| Hypselorhachis Temporal range:Middle Triassic | |
|---|---|
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| Skeletal reconstruction and hypothetical size comparison ofCtenosauriscus (left) andHypselorhachis (right). Known remains shown in white | |
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| Close-up of the holotype | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Archosauria |
| Clade: | Pseudosuchia |
| Clade: | Paracrocodylomorpha |
| Clade: | †Poposauroidea |
| Family: | †Ctenosauriscidae |
| Genus: | †Hypselorhachis Butleret al.,2009 |
| Species | |
Hypselorhachis is agenus of extinctreptile, possibly a ctenosauriscidarchosaur related toCtenosauriscus. It lived during theTriassic Period. It is currently known only from a singlevertebra found from theMiddle TriassicManda Beds inTanzania. 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.
Thetype species isH. mirabilis, mentioned but never fully described by EnglishpaleontologistAlan J. Charig.Hypselorhachis was assigned to theCtenosauriscidae, a group of sail-backed archosaurs, in 1988.[1] It was formally described byRichard J. Butler and co-workers in 2009.[2] 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 theAnisian stage. The only characteristic that diagnoses the genus is a feature seen in theprezygapophysis - 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 theneural spine of the holotype is elongate, being over five times the height of thecentrum,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 asArizonasaurus andCtenosauriscus possessed characteristically largesails that were formed from elongate neural spines, and the shape of the neural spine is also similar. InHypselorhachis, 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. Theneural arch is very solid and quite compact, and the neural canal rather thin, so thespinal cord would have been relatively well protected.[2]
Other features of the vertebra include multiplefossae 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 assaurischian dinosaurs,pterosaurs,Erythrosuchus,Postosuchus andArizonasaurus. The fact thatHypselorhachis shares them withArizonasaurus 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 ofArizonasaurus is strong evidence to suggest thatHypselorhachis was indeed a ctenosauriscid, or at least a relative of the main ctenosauriscid group.[2]
