''Kalthifrons'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

monospecific In biology, a monotypic taxon is a taxonomic group (taxon) that contains only one immediately subordinate taxon. A monotypic species is one that does not include subspecies or smaller, infraspecific taxa. In the case of genera, the term "unispec ...

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

mekosuchine Mekosuchinae is an extinct clade of Crocodilia, crocodilians from the Cenozoic of Australasia. They first appear in the fossil record in the Eocene in Australia, and survived until the arrival of humans: in the Pleistocene in Australia and within ...

crocodylian Crocodilia (or Crocodylia, both ) is an order of mostly large, predatory, semiaquatic reptiles, known as crocodilians. They first appeared 95 million years ago in the Late Cretaceous period (Cenomanian stage) and are the closest living ...

known from the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Tirari Formation in

Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...

. It contains a single species, ''Kalthifrons aurivellensis''.

Discovery and naming

The remains of ''Kalthifrons'' were discovered on the western shores of Lake Palankarinna in

South Australia South Australia (commonly abbreviated as SA) is a state in the southern central part of Australia. It covers some of the most arid parts of the country. With a total land area of , it is the fourth-largest of Australia's states and territories ...

Lake Eyre Basin The Lake Eyre basin ( ) is a drainage basin that covers just under one-sixth of all Australia. It is the largest endorheic basin in Australia and amongst the largest in the world, covering about , including much of inland Queensland, large porti ...

. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

(specimen ''SAM P35062''), a cranium and two mandibular rami, was specifically recovered from the Mampuwordu Sand Member of the

Pliocene The Pliocene ( ; also Pleiocene) is the epoch in the geologic time scale that extends from 5.333 million to 2.58Tirari Formation. It was found upside down with most of the ventral surface and both mandibular elements being in poor condition. The Golden Fleece Locality, where the fossil was discovered, also yielded a variety of isolated teeth, osteoderms and vertebrae remains, none of which however can be confidently assigned to ''Kalthifrons''. The generic name derives from the

Dieri The Diyari (), alternatively transcribed as Dieri (), is an Indigenous Australian group of the South Australian desert originating in and around the delta of Cooper Creek to the east of Lake Eyre. Language Diyari is classified as one of the Ka ...

word for "spear" (kalthi) and fons, meaning forehead, a name chosen in reference to the animal's elongated frontal process. The species epithet is a combination of the Latin "aurum" for gold and "vellus" meaning fleece, chosen to reflect the locality where the fossil has been discovered.

Description

''Kalthifrons'' possessed a small skull, however still larger than that of the smallest members of

Mekosuchinae Mekosuchinae is an extinct clade of crocodilians from the Cenozoic of Australasia. They first appear in the fossil record in the Eocene in Australia, and survived until the arrival of humans: in the Pleistocene in Australia and within the Holocene ...

such as ''

Volia ''Volia'' is an extinct monospecific genus of mekosuchine crocodylians from Fiji named in 2002.; ; 2002: An extinct Pleistocene endemic mekosuchine crocodylian from Fiji. ''Journal of vertebrate paleontology'', 22: 612–628. It was around ...

'', ''

Trilophosuchus ''Trilophosuchus'' ("Triple Crest Crocodile") is an extinct genus of mekosuchine crocodilian from Australia. Unlike living crocodilians, it is hypothesized to have been terrestrial. ''Trilophosuchus'' was approximately in length. It had a short ...

'' and ''

Mekosuchus ''Mekosuchus'' is a genus of extinct Australasian crocodiles within the subfamily Mekosuchinae. They are believed to have been made extinct by the arrival of humans on the South Pacific islands where they lived. The species of this genus were s ...

'' itself. It's relatively short and broad with a roughly triangular shape and platyrostral (however crushing of the fossil makes it hard to determine to which degree), suggesting a semi-aquatic generalist lifestyle. The skull was at most moderately deep with premaxilla that are narrower than those of most other mekosuchines. The external nares are elongated, longer than wide and almost entirely enclosed by the premaxillae safe for a small area at their posterior margin where the nasals contribute. At their posterior end the premaxillae form a triangular process that extends between the nasals and the maxilla. The maxilla possesses a large dorsal swelling similar to other crocodilians above the enlarged 5th maxillary tooth position. The exact number of teeth is hard to determine as the ventral surface was heavily damaged prior to burial, but based on the remains each maxilla held at least 11 teeth, possibly more, while each premaxilla held between 4 and 5 teeth. At the anterior end of the maxilla these teeth are closely spaced, possibly too closely for the presence of reception pits. Preservation however prevents any definitive statements on the matter. A large, open notch is present at the anterior end of the maxilla at the transition to the premaxillae to receive the 4th dentary tooth when the jaws are closed. This differs from ''

Australosuchus ''Australosuchus'' is an extinct monospecific genus of crocodylian belonging to the subfamily Mekosuchinae. The type and only known species ''Australosuchus clarkae'' lived during the Late Oligocene and the Early Miocene of southern Australia. ...

'' with its semi-enclosed notch. A second notch is present between the 6th and 7th maxillary tooth, which overall suggests that the dentition of ''Kalthifrons'' was at least partly interlocking. The nasal bone extends up to the external nares anteriorly and is split by the frontal process at its posterior end. The lacrimals are elongated and irregular triangular in shape with thick preorbital ridges reaching from the anterior end of the lacrimals to the anterior end of the orbits. These ridges are the result of a distinct change in slope, unlike in the

Saltwater Crocodile The saltwater crocodile (''Crocodylus porosus'') is a crocodilian native to saltwater habitats and brackish wetlands from India's east coast across Southeast Asia and the Sundaic region to northern Australia and Micronesia. It has been listed ...

, which has preorbital ridges raised above the rest of the skull. In general the ridges seen on ''Kalthifrons'' resemble more those of '' Baru wickeni''. The prefrontals form convex lateral projections as part of the orbital margin. The orbits are located more anteriorly compared to ''Australosuchus'' with pointed anterior margins. The supratemporal fenestrae are small, but largely undisturbed by the overhanging bone of the trapezoid skull table. The namegiving feature of ''Kalthifrons'' is the peculiar frontal bone, which can be separated into two main areas. The posterior end of the frontal is broad and makes up the intraorbital space. Meanwhile, the anterior end forms a long anterior process that extends far down the rostrum and makes up 64% of the frontal. This process is relatively smooth for most of its dorsal surface, but is covered in rugose ornamentation on its posterior-most centimeter which carries over to the rest of the bone. Unlike in other mekosuchines such as ''Baru'', the supraoccipital does not contribute much to the skull roof and is dorsally restricted to a small roughly semilunar sliver at the posterior margin of the parietal. The mandible is poorly preserved and provides little information. Notably however, the broad and flat ventral surface is offset from the lateral surface by a right angled bend along the ventrolateral margin, a condition also seen in referred mandibles of ''

Quinkana ''Quinkana'' is an extinct genus of mekosuchine crocodylians that lived in Australia from about 28 million to about 10,000 years ago. Most attributed specimens have been found in Queensland. It is speculated to have been one of the top predators ...

''.

Phylogeny

Poor preservation and incongruent nature of morphological features renders it difficult to place ''Kalthifrons'' within

. Although its placement is weak, Yates and Pledge (2016) rule out the possibility that ''Kalthifrons'' could belong to any other

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

of crocodilian, rendering a position as a mekosuchine as the most probable scenario. In their analysis ''Kalthifrons position varied greatly between trees, with its uncertain nature creating a large

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...

at the base of the group that is Mekosuchinae minus ''

'' (which was recovered as a

sister taxon In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and t ...

to all other taxa). Some trees recovered ''Kalthifrons'' as an early diverging member positioned outside the clade ''

Kambara ''Kambara'' is an extinct genus of mekosuchine crocodylian that lived during the Eocene epoch in Australia. Description At around 55 million years old, remains of ''Kambara'' are among the oldest Tertiary fossils found in Australia (although th ...

'' + all other mekosuchines, while others found it as a derived member of Mekosuchini (''

Baru ''Baru'' is an extinct genus of Australian mekosuchine crocodilian. It was semi-aquatic, around 4 m (13 ft) in length. Being semi-aquatic its habitat was around fresh pools of water in wet forests, ambushing their prey, much like moder ...

'', ''

'' and the dwarf mekosuchines). The strict consensus tree of 566 most parsimonious trees recovered by Yates and Pledge is depicted below.

Paleobiology

Based on the platyrostral, short and triangular skull ''Kalthifrons'' was most likely a semi-aquatic generalist predator living in the waterways of what is now South Australia. The specific locality where ''Kalthifrons'' was found also preserves a variety of other crocodilian remains, mostly osteoderms and teeth as well as the remains of turtles ('' Elseya sp. cf. E. lavarockorum''). Furthermore a maxilla assigned to ''Quinkana'' has been found in the Woodard Quarry of the Mampuwordu Sand Member. The combination of disassociated remains of freshwater turtles and crocodilians and a thin layer of

selenite Selenite may refer to: Substances containing selenium *A selenium-containing anion or ionic compound with the SeO32− anion: **Selenite (ion), anion is a selenium oxoanion with the chemical formula SeO32− ***Selenous acid, the conjugate acid, w ...

underlying the sand have been interpreted as a sign that the Golden Fleece Locality represents a waterhole that dried up sometime during the Late Pliocene, causing the mass death of the animals inhabiting the waters including ''Kalthifrons''. During the late

Neogene The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...

mekosuchine diversity experienced a significant drop with many taxa going extinct in mainland Australia, the survivors including the large ''Paludirex'' and ''Quinkana'' as well as smaller species that survived elsewhere on islands. Around the same time the first members of the genus ''

Crocodylus ''Crocodylus'' is a genus of true crocodiles in the family Crocodylidae. Taxonomy The generic name, ''Crocodylus'', was proposed by Josephus Nicolaus Laurenti in 1768. ''Crocodylus'' contains 13–14 extant (living) species and 5 extinct species ...

'' arrived in Australia, with fossil evidence indicating they appeared in Australia during the Pliocene, probably having migrated there via the Malay archipelago. Yates notes that the overlying Pompapillina Member of the Tirari Formation preserves the bones of an undescribed, generalist member of ''Crocodylus'', but no mekosuchine remains have been found. The Pompapillina Member dates to the Late Pliocene, between 3.9 to 3.4 Ma, which means the discovery of ''Crocodylus'' remains could be the first direct evidence of the faunal turnover from the endemic mekosuchines to crocodylines. However there is no evidence that the extinction of ''Kalthifrons'' was directly caused by competition with invasive crocodylines and it is equally possible that the later simply colonized newly vacant niches after cooling climate and aridification drove the previously present generalists to extinction.

References

{{taxonbar, from=Q109546662 Mekosuchinae Prehistoric pseudosuchian genera