Lower Cretaceous Marine Invertebrates

Australia: The Land Where Time Began

Lower Cretaceous Marine Invertebrates

The fossil record for the macroinvertebrates of Australia is said by the authors¹ to be documented very well. In the Nanutarra (Nanutarra?) Formation that outcrops in the Ashburton River region, Carnarvon Basin of northwestern Western Australia contains assemblages from the Berriasian-Barremian age. The fauna recovered from this deposit includes bivalves, gastropods, belemnites, that are squid-like animals, and corals, Actinastraea. In this assemblage some of the most commonly found taxa are the bivalves Camptonectes and Trigonia and Hibolithes, a belemnopsid belemnite. These taxa represent lineages from the Jurassic that are believed to have been well-established in the Australian region since the early part of the Mesozoic.

The macroinvertebrate assemblages became extremely diverse after the epicontinental environments underwent such a massive expansion during the Aptian. In the Bulldog Shale of South Australia and the Wallumbilla Formation in Queensland-northern New South Wales, that were laterally equivalent, more than 63 taxa of benthic molluscs have been found. These included gastropods, bivalves and scaphopods, as well as cephalopods, echinoderms - brittle stars, starfish, crinoids and sea urchins, decapod crustaceans, brachiopods (lamp shells) and sponges. A warm-water origin has been inferred for the lineages that are mostly representatives of forms that were widespread or globally distributed. These included most of the identified ammonites - Australiceras, Sanmartinoceras, Tropaeum, bivalves - such as Eyrena, Laevitrigonia, Maccoyella, Panopea, Teredo, gastropods such as Euspira, and scaphopods such as Dentalium.

At the genus and species level the fauna is almost completely endemic, among which are unique bivalves such as Cyrenopsis, Fissilunula, Pseudavicula, Tatella and Tancretella. The authors¹ suggest that this implies that after immigrant taxa became established in the epicontinental seaway of Australia speciation occurred rapidly, possibly as a result of geographic isolation and the onset of a cooling phase.

In units such as the Toolebuc Formation and Allaru Mudstone assemblages of macroinvertebrates from the Albian underwent a reduction in diversity to about 43 taxa when compared to the higher diversity of the Aptian. Particularly benthic molluscs were restricted to a few that were abundant and widely distributed, such as the bivalve Inoceramus from the Cretaceous. There was also a reduction of faunal endemism when compared to the levels of the Aptian. The taxa that are widespread in both northern and southern high latitudes, as is the case with the bivalve taxa Anopaea, and crustaceans such as Hoploparia and Torynomma and echinoderms, where diagnostic, are mostly forms that are cosmopolitan. In the Toolebuc Formation and Allaru Mudstone, of upper Albian age, the high proportion of epifaunal bivalves is suggested by the authors¹ to indicate seafloor sediments that were poorly oxygenated.

Ammonites, nautiloids, belemnites and vampyromorph squid were pelagic macroinvertebrates from the Albian. The particularly diverse ammonites include a mixture of cosmopolitan forms such as Anisoceras, Beudanticeras, Hamites and Scaphites and southern endemics such as Labeceras and Myloceras. The latter are most dominant in the marine environments of the margins of the Australia continent such as the Bathurst Island Group of the Money Shoals Platform in the Northern Territory, that The authors¹ suggest appear to have been derived from contemporary European assemblages, as well as other parts of the Northern Hemisphere. The authors¹ also suggest a northern origin for the nautiloids Cimomia and Kummelonautilus and The vampyromorphs Boreopeltis and Trachyteuthis. The authors¹ suggest that the endemism of the assemblages of ammonites and belemnites from the epicontinental seaway of Australia appear to have evolved as a result of the increasing level of isolation experienced in the epicontinental basin. There was a successful radiation from the epicontinental seaway of Australia where they evolved, to the coastal shelf environments of the adjacent parts of Gondwana of the labeceratid ammonites - Labeceras and Myloceras and the dimitobelid belemnites - Dimitobelus.