Aenigmaspina

Aenigmaspina; Temporal range: Late Triassic,; ~ Rhaetian PreꞒ Ꞓ O S D C P T J K Pg N
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Pseudosuchia
Family:	† Ornithosuchidae (?)
Genus:	† Aenigmaspina ; Patrick et al. 2019
Species:	†A. pantyffynnonensis
Binomial name
	†Aenigmaspina pantyffynnonensis; Patrick et al. 2019

Last updated September 02, 2023

Aenigmaspina (from Latin aenigma and spina, meaning "enigmatic spine") is an extinct genus of enigmatic pseudosuchian (=crurotarsan) archosaur from the Late Triassic of the United Kingdom. Its fossils are known from the Pant-y-ffynnon Quarry in South Wales, of which its type and only known species is named after, A. pantyffynnonensis. Aenigmaspina is characterised by the unusual spines on its vertebrae, which are broad and flat on top with a unique 'Y' shape. Although parts of its skeleton is relatively well known, the affinities of Aenigmaspina to other pseudosuchians are unclear, although it is possibly related to families Ornithosuchidae, Erpetosuchidae or Gracilisuchidae.^[1]

Description

Aenigmaspina was a small (<1 m long) archosaur with a slender skeleton and build. It is mostly known from the front half of its body, including its vertebral column, ribs, shoulder and parts of the forelimb, as well as possibly pieces of skull and pelvis that may belong to it.^[1]

The vertebrae of Aenigmaspina are its most distinctive feature. These are characterised by their spine tables, where the tops of the neural spines split into a broad 'V' shape with a deep groove between them. In the cervicals of the neck, each leaf of the table is angled up at 45° and forms a roughly rectangular shape from above, while those of the dorsals are flatter and the table is more shield-shaped, with a straight front edge and pointed at the back. These 'Y'-shaped neural spines are only found in the cervicals and first four dorsals, behind them the neural spines are only slightly expanded at their tips and became more slender down the spine.

Nine osteoderms are known, forming a single row running down the spine that split into two long pointed spines at the back with smooth, thin surfaces. The osteoderms of other archosaurs are usually found in a paired row, and the symmetrical shape with two pointed spines (as well as furrows and notches on the midline of some) suggests they are actually fused pairs of osteoderms. The osteoderms also sport keels on their underside that lock neatly into the furrow in their vertebrae, which could suggest the osteoderms were restricted to the neck and front-most section of the back as the 'Y'-shaped spine tables are not found past the 4th dorsal vertebra. This would be unusual, but not completely unprecedented as some pseudosuchians (particularly ornithosuchids) are known to have only had osteoderms in front of the hips, although not to the extent in Aenigmaspina.^[1]

The scapula is very long and slender with little curve along its length, suggesting a tall, narrow body shape. The glenoid (shoulder joint) faces down and slightly back, suggesting upright forelimbs. The humerus and ulna are both very long and slender, and the ulna is noticeably longer than the humerus. They are similarly proportioned to those of the contemporary long-legged crocodylomorph Terrestrisuchus but differs in having a rounded deltopectoral crest at the very top of the humerus. A pelvis that may belong to Aenigmaspina is lightly built, with slender pubes and ischia and an ilium with a long, pointed backwards process and deep hip socket (acetabulum).^[1]

If the few known skull elements are correctly referred to Aenigmaspina, the parietal bone suggests that the roof of its skull may have been flat and roughly textured.^[1]

History of discovery

Aenigmaspina was collected from Pant-y-ffynnon quarry in South Wales, a quarry of Carboniferous limestone that contains fissures filled with Triassic-Early Jurassic aged sediment, from which Aenigmaspina and other Triassic-aged reptiles are known from. The exact age of the fissures is uncertain, but a latest Triassic Rhaetian age has been suggested based on biostratigraphy between the Pant-y-ffynnon fissures and other similar sites.^[2] The specimens were collected by palaeontologists Kenneth Kermack and Pamela Robinson of University College London between 1951 and 1952, along with four other species of reptile, and were first presented at a talk in 1953 and later written on briefly in 1956 where the specimen was informally nicknamed 'Edgar'.^[3] The specimen would eventually be fully described and formally named as part of a thesis by Erin Patrick and published in 2019.^[1]

At least one individual of Aenigmaspina is definitively known. This individual, 'Edgar', consists of a pair of split blocks (the holotype NHMUK P9/3a) that together contain osteoderms, vertebrae, ribs and a scapula clustered tightly together, as well as more pieces from the forelimb and additional vertebrae that likely belong to this individual. The specimen was found tightly curled up, possibly because the animal died in a burrow before being preserved. This specimen was CT scanned to examine details of the bones more closely without risking further damage to the fossil. Other isolated pieces, including pieces of skull and a pelvis, may belong to Aenigmaspina, however, because they are labelled similarly to fossils of other species from Pant-y-ffynnon their identity cannot be confirmed in isolation. Nonetheless, they do show similarities in both size and form (i.e. slender limbed), although some must come from at least one other individual based on duplicated bones from the forelimb.^[1]

A single vertebra from a quarry in Cromhall in South Gloucestershire, England closely matches the cervicals of Aenigmaspina, including what appears to be the 'Y'-shaped spine table.^[1]

The generic name, from the Latin aenigma for "enigma" or "puzzle" and spina for "spine", was chosen to refer to the difficulty of identifying the fossil pieces belonging to it and its uncertain evolutionary relationships, and also for its uniquely characteristic spine tables on its vertebrae. The species name is from the Pant-y-ffynnon (Welsh 'spring in a hollow') quarry, where it was found. The specimens of Aenigmaspina are all stored in the Natural History Museum, London, UK.^[1]

Classification

The affinities of Aenigmaspina to other archosaurs is, as the name suggests, enigmatic. It has been identified as a crurotarsan (pseudosuchian) archosaur from its short cervicals, short cervical ribs, and broad spine tables with associated osteoderms, as opposed to an avemetatarsalian. However, it does not preserve any unique traits (autapomorphies) that would ally it with any known major group of pseudosuchians. Despite this, it has been determined to unlikely be a phytosaur, aetosaur, 'rauisuchian' or a crocodylomorph, as it either shares no traits with them or is much smaller than would be expected for these forms.^[1]

Possible relatives of Aenigmaspina could be Erpetosuchus , Ornithosuchus and Gracilisuchus .

Its relationships were tested using two phylogenetic analyses, one based on Ezcurra (2016)^[4] and another from Ezcurra et al. (2017),^[5] however this resulted in similarly unclear results, and only showed weak affinities to the basal pseudosuchian Ornithosuchidae in the former and surprisingly to the non-archosaur archosauromorph Proterochampsia in the latter. Indeed, even after the trees were modified by removing unstable taxa, Aenigmaspina remained paired with the proterochampsian Rhadinosuchus despite being removed from all other proterochampsians and allied with Ornithosuchidae (shown in the cladogram below).^[1]

Ornithosuchidae

	Rhadinosuchus

	Aenigmaspina

Ornithosuchus

	Riojasuchus

	Venaticosuchus

A relationship amongst proterochampsians is unlikely, as the only similarity they share is a rough skull roof, and proterochampsians lack the characteristic spine tables and often have much thicker, sculpted osteoderms. The relationship to ornithosuchids is more plausible, as they share some characteristic features, but not all. However, it is also equally plausible that Aenigmaspina is related erpetosuchids, a group of small pseudosuchians with similarly enigmatic and variable affinities, as they both share broad spine tables, although none are as prominently 'Y'-shaped like in Aenigmaspina. Erpetosuchid osteoderms are also typically thicker and more ornamented than the thin, smooth ones of Aenigmaspina. Gracilisuchids are another small group it could belong to, however they can only be compared to by their osteoderms and vertebrae, both of which differ from Aenigmaspina (particularly the neural spines, which are uniquely trapezoid-shaped in gracilisuchids, unlike the rectangular spines of Aenigmaspina). However, osteoderm texture is known to vary throughout Pseudosuchia, so this trait may not be significant. The relationships of Aenigmaspina are likely to remain unresolved until more fossils (particularly from the skull) are found.^[1]

Palaeoecology

In the Late Triassic, the Pant-y-ffynnon Quarry was part of an ancient island as part of a palaeo-archipelago across southern Wales and England to Bristol. The island was a forested karstic environment, riddled with fissures and caves in the limestone, to which long limbed, agile reptiles like Aenigmaspina may have been well suited for. Other archosaurs that coexisted with Aenigmaspina were the small, fast-running predatory crocodylomorph Terrestrisuchus , the herbivorous sauropodomorph dinosaur Pantydraco , and the coelophysoid theropod Pendraig .^[6] Rhynchocephalians (relatives of modern tuataras) are abundant, known from at least three species including Clevosaurus cambrica , Diphydontosaurus and one or two unnamed species.^[1]^[2]

Related Research Articles

<i>Gracilisuchus</i> Genus of fossil reptiles

Gracilisuchus is an extinct genus of tiny pseudosuchian from the Late Triassic of Argentina. It contains a single species, G. stipanicicorum, which is placed in the clade Suchia, close to the ancestry of crocodylomorphs. Both the genus and the species were first described by Alfred Romer in 1972.

<i>Riojasuchus</i> Extinct genus of reptiles

Riojasuchus is an extinct genus of ornithosuchid archosaur from the Late Triassic (Norian) of Argentina. Ornithosuchidae was a widespread family of facultatively bipedal pseudosuchians with adaptations for scavenging. Riojasuchus is notable as one of the youngest and most complete members of the family. The type and only known species, Riojasuchus tenuisceps, was named and described by José Bonaparte in 1967. It was one of the first of many well-preserved Triassic archosaurs to be discovered in Argentina. The holotype specimen, PVL 3827, was found in the Los Colorados Formation of the Ischigualasto-Villa Unión Basin in northwestern Argentina.

<i>Terrestrisuchus</i> Genus of terrestrial crocodylomorph

Terrestrisuchus is an extinct genus of very small early crocodylomorph that was about 76 centimetres (30 in) long. Fossils have been found in Wales and Southern England and date from near the very end of the Late Triassic during the Rhaetian, and it is known by type and only known species T. gracilis. Terrestrisuchus was a long-legged, active predator that lived entirely on land, unlike modern crocodilians. It inhabited a chain of tropical, low-lying islands that made up southern Britain, along with similarly small-sized dinosaurs and abundant rhynchocephalians. Numerous fossils of Terrestrisuchus are known from fissures in limestone karst which made up the islands it lived on, which formed caverns and sinkholes that preserved the remains of Terrestrisuchus and other island-living reptiles.

Hesperosuchus is an extinct genus of crocodylomorph reptile that contains a single species, Hesperosuchus agilis. Remains of this pseudosuchian have been found in Late Triassic (Carnian) strata from Arizona and New Mexico. Because of similarities in skull and neck anatomy and the presence of hollow bones Hesperosuchus was formerly thought to be an ancestor of later carnosaurian dinosaurs, but based on more recent findings and research it is now known to be more closely related to crocodilians rather than dinosaurs.

<span class="mw-page-title-main">Ornithosuchidae</span> Extinct family of reptiles

Ornithosuchidae is an extinct family of pseudosuchian archosaurs from the Triassic period. Ornithosuchids were quadrupedal and facultatively bipedal, meaning that they had the ability to walk on two legs for short periods of time. They had distinctive, downturned snouts, unique, "crocodile-reversed" ankle bones, and several other features that distinguish them from other archosaurs. Ornithosuchids were geographically widespread during the Carnian and Norian stages of the Late Triassic with members known from Argentina, Brazil, and the United Kingdom. Four genera, comprising Ornithosuchus, Venaticosuchus, Dynamosuchus, and Riojasuchus are presently known. The family was first erected by German paleontologist Friedrich von Huene in 1908.

Turfanosuchus is a genus of archosauriform reptile, likely a gracilisuchid archosaur, which lived during the Middle Triassic (Anisian) of northwestern China. The type species, T. dabanensis, was described by C.C. Young in 1973, based on a partially complete but disarticulated fossil skeleton found in the Kelamayi Formation of the Turfan Basin.

Yarasuchus is an extinct genus of avemetatarsalian archosaur that lived during the Anisian stage of the Middle Triassic of India. The genus was named and described in 2005 from a collection of disarticulated but fairly complete fossil material found from the Middle Triassic Yerrapalli Formation. The material is thought to be from two individuals, possibly three, with one being much more complete and articulated than the other. The type and only species is Y. deccanensis. Yarasuchus was a quadruped roughly 2–2.5 metres (6.6–8.2 ft) long, with an elongated neck and tall spines on its vertebrae. Unlike other quadrupedal Triassic reptiles, the limbs and shoulders of Yarasuchus were slender, and more like those of ornithodirans.

<i>Chanaresuchus</i> Extinct genus of reptiles

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References

1 2 3 4 5 6 7 8 9 10 11 12 13 Patrick, Erin L.; Whiteside, David I.; Benton, Michael J. (2019). "A new crurotarsan archosaur from the Late Triassic of South Wales" (PDF). Journal of Vertebrate Paleontology . 39 (3): e1645147. doi:10.1080/02724634.2019.1645147. S2CID 202848499. Archived from the original (PDF) on 30 August 2019.
1 2 Keeble, Emily; Whitestide, David I.; Benton, Michael J. (2018). "The terrestrial fauna of the Late Triassic Pant-y-ffynnon Quarry fissures, South Wales, UK and a new species of Clevosaurus (Lepidosauria: Rhynchocephalia)" (PDF). Proceedings of the Geologists' Association . 129 (2): 99–119. doi:10.1016/j.pgeola.2017.11.001. hdl:1983/5afdc677-3ea0-4519-813d-6052ef8370ec.
↑ Kermack, K. A. (1956). "An ancestral crocodile from South Wales" (PDF). Proceedings of the Linnean Society of London . 166: 1–2. doi:10.1111/j.1095-8312.1956.tb00747.x.
↑ Ezcurra, Martín D. (2016-04-28). "The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms". PeerJ. 4: e1778. doi: 10.7717/peerj.1778 . ISSN 2167-8359. PMC 4860341 . PMID 27162705.
↑ Martín D. Ezcurra; Lucas E. Fiorelli; Agustín G. Martinelli; Sebastián Rocher; M. Belén von Baczko; Miguel Ezpeleta; Jeremías R. A. Taborda; E. Martín Hechenleitner; M. Jimena Trotteyn; Julia B. Desojo (2017). "Deep faunistic turnovers preceded the rise of dinosaurs in southwestern Pangaea". Nature Ecology & Evolution. 1 (10): 1477–1483. doi:10.1038/s41559-017-0305-5. PMID 29185518. S2CID 10007967.
↑ Spiekman, S. N.; Ezcurra, M. D.; Butler, R. J.; Fraser, N. C.; Maidment, S. C. (2021). "Pendraig milnerae, a new small-sized coelophysoid theropod from the Late Triassic of Wales". Royal Society Open Science . 8 (10): Article ID 210915. doi: 10.1098/rsos.210915 . PMC 8493203 . PMID 34754500.

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[Edgar-1] 1 2 3 4 5 6 7 8 9 10 11 12 13 Patrick, Erin L.; Whiteside, David I.; Benton, Michael J. (2019). "A new crurotarsan archosaur from the Late Triassic of South Wales" (PDF). Journal of Vertebrate Paleontology . 39 (3): e1645147. doi:10.1080/02724634.2019.1645147. S2CID 202848499. Archived from the original (PDF) on 30 August 2019.

[Clevo-2] 1 2 Keeble, Emily; Whitestide, David I.; Benton, Michael J. (2018). "The terrestrial fauna of the Late Triassic Pant-y-ffynnon Quarry fissures, South Wales, UK and a new species of Clevosaurus (Lepidosauria: Rhynchocephalia)" (PDF). Proceedings of the Geologists' Association . 129 (2): 99–119. doi:10.1016/j.pgeola.2017.11.001. hdl:1983/5afdc677-3ea0-4519-813d-6052ef8370ec.

[3] Kermack, K. A. (1956). "An ancestral crocodile from South Wales" (PDF). Proceedings of the Linnean Society of London . 166: 1–2. doi:10.1111/j.1095-8312.1956.tb00747.x.

[4] Ezcurra, Martín D. (2016-04-28). "The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms". PeerJ. 4: e1778. doi: 10.7717/peerj.1778 . ISSN 2167-8359. PMC 4860341 . PMID 27162705.

[5] Martín D. Ezcurra; Lucas E. Fiorelli; Agustín G. Martinelli; Sebastián Rocher; M. Belén von Baczko; Miguel Ezpeleta; Jeremías R. A. Taborda; E. Martín Hechenleitner; M. Jimena Trotteyn; Julia B. Desojo (2017). "Deep faunistic turnovers preceded the rise of dinosaurs in southwestern Pangaea". Nature Ecology & Evolution. 1 (10): 1477–1483. doi:10.1038/s41559-017-0305-5. PMID 29185518. S2CID 10007967.

[6] Spiekman, S. N.; Ezcurra, M. D.; Butler, R. J.; Fraser, N. C.; Maidment, S. C. (2021). "Pendraig milnerae, a new small-sized coelophysoid theropod from the Late Triassic of Wales". Royal Society Open Science . 8 (10): Article ID 210915. doi: 10.1098/rsos.210915 . PMC 8493203 . PMID 34754500.

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