Hypselorhachis mirabilis
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''Hypselorhachis'' is a
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 ...
of extinct
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
, possibly a ctenosauriscid
archosaur Archosauria () is a clade of diapsids, with birds and crocodilians as the only living representatives. Archosaurs are broadly classified as reptiles, in the cladistic sense of the term which includes birds. Extinct archosaurs include non-avian d ...
related to ''
Ctenosauriscus ''Ctenosauriscus'' is an extinct genus of sail-backed poposauroid archosaur from Early Triassic deposits of Lower Saxony in northern Germany. It gives its name to the family Ctenosauriscidae, which includes other sail-backed poposauroids such as ...
''. It lived during the
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
Period. It is currently known only from a single
vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...
found from the
Middle Triassic In the geologic timescale, the Middle Triassic is the second of three epochs of the Triassic period or the middle of three series in which the Triassic system is divided in chronostratigraphy. The Middle Triassic spans the time between Ma and ...
Manda Beds in
Tanzania Tanzania (; ), officially the United Republic of Tanzania ( sw, Jamhuri ya Muungano wa Tanzania), is a country in East Africa within the African Great Lakes region. It borders Uganda to the north; Kenya to the northeast; Comoro Islands and ...
. The vertebra is preserved in reasonably good condition, as although the tall neural spine is chipped in several places it is not broken despite being quite slender, only around 20 mm thick transversely. The
type species In zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the species that contains the biological type specimen ...
is ''H. mirabilis'', mentioned but never fully described by English
paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...
Alan J. Charig Alan Jack Charig (1 July 1927 – 15 July 1997) was an English palaeontologist and writer who popularised his subject on television and in books at the start of the wave of interest in dinosaurs in the 1970s. Charig was, though, first and fo ...
. ''Hypselorhachis'' was assigned to the
Ctenosauriscidae Ctenosauriscidae is an extinct family (biology), family of pseudosuchian archosaurs within the clade Poposauroidea. Ctenosauriscids existed in Africa, Asia, Europe and North America during the Early Triassic to the Middle Triassic period (latest ...
, a group of sail-backed archosaurs, in 1988.Carroll, R.L. (1988). ''Vertebrate Paleontology and Evolution''. W.H. Freeman and Company It was formally described by
Richard J. Butler Richard James Butler is a vertebrate palaeontologist at the University of Birmingham, where he holds the title of professor of palaeobiology. His research focuses on ornithischian dinosaur evolution, dinosaur origins, and fossil tetrapod macroevo ...
and co-workers in 2009. The name means 'wonderful high spine', from the Latin 'mirabilis' 'wonderful' and the Greek 'ὑπσελος', 'high' and 'ῥαχις' 'spine' or 'backbone'. ''Hypselorhachis'' was probably at least 3 metres long, maybe up to 4 or 5 metres, as the vertebra is certainly from quite a large animal. ''Hypselorhachis'' is known from a single anterior dorsal vertebra found from the Lifua Member of the Manda Beds, which is thought to have been deposited during the
Anisian In the geologic timescale, the Anisian is the lower stage or earliest age of the Middle Triassic series or epoch and lasted from million years ago until million years ago. The Anisian Age succeeds the Olenekian Age (part of the Lower Triassic Ep ...
stage. The only characteristic that diagnoses the genus is a feature seen in the
prezygapophysis The articular processes or zygapophyses (Greek ζυγον = "yoke" (because it links two vertebrae) + απο = "away" + φυσις = "process") of a vertebra are projections of the vertebra that serve the purpose of fitting with an adjacent vertebr ...
- a small part of the bone projects dorsally. Because of the lack of any other material, comparisons between it and other early archosaurs can only be based on features seen in the vertebra, making any current phylogenetic classification tentative. Because the neural spine of the holotype is elongate, being over five times the height of the
centrum (Latin for ''center'') may refer to: Places In Greenland * Nuuk Centrum, a district of Nuuk, Greenland * Centrum Lake, Greenland In the Netherlands * Amsterdam-Centrum, the inner-most borough of Amsterdam, Netherlands * Rotterdam Centrum, a borou ...
, ''Hypselorhachis'' may be a ctenosauriscid. The neural spine is around 305 mm high, or possibly greater than this as some of the end may be missing, while the centrum is only around 60 mm high even at its tallest points. As is visible on the picture, the centrum has deeply concave lower, left and right surfaces. Other ctenosauriscids such as ''
Arizonasaurus ''Arizonasaurus'' was a ctenosauriscid archosaur from the Middle Triassic (243 million years ago). ''Arizonasaurus'' is found in the Middle Triassic Moenkopi Formation of northern Arizona. A fairly complete skeleton was found in 2002 by Sterling ...
'' and ''
Ctenosauriscus ''Ctenosauriscus'' is an extinct genus of sail-backed poposauroid archosaur from Early Triassic deposits of Lower Saxony in northern Germany. It gives its name to the family Ctenosauriscidae, which includes other sail-backed poposauroids such as ...
'' possessed characteristically large sails that were formed from elongate neural spines, and the shape of the neural spine is also similar. In ''Hypselorhachis,'' as in the ctenosauriscids mentioned, the neural spine is wider at the distal end than at the proximal end, and if, as seems certain, it did have a full sail the sail would have been quite tough due to mostly being transversely compressed bone with little space in between. The neural arch is very solid and quite compact, and the neural canal rather thin, so the
spinal cord The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column (backbone). The backbone encloses the central canal of the spi ...
would have been relatively well protected. Other features of the vertebra include multiple fossae or pits around the neural canal, especially just above it on the posterior side of the vertebra. These were present in many archosaurs and archosauromorphs, such as saurischian dinosaurs,
pterosaur Pterosaurs (; from Greek ''pteron'' and ''sauros'', meaning "wing lizard") is an extinct clade of flying reptiles in the order, Pterosauria. They existed during most of the Mesozoic: from the Late Triassic to the end of the Cretaceous (228 to ...
s, ''
Erythrosuchus ''Erythrosuchus'' (from el, ἐρυθρός , 'red' and el, σοῦχος , 'crocodile') is an extinct genus of archosauriform reptile from the Triassic of South Africa. Remains have been found from the Cynognathus Assemblage Zone of the Beaufo ...
,
Postosuchus ''Postosuchus'', meaning "Crocodile from Post", is an extinct genus of rauisuchid reptiles comprising two species, ''P. kirkpatricki'' and ''P. alisonae'', that lived in what is now North America during the Late Triassic. ''Postosuchus'' is a me ...
'' and ''Arizonasaurus.'' The fact that ''Hypselorhachis'' shares them with ''Arizonasaurus'' may also be further evidence of its being closely related - although on their own they were so common that they would not suggest this, the presence of both the deep fossae and the high neural spine resembling that of ''Arizonasaurus'' is strong evidence to suggest that ''Hypselorhachis'' was indeed a ctenosauriscid, or at least a relative of the main ctenosauriscid group.


References


External links


Discussion of "Hypselorhachis" on the DML
{{Taxonbar, from=Q3787996 Ctenosauriscids Triassic archosaurs Anisian life Middle Triassic reptiles of Africa Triassic Tanzania Fossils of Tanzania Fossil taxa described in 2009 Prehistoric pseudosuchian genera