| Casineria | |
|---|---|
 | |
| The counterslab of the holotype fossil | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Eotetrapodiformes |
| Clade: | Elpistostegalia |
| Clade: | Stegocephali |
| Genus: | †Casineria Paton, Smithson & Clack, 1999 |
| Type species | |
| †Casineria kiddi Paton, Smithson & Clack, 1999 | |
Casineria is an extinctgenus oftetrapodomorph which lived about 340–334 million years ago in theMississippian epoch of theCarboniferous period. Itsgeneric name,Casineria, is a latinization of Cheese Bay, the site nearEdinburgh,Scotland, where theholotype fossil was found.[1] When originally described in 1999, it was identified as atransitional fossil noted for its mix of basal (amphibian-like) and advanced (reptile-like) characteristics, putting it at or very near the origin of theamniotes, the group containing allmammals,birds, modern reptiles, and other descendants of their reptile-likecommon ancestor. However, the sole known fossil is lacking key elements such as askull, making exact analysis difficult.[2] As a result, the classification ofCasineria has been more controversial in analyses conducted since 1999. Other proposed affinities include a placement among thelepospondyls,[3]seymouriamorphs,[4] "gephyrostegids",[5] or as a synonym ofCaerorhachis, another controversial tetrapod which may have been an earlytemnospondyl.[6]
In 1992, an amateur fossil collector spotted the remnants of this four-legged creature on the shore of Cheese Bay, Scotland.[7] For the next five years, the fossil languished at theNational Museum of Scotland in Edinburgh while researchers focused on other projects. Consisting of a slab and counterslab (compression fossil) of a single partial skeleton, the fossil was cataloged with the specimen designation NMS G.1993.54.1. Around 1997, work began to expose the remainder of the fossil from the surrounding matrix. The work revealed that the animal probably lived in an environment much drier than previously understood. The findings were first reported in the April 8, 1999 edition ofNature.[1]
Casineria was a small animal with apostcrania length estimated to have been 15 centimeters. Its small size would have made it ideal for hunting the invertebrates of theCarboniferous.[1]
Casineria had five fingers, with the bones at the finger tip being tapering and curved. This spurred the initial describers to argue that it was the oldest known animal with claws on each hand, and marks the earliest clawed foot.[1] Claws are extremely rare among amphibians, but ubiquitous amongamniotes, so their supposed presence inCasineria has been considered to be evidence towards its amniote identity. Claws are also a feature intimately bound to the formation ofkeratinous scales in reptiles, so in life,Casineria would have borne scaly, reptilian-type skin, and would have resembled a small lizard (despite being only distantly related to truelizards).[8]
Under the hypothesis that it was among the first amniotes in the biological sense, it would have laid an amnioticegg not dependent on being laid in water to survive, possibly hiding them in damp vegetation or hollowed out tree stumps. This has been inferred from the fact thatCasineria was found in rocks showing a rather dry environment.[1] In the early Carboniferous period before the appearance ofCasineria, vertebrates were primarily aquatic, only spending part of their time on land.Casineria was believed to be among the first vertebrates to live and reproduce on land.
However, later studies have not consistently placedCasineria as an early amniote.[3][4][6] Even the presence of claws has been considered doubtful, as Marjanovic & Laurin (2019) noted that the finger tips were squared-off, rather than pointed.[5]
While retaining a general build like those found in the amphibianreptiliomorph groups likeSeymouriamorpha andDiadectomorpha,Casineria also shows features that tie it in with early reptiles, notably a generally gracile build with light leg bones, unfused ankles and toes terminating in claws. This would enable the animal to use their feet actively in traction, rather than asholdfasts, an indication of a primarily terrestrial lifestyle.[1] These traits have been argued to show that it was more closely related toamniotes than to other knownreptiliomorph amphibians.
With its advanced features,Casineria may have been one of the first trueamniotes, and thus the firstreptile under traditional classification. Inphylogenetic parlance it would have been astem amniote, close to, but outside thecrown group Amniota (the group consisting of the last common ancestor ofsynapsids andsauropsids and all its descendants).Casineria pushes back the origin of amniote lineages much earlier than was previously assumed.[1] However, like with much of the basal tetrapod stock, the phylogenetic position is uncertain, and it has also been suggestedCasineria is alepospondyl,[3]seymouriamorph,[4] synonymous withCaerorhachis (which is possibly a basaltemnospondyl amphibian),[6] or part of a grade of small tetrapods traditionally considered the familyGephyrostegidae.[5]
{{cite web}}: CS1 maint: archived copy as title (link)
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