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Koolasuchus; Temporal range: Early Cretaceous (Barremian-Aptian), 125–120 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
	Holotype mandibles
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Order:	†Temnospondyli
Suborder:	†Stereospondyli
Family:	†Chigutisauridae
Genus:	†Koolasuchus; Warren et al., 1997
Type species
	†Koolasuchus cleelandi; Warren et al., 1997

Koolasuchus is an extinct genus of brachyopoid temnospondyl in the family Chigutisauridae. Fossils have been found from Victoria, Australia and date back 125-120 million years ago to Barremian-Aptian stages of the Early Cretaceous. Koolasuchus is the youngest known temnospondyl. It is known from several fragments of the skull and other bones such as vertebrae, ribs, and pectoral elements. The type species Koolasuchus cleelandi was named in 1997. K. cleelandi was adopted as the fossil emblem for the state of Victoria, Australia on 13 January 2022.^[1]

History

The first fossil of temnospondyls found in the Strzelecki Group was NMV-PI56988, the posterior fragment of a jaw, collected around 1980. The jaw fragment was first mentioned in a 1986 publication by Anne Warren and R. Jupp, who did not definitively identify it as that of a temnospondyl due to the Cretaceous age of the specimen, much younger than any other known temnospondyl specimen at the time.^[2] In 1991, additional remains were reported including NMV-PI86040, an intercentrum (part of the vertebra) and NMV-PI86101, an isolated skull roof bone, likely representing either a frontal, a supratemporal or a parietal. The intercentrum unquestionably confirmed that temnospondyls were present in the Strzelecki Group. The morphology of the skull roof bone lead to the authors suggesting that the temnospondyl was either a member of Plagiosauridae or Brachyopoidea.^[3]

Koolasuchus was named in 1997 from the Aptian aged Wonthaggi Formation of Strzelecki Group in Victoria.^[4] It is known from four fragments of the lower jaw and several postcranial bones, including ribs, vertebrae, a fibula, and parts of the pectoral girdle. A jawbone was found in 1978 in a fossil site known as the Punch Bowl near the town of San Remo. Later specimens were found in 1989 on the nearby Rowell's Beach. A partial skull is also known but has not been fully prepared. Koolasuchus was named for the palaeontologist Lesley Kool. The name is also a pun on the word "cool" in reference to the cold climate of its environment.^[5] The type species K. cleelandi is named after geologist Mike Cleeland.^[6]

Description

Koolasuchus was a large, aquatic temnospondyl, measuring up to 3 metres (9.8 ft) in length and weighing up to 500 kilograms (1,100 lb).^[7]^[8]^[9] Like other chigutisaurids, it had a wide, rounded head and tabular horns projecting from the back of the skull.^[10] Although represented by incomplete material, the skull was likely 65 centimetres (26 in) long.^[11]

Koolasuchus is distinguished from other temnospondyls aside from Siderops and Hadrokkosaurus by having the ramus of the mandible "articular is excluded from the dorsal surface of the postglenoid area by a suture between the surangular and the prearticular", and is distinguished from those two taxa by a lack of coronoid teeth.^[4]

Paleobiology

Koolasuchus inhabited rift valleys in southern Australia during the Early Cretaceous. During this time the area was below the Antarctic Circle, and temperatures were relatively cool for the Mesozoic. Based on the coarse-grained rocks in which remains were found, Koolasuchus likely lived in fast-moving streams. As a large aquatic predator, it was similar in lifestyle to crocodilians. Although eusuchians and kin were common during the Early Cretaceous, they were absent from southern Australia 120 million years ago, possibly because of the cold climate. By 110 Mya, represented by rocks in the Dinosaur Cove fossil locality, temperatures had warmed and crocodilians had returned to the area. These crocodilians likely displaced Koolasuchus, leading to its disappearance in younger rocks.^[5]

References

^ Government of Victoria (13 January 2022). "Proclamations" (PDF). Victoria Government Gazette. p. G42. Retrieved 15 January 2022.
^ Jupp, R.; Warren, A. A. (1986-01-01). "The mandibles of the Triassic temnospondyl amphibians". Alcheringa: An Australasian Journal of Palaeontology. 10 (2): 99–124. doi:10.1080/03115518608619164. ISSN 0311-5518.
^ Warren, A.A.; Kool, L.; Cleeland, M.; Rich, T.H.; Rich, P. Vickers (January 1991). "An Early Cretaceous labyrinthodont". Alcheringa: An Australasian Journal of Palaeontology. 15 (4): 327–332. doi:10.1080/03115519108619027. ISSN 0311-5518.
^ ^a ^b Warren, A.A.; Rich, P.V.; Rich, T.H. (1997). "The last, last labyrinthodonts?". Palaeontographica A. 247 (1–4): 1–24. doi:10.1127/pala/247/1997/1. S2CID 247068275.
^ ^a ^b Rich, T.H.V.; Rich, P.V. (2000). Dinosaurs of Darkness. Bloomington: Indiana University Press. p. 222. ISBN 978-0-253-33773-3.
^ "Life in the Shadows, Non-reptilian life in Mesozoic Australia". geocities. Archived from the original on 2008-02-20. Retrieved 2008-08-09.
^ Kear, Benjamin P.; Hamilton-Bruce, Robert J. (2011-04-19). Dinosaurs in Australia: Mesozoic Life from the Southern Continent. Csiro Publishing. ISBN 978-0-643-10231-6.
^ Martin, A.J. (2009). "Dinosaur burrows in the Otway Group (Albian) of Victoria, Australia, and their relation to Cretaceous polar environments" (PDF). Cretaceous Research. 30 (2009): 1223–1237. doi:10.1016/j.cretres.2009.06.003. Archived from the original (PDF) on 2011-07-19. Retrieved 2011-04-20.
^ Hart, Lachlan J.; Gee, Bryan M.; Smith, Patrick M.; McCurry, Matthew R. (2023-08-03). "A new chigutisaurid (Brachyopoidea, Temnospondyli) with soft tissue preservation from the Triassic Sydney Basin, New South Wales, Australia". Journal of Vertebrate Paleontology. doi:10.1080/02724634.2023.2232829. ISSN 0272-4634.
^ Warren, A.; Marsicano, C. (2000). "A phylogeny of the Brachyopoidea (Temnospondyli, Stereospondyli)". Journal of Vertebrate Paleontology. 20 (3): 462–483. doi:10.1671/0272-4634(2000)020[0462:APOTBT]2.0.CO;2. hdl:11336/93649. S2CID 86107783.
^ Steyer, J.S.; Damiani, R. (2005). "A giant brachyopoid temnospondyl from the Upper Triassic or Lower Jurassic of Lesotho". Bulletin de la Société Géologique de France. 176 (3): 243–248. doi:10.2113/176.3.243.