches, long). Its specific name is in honour of Peter Trusler, the artist responsible for many illustrations and paintings of Victorian dinosaurs (see the Ausktribosphenid images below). As of the 2004 season, at least 10 Teinolophos jaws have been recovered from the Flat Rocks site.
The Ausktribosphenids are a controversial family of mammals known only from the sites in the Strzelecki Group of Victoria. Ausktribosphenos nyktos ("southern Cretaceous tribosphenic nocturnal mammal") was named in 1998, based on a 16 mm (0.6 inches) lower jaw with teeth. Previously, one such jaw had been discovered in 1993, with a further two in 1995, however it was not until 1997 that the holotype was discovered, that was preserved well enough to be officially described and named. Well over a dozen Ausktribosphenid lower jaws are now known, although not all diagnostic to the species level. The form of the mandible and the number and shape of the teeth suggest that Ausktribosphenids are primitive placental mammals, perhaps related to the Erinaceidae, the family that includes hedgehogs.
Other researchers have cast doubt on the placental status of Ausktribosphenids. The most serious objection was that the holotype specimen lacked a mandibular angle, something characteristic of placental mammals. During the 2000 season a new specimen was discovered that has since been named Bishops whitmorei. It was named in honour of Barry Bishops, the former chairman of the National Geographic Society, and Frank Whitmore, a member of the Society's Commitee of Research and Exploration, who helped to fund the Victorian palaeontological work. Bishops is also an Ausktribosphenid, although much better preserved than any of the A.nyktos specimens. It has a total of nine teeth, and clearly shows a mandibular angle. Further study of the original A.nyktos holotype indicates that the area of the jaw where the mandibular angle is located was severely worn, although once it is known what to look for, there seems to be a slight indication of where the structure once was. The Bishops lower jaw type specimen is about 16 mm (0.6 inches) long. Another specimen attributed to B.whitmorei measures about 20 mm (0.8 inches) long. So far at least 7 Bishops jaws have been recovered from Flat Rocks. An even tinier specimen of Ausktribosphenos has also been found, only half the size of A.nyktos. It may also be a new species.
Another theory suggests that southern hemisphere tribosphenic mammals (the name "Australosphenids" has been proposed) may represent a line of evolution leading to monotremes that evolved the tribosphenic condition independently of northern hemisphere Boreosphenids (that include placental mammals). It has been proposed that the Australosphenids are a paraphyletic group that includes Shuotherium, Asfaltomylos, Ambondro, Ausktribosphenos and Bishops. Asfaltomylos patagonicus is from the Canadon Asfalto Formation in Patagonia, South America, dating to the Mid Jurassic. Shuotherium dongi is from the Late Jurassic of China. Ambondro mahabo is from the Middle Jurassic of Madagascar. It would seem that the Australosphenida was proposed due to the fact that tribosphenic teeth were showing up in the Southern Hemisphere, where placentals weren't supposed to be, and during the Jurassic, when they weren't supposed to exist.
When Ausktribosphenos was first described, and argued to be a placental mammal, a theory was put forward to explain how placentals came to appear in the Late Cretaceous in other parts of the world, separated from the Gondwanan countries by ocean barriers in most cases. It was thought that micro-plate rafting could explain their vast sea voyages. Small micro-plates to the northwest of Australia and New Guinea detached from Gondwana some time after the Late Jurassic, "docking" with southeast Asia during the Late Cretaceous. However, if the other tribosphenic mammals included in the Australosphenida were indeed related to the Australian animals, then the micro-plate rafting theory has a problem: one of the Australosphenida (Shuotherium) dates to the Late Jurassic of China.
In 2002 a new mammal was described from China, from the same deposits that the "feathered" dinosaurs were recovered. Eomaia scansori ("dawn mother") is exquisitely preserved, and lived ten million years before Ausktribosphenos or Bishops. It is said to be the most primitive placental mammal known. If the Australian animals are indeed placentals, then they must have spread from Gondwana a lot earlier than micro-plate rafting allows for, at least by the Late Jurassic (if the Chinese Shuotherium is related to the Ausktribosphenids).
Labyrinthodonts were giant amphibians distantly related to modern newts and salamanders. In outward appearance labyrinthodonts looked much like modern salamanders, but with two main differences. One: they had jaws full of sharp teeth. Two: they often grew to enormous sizes.
Labyrinthodonts are named for the pattern of infolding of the dentine and enamel of the teeth, that resembles a maze (or labyrinth). Most species became extinct during the Permian and Triassic periods, even before dinosaurs or mammals evolved in some cases. However a few species managed to hang on in isolated parts of the world, southeastern Australia being one such place. Whereas most labyrinthodonts died off around 250 million years ago at the latest, the Australian animals lived on until the Early Cretaceous, an extra 130 million years. All of the Australian animals belong to the Brachyopoidea; indeed it seems that when Australia was part of Pangea the brachyopoids seem to have originated there, spreading out to other parts of the ancient super-continent. Australia would have been the ancestral home of Brachyopoid labyrinthodonts, and as it turned out they clung on in their ancestral lands long after other labyrinthodonts became extinct elsewhere.
The Australian examples all seem to belong to the Temnospondyls, one of the most diverse and successful orders of labyrinthodonts, which included the brachyopoids. Some temnospondyls were completely aquatic, with external gills much like an axolotl, or "Mexican walking fish". Some others became as terrestrial as reptiles, only returning to the water to breed. Temnospondyls had large heads with advanced hearing abilities similar to frogs, capable of hearing sounds moving through air. Channels in the bone of the skull roof helped them to detect vibrations within the water as well.
There are many species of labyrinthodont known from Australia, but only three that post-date the Triassic Period. Austropelor wadleyi is known only from a fragment of lower jaw, from Jurassic deposits in Queensland. When first found, and published as a Jurassic labyrinthodont, there was such an outcry by other researchers that the deposits were re-dated as Triassic based on the presence of this one bone. It was not until the near-complete Siderops material was found, and after the Austropelor deposits were dated to the Jurassic via pollen analysis, that people began to believe that some labyrinthodonts survived the Triassic period.
Siderops kehli lived in the Early Jurassic of south-central Queensland. It is known from an almost complete skeleton preserved in iron stone (hence the name Siderops: "iron face"). In fact the surrounding matrix was harder than the bones themselves, and the skeleton took quite a long time to be prepared. It measured about 2.5 metres (8 feet) long, with a shovel-shaped head 57 cm (22 inches) long and 67 cm (26 inches) wide. At the time of discovery it was one of the largest known labyrinthodont species. It's teeth were truly impressive, especially by labyrinthodont standards. On the palate a row of about 100 teeth were small towards the back, but rapidly increase in size to 30 mms (1.2 inches) towards the front. Inside these is a continuous row of approximately 80 teeth, a bit bigger than the outer row and projecting beneath them. In this inner row of teeth are three sets of tusks on either side, the largest of which extends almost 70 mm (2.75 inches) below the jaw line. The teeth are shaped like those of other aggressive carnivore species, like monitor lizards. On each side of the tooth a sharp ridge extends towards the tooth tip. When the upper and lower jaws were brought together, the teeth slid past each other like a series of scissor-like blades. The surface of the palate in the area of the palatal bones was covered by tiny 1-2 mm denticles, similar to shark skin. Once Siderops got something into its mouth, there was little hope of escape. With its large head and impressive teeth, but with a relatively weak body, it probably hunted food underwater, and perhaps took land animals (like small dinosaurs) at the water's edge much like a crocodile would. To capture small prey in the water, all it needed to do was to open its huge mouth, and the in-rush of water would have sucked prey straight in. That it had teeth at all tends to indicate that Siderops was not just content on sucking in small prey.
Koolasuchus lower jaw
The Koonwarra site in southeastern Victoria (Strzelecki Group) has yielded exquisitely preserved plant, fish and invertebrate fossils dating to 115-118 million years ago. It also contains the impressions of several feathers (either from birds or theropod dinosaurs). A wide range of insects are known from the site, including wasps, ants, fleas, flies, mayflies, bugs, and water beetles. It is tempting to imagine the fleas infesting the local mammal species, although they may have also bothered birds and "feathered" dinosaurs, or perhaps the flying pterosaurs. Spiders, earthworms, and a horseshoe crab are also known from the deposits. The range of insects is similar to some modern aquatic insect ecosystems, especially that of alpine areas, indicating they were adapted to cold waters. It is thought that these fossil deposits formed when the shallow parts of a lake froze over during the winter, killing fish and insects alike.
The fish are so well preserved that it is possible to count the number of scales on the exposed sides, and to see every single stiffening vein in the fins. They look for all the world like fish that have died and dried out only a matter of days ago. It is more like 42 billion days since they actually died. So far several species have been described from the site. Ceratodus avus was a lungfish, not too different from the kind that are still alive today. The characteristic tooth plates of lungfish are known from several of the Victorian sites. The Palaeoniscoid Coccolepsis woodwardi, the teleosts Koonwarria manifrons and Leptolepis koonwarriensis, and the Archaeomaenid holostean Wadeichthys oxyops have also been described. Remains of a Clupeiform are known, but have not been given a formal name.
|Ginkgoites australis||Leptolepis koonwarriensis|
|© Jim Frazier||© Frank Coffa|
Numerous plant fossils are preserved in the Koonwarra deposits, giving a good indication of the type of landscape surrounding the ancient lake. They include several types of ferns, araucarian and podocarp conifers, liverworts, horsetails, ginkgoes, club mosses, and even the flower and leaves of an angiosperm. Angiosperms (flowering plants) where only just beginning to spread about the world in the Early Cretaceous. This specimen represents one of the oldest flower fossils known from anywhere in the world. Archaefructus from China is older by "only" ten million years.
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