
Bluff Downs fossil site

The Bluff Downs fossil site is a paleontological site of Pliocene age in northern Queensland, Australia, and is the most significant fossil site of the Pliocene age in Australia. The fossil site lies on the banks of Allingham Creek on the pastoral property of Bluff Downs Station.
Geology.
Volcanic Activity and Nulla Basalt Province.
Bluff Downs Fossil Site is located within the Nulla Basalt Province, one of four late Cenozoic basalt provinces identified in 1956. The Nulla Basalt Province is dated to the late Pliocene (3.6 to 2.58 million years ago) or early Pleistocene (2.58 – 0.8 million years ago). and consists of multiple olivine basalt flows from lava flows associated with four periods of volcanic activity which occurred 4.5-4.0, 2.3, 1.3 and 1.1 million years ago (according to radiogenic argon determination methods of dating). The dating of the flows matches the ordering suggested by analysis of aerial photographs. These flows of olivine basalt lavas are located on the eastern flank of the Great Diving Range, and overlie Palaeozoic rock formations as well as sediments from the early Cenozoic, all of which have been partially weathered due to a humid and tropical palaeoenvironment. The most recent volcanic activity in the region occurred 13, 000 years ago, according to sediments underlying the youngest of flow, the Toomba Flow. These flows are relatively thin and were similar to flows which can be observed in present-day Hawai’i. The Bluff Downs Flow directly overlies the fossiliferous sediment (known as the Allingham Formation) in which the Bluff Downs Local Fauna are found, and helped to protect the fossils from erosion
The Allingham Formation.
The Allingham Formation, named by Archer and Wade in 1976, is the section of the Nulla Basalt Province which contains the fossils which the Bluff Downs Local Fauna are attributed to. It consists of a mixture of sediment that originated on land and was washed away after eroding into nearby waterbodies (terrigenous sediment), clays, silts, sands (including calcareous sands), and Chara limestones (calcareous nodules that were deposited directly over the fossiliferous sediment and consequently overlain by basalt). These sediments were formed in lakes and rivers (i.e. are lacustrine and fluviatile), indicating the presence of various water bodies such as lakes, rivers and streams in the palaeoenvironment at the time of deposition. There were several different depositional events and analysis of the sediments suggests that during the early Pliocene, a stream widened to form a shallow lake.
Fossils.
Many vertebrate fossils have been found in the terrigenous sediments of Bluff Downs Fossil Site, including both broken and complete bones and skulls, though articulated skeletons (that is, with the bones in the same position as upon the organism’s death) are rare. The Bluff Downs Local Fauna is derived from the vertebrate species found in these fossils.
Dating.
Precise dating of vertebrate fossil sites in Australia is rare, and many cenozoic-age sediments remain undated. Unusually, the Bluff Downs Local Fauna have been specifically dated. This is because a basalt layer (the Bluff Downs Flow) directly overlies the fossiliferous deposit, allowing a minimum age to be calculated. This basalt flow has been dated to 3.62 ±0.5 million years old; the fossils having been deposited between 5.2 and 2.6 million years ago, during the late Pliocene period. The Allingham Formation has been radiometrically dated to no less than 4 ±0.12 million years old, placing it in the early Pliocene.
Bluff Downs Local Fauna.
The Bluff Downs Local Fauna, named and described by Archer in 1976, includes numerous vertebrate species found at the Bluff Downs Fossil Site, many of which are similar to but slightly older than the Chinchilla fauna (associated with Chinchilla Fossil Site, also in Queensland), according to more ancestral physical features. This collection of vertebrate species was noted to be considerably biodiverse by Archer, and features many ancestors of the species we now recognise as uniquely Australian, as well as unusual extinct species of megafauna, such as Diprotodonts and Thylacoleonids.
The Bluff Downs Local Fauna includes several large reptilian predators, such as crocodiles and a giant varanid (monitor lizard), which has long puzzled palaeontologists as no large predatory terrestrial mammals have been found at the site.
The fauna include species such as the:
Palaeoenvironment.
The palaeoenvironment of Bluff Downs during the Pliocene featured large bodies of water. This is inferred from the presence of fluviatile and lacustrine sedimentary deposits, as well as the presence of certain species from the site including pygmy geese and darters, short-necked turtles and long-necked tortoises, which suggest shallow, turbid lagoons were a feature of the prehistoric landscape. The presence of crocodiles and tortoises indicates that these water bodies were at the very least seasonal, as these animals migrate for water. The current understanding is that many of these water bodies were permanent features.
There may have also been a river-based, or riparian, rainforest as species including a land snail and ringtail possum suggest that there were at least patches of closed forests present.
The palaeoenvironment as a whole has been compared to present-day Kakadu, with permanent water bodies, patches of forest and an overall high level of precipitation and humidity, features central to Kakadu’s geography, and the species found at the site mirror this.