The Pachyrhizodus grawi specimen 'Morree' on display at Kronosaurus Korner.
Etymology
The scientific name for fish is Osteichthyes - meaning ‘bone fish’ in Greek. This refers to the largely bony skeletons of osteichthyans (as opposed to the cartilaginous skeletons of sharks).
Relationships
Most of osteichthyan fossils found in Richmond belong to actinopterygian teleost fish. Actinopterygians are known as ray-finned fish due to the presence of lepidotrichia (thin bony segments) within their fins. Most modern osteichthyans are members of the Actinopterygii, with only a small proportion – lungfishes and coelacanths belonging to the lobed-finned Sarcopterygii (this group also contains tetrapods). The only known sarcopterygian fossils from Richmond include isolated lungfish tooth plates recovered from the Mackunda Formation (these specimens likely came from freshwater creatures washed into the Eromanga Sea).
The first actinopterygians, sometimes referred to as palaeonisciformes, evolved over 400 million years ago during the Late Silurian. Following a mass extinction event 250 million years ago at the end of the Permian, a newly evolved actinopterygian group known as the Neopterygii rose to dominance. Neopterygians (meaning ‘new fins’) differed from their earlier predecessors by evolving faster, more manoeuvrable bodies. By the Late Triassic/Early Jurassic, a group of neopterygians known as the Teleostei developed movable mouthparts enabling them to extend their jaws outwards – a successful adaptation for feeding.
Several different types of teleosts lived in the Eromanga Sea, including primitive forms such as the pachycormiform Australopachycormus and the aspidorhynchiform Richmondichthys. They were accompanied by several elopiform genera including Pachyrhizodus, Marathonichthys and Flinderishchtys and the osteoglossomorph ichthyodectid Cooyoo. Members of the Elopiformes and Osteoglossomorpha are still alive today.
Many undescribed fossil teleosts are also known from Richmond. The relationships of these forms are yet to be established.
Modern elopiforms like the tarpon (left) are distantly related to extinct elopiforms like Pachyrhizodus grawi (right) from the Eromanga Sea (reconstruction of P. grawi modified from Bartholomai 2012).
Discoveries
The first teleost fossil discovered from Northwest Queensland was reported by British palaeontologist Mr Robert Etheridge in 1872. This heavily scaled form was attributed to the genus Aspidorhynchus until the 1890s when it was reassigned as a new species of Belonostomus ― B. sweeti by Dr Robert Logan Jack and Mr Robert Etheridge Jnr. The subsequent discovery of better preserved specimens enabled Dr Alan Bartholomai to discern significant differences between this form and Belonostomus, resulting in the erection a new genus Richmondichthys (named after the town of Richmond).
Mr Robert Etheridge Jnr. also described a different fossil teleost from Marathon Station in 1905. Based on a heavily weathered skull, this specimen was assigned to a new species of Ichthyodectes ― I. marathonensis. This placement within Ichthyodectes was challenged by Dr David Bardack who reassigned the form to Pachyrhizodus marathonensis. Research published in 2012 by Dr Alan Bartholomai described a second species of Pachyrhizodus from Richmond named P. grawi. Due to the efforts Mr Gary and Mrs Barb Flewelling, several complete specimens of P. grawi have subsequently been found and displayed at Kronosaurus Korner.
The renowned British palaeontologist Sir Arthur Smith Woodward also described fossil teleost remains from Richmond in 1894. This material, consisting of an incomplete set of ichthyodectid jaws, was collected from by Dr Jack from Clutha Station. Sir Woodward described this specimen as a new species of Portheus ― P. australis. This genus was later rendered a junior synonym of the Xiphactinus, the largest known osteichthyan. The discovery of better preserved ichthyodectid specimens from Richmond allowed Ms Tempe Lees and Dr Bartholomai to reassign Woodward’s form to the new taxon Cooyoo australis.
The largest semi-articulated Cooyoo specimen was discovered by Mr Gary and Mrs Barb Flewelling and Mr Anthony Saffioti in 2011 at Richmond’s Free Fossil Hunting Site 1. The 2.5 metre long specimen nicknamed ‘Wandah’ is currently on display at Kronosaurus Korner.
The Cooyoo specimen 'Wandah' - note the use of Mr Gary Flewelling for scale.
Description
Of the Elopiformes from the Eromanga Sea, the two species of Pachyrhizodus have tuna-like fusiform (spool-shaped) bodies that are less than one metre in length. The robust P. marathonensis has large jaws and teeth while the more gracile P. grawi has relatively large orbits, a pinched snout and slender teeth.
Flindersichthys superficially resembles a giant grouper - measuring up to 1.4 metres in length. Its large jaws feature an underbite and are lined with thousands of small comb-like teeth.
Stretching between 2.5-3 metres in length, Cooyoo represents the largest elopiform from the Eromanga Sea. It has a blunt bulldog-like skull adorned with a large crest. Its robust jaws feature a pronounced underbite and are lined anteriorly with large conical (cone-shaped) teeth. Its elongate body ends in a homocercal (symmetrical), forked caudal fin.
The pachycormiform Australopachycormus has a long rostrum (snout) giving it the superficial appearance of a swordfish (although both forms are unrelated). Its jaws were lined with forward-facing, arrow-shaped teeth that vary in size. Its streamlined body has highly modified paired fins – the large pectoral fins have a jagged saw-like leading edge while the pelvic fins were probably ribbon-like structures.
The aspidorhynchiform Richmondichthys has a narrow toothless skull. Its elongate, 1.8-metre-long body is covered in bands of deep scales that superficially resembles the armour of armadillos. These protective scales are coated in a shiny mineralised tissue known as ganoine.
Left to right: reconstructions of Cooyoo australis (modified from modified from Lees and Bartholomai, 1987) and Richmondichthys sweeti (modified from Bartholomai, 2004).
Palaeobiology
Pachyrhizodus may have had a similar lifestyle to modern tuna. Its fast-moving body was well adapted for chasing small fish and cephalopods in the open water.
In contrast, Richmondichthys was probably slow moving, filter feeder – swimming through swarms of plankton with its toothless jaws agape.
It is thought that the large jaws of Flindersichthys enabled it to swallow small-bodied prey whole.
Australopachycormus was probably a fast-moving predator which used its long pointed rostrum and pectoral fins to kill small prey. Wear on its cutting teeth suggest that Australopachycormus bit through the hard scales of other fish or ammonite shells.
It is also likely that Cooyoo was predator – using its strong forked tail to quickly pursue prey. Its large conical teeth were well-adapted for catching and holding the slippery bodies of other fish.
Fossil evidence suggests that Cooyoo may have fallen prey to larger predators from the Eromanga Sea. Ms Lovisa Wretman and Dr Benjamin Kear posited that potential bite marks on a specimen at Kronosaurus Korner match the size and spacing for teeth of a polycotylid plesiosaur, pterosaur or Platypterygius. The lack of bone growth around this area suggests that the Cooyoo specimen died as a result of its wounds.
Cooyoo specimen KK F0457 with possible bite marks from a large predator. This specimen was discovered by Mr David Goodman and Mr Wayne Rhodes in 2008 from Richmond.
References
Bartholomai, A. 2004. The large aspidorhynchid fish Richmondichthys sweeti (Etheridge Jr and Smith Woodward 1891) from Albian marine deposits of Queensland Australia. Memoirs of the Queensland Museum 49: 521–536.
Bartholomai, A. 2010. Revision of Flindersichthys denmeadi Longman 1932, a marine teleost from the Lower Cretaceous of the Great Artesian Basin, Queensland. Memoirs of the Queensland Museum – Nature 55: 43-68.
Bartholomai, A. 2012. The pachyrhizodontid teleosts from the marine Lower Cretaceous (latest mid to Late Albian) sediments of the Eromanga Basin, Queensland, Australia. Memoirs of the Queensland Museum - Nature 56: 119-48.
Bartholomai, A. 2013. New teleosts (Elopomorpha: Albuliformes) from the. Lower Cretaceous (Late Albian) of the Eromanga. Basin, Queensland, Australia. Memoirs of the Queensland Museum – Nature 58: 73-94.
Kear, B. P. 2007. First record of a pachycormid fish (Actinopterygii: Pachycormiformes) from the Lower Cretaceous of Australia. Journal of Vertebrate Paleontology 27: 1033-1038.
Lees, T. and Bartholomai, A. 1987. Study of a Lower Cretaceous Actinopterygian (Class Pisces) Cooyoo australis from Queensland, Australia. Memoirs of the Queensland Museum 25: 177–192.
Wretman, L. and Kear, B. P. 2013. An Australasian Journal of Palaeontology (2013): Bite marks on an ichthyodectiform fish from Australia: possible evidence of trophic interaction in an Early Cretaceous marine ecosystem. Alcheringa 38: 170-176.