Haplocanthosaurus Priscus

''Haplocanthosaurus'' (meaning "simple spined lizard") is a genus of intermediate sauropod dinosaur. Two species, ''H. delfsi'' and ''H. priscus'', are known from incomplete fossil skeletons. It lived during the late Jurassic period (Kimmeridgian stage), 155 to 152 million years ago.Turner, C.E. and Peterson, F., (1999). "Biostratigraphy of dinosaurs in the Upper Jurassic Morrison Formation of the Western Interior, U.S.A." Pp. 77–114 in Gillette, D.D. (ed.), ''Vertebrate Paleontology in Utah''. Utah Geological Survey Miscellaneous Publication 99-1. The type species is ''H. priscus'', and the referred species ''H. delfsi'' was discovered by a young college student named Edwin Delfs in Colorado, United States. ''Haplocanthosaurus'' specimens have been found in the very lowest layer of the Morrison Formation, along with '' Hesperosaurus mjosi'', '' Brontosaurus yahnahpin'', and '' Allosaurus jimmadseni''.Foster, J. (2007). ''Jurassic West: The Dinosaurs of the Morrison Formation and Their World''. Indiana University Press. 389pp.

Discovery

There are four known specimens of ''Haplocanthosaurus'': one of ''H. delfsi'', and three of ''H. priscus''. Of these, the type of ''H. delfsi'' is the only one complete enough to mount. The mounted specimen of ''H. delfsi'' now stands in the Cleveland Museum of Natural History, albeit with a completely speculative replica skull, as the actual skull was not recovered. Present in stratigraphic zones 1, 2, and 4.Foster, J. (2007). "Appendix." ''Jurassic West: The Dinosaurs of the Morrison Formation and Their World''. Indiana University Press. pp. 327-329. Recently described specimens from a different region of the Morrison Formation were assigned to ''Haplocanthosaurus'' in 2014. The study describing them noted that ''Haplocanthosaurus'' is known for certain from at least four specimens, assigned to ''H. priscus'' (CM 572), ''H. utterbacki'' (=''H. priscus''; CM 879), ''H. delfsi'' (CMNH 10380), and ''H.'' sp. (MWC 8028). Up to seven additional specimens have been assigned to ''Haplocanthosaurus''? or Haplocanthosauridae indet. One potential specimen has been nicknamed "Big Monty," for its discovery in Montana. It's been claimed to measure an incredible long. However, much controversy surrounds the specimen and, as such, little is truly known about it.

Description

''Haplocanthosaurus'' was one of the smallest sauropods of the Morrison. While some Morrison sauropods could reach lengths of over 20 meters (or over 66 feet), ''Haplocanthosaurus'' was smaller, reaching a total length of 14.8 meters (49 feet) and an estimated weight of .

''Haplocanthosaurus'' is known from many elements, mostly of vertebra. In the middle and cervical caudals of ''Apatosaurus'', ''Camarasaurus'', ''Cetiosaurus'' and ''Haplocanthosaurus'', the intraprezygapophyseal lamina is separate from the root of the neural canal by a vertical midline lamina. In the last few caudals and the most cranial dorsals, the lateral edge of the prezygapophyseal lamina becomes widened and roughened. Hatcher (1901) interpreted this as forming the attachment area for the muscles from which the scapular blade was suspended.Weishample, D.B.; Dodson, P.; Osmolska, H. (2004). "The Dinosauria: Second Edition". ''Berkeley, University of California Press''. 2: 266, 281-286, 288, 296-299, 302-302.

The dorsoventrally elongate oval outlines are characteristic of ''Haplocanthosaurus'' with only ''Camarasaurus'' also possessing them. The parapophyses remaining as oval facets on the craniolateral margin, and the sacral spines 1-3 fused are also found in both ''Haplocanthosaurus'' and ''Camarasaurus''.

The ''Cetiosaurus'' specimen OUMNH J13695 has a low horizontal ridge on each of its lateral surfaces, creating a slightly subhexagonal transverse cross-section, and that feature is also seen on ''Cetiosauriscus'', the anterior caudals of ''Haplocanthosaurus'', and caudals 15-30 of ''Omeisaurus''. Also, the area around the periphery of each articular face is flattened, creating a ‘bevelled’ appearance, and also occurs in ''Haplocanthosaurus'' and ''Cetiosauriscus''.

Distinguishing characteristics

''Haplocanthosaurus'' is distinguished by dorsal vertebra lacking cranial centrodiapophyseal laminae. Also, it is distinguished by elongate intrapostzygapophyseal laminae, dorsoventrally directed dorsal transverse processes that approach the height of the neural spines, and the distal end of the scapular blade being dorsally and ventrally expanded.

Classification

''Haplocanthosaurus priscus'' was originally named ''Haplocanthus priscus'' by John Bell Hatcher in 1903. Soon after his original description, Hatcher came to believe the name ''Haplocanthus'' had already been used for a genus of acanthodian fish (''Haplacanthus'', named by Louis Agassiz in 1845), and was thus preoccupied. Hatcher re-classified his sauropod later in 1903, giving it the new name ''Haplocanthosaurus''. However, the name was not technically preoccupied at all, since there was a variation in spelling: the fish was named ''Haplacanthus'', not ''Haplocanthus''. While ''Haplocanthus'' technically remained the valid name for this dinosaur, Hatcher's mistake was not noticed until many years after the name ''Haplocanthosaurus'' had become fixed in scientific literature. When the mistake was finally discovered, a petition was sent to the ICZN (the body which governs scientific names in zoology), which officially discarded the name ''Haplocanthus'' and declared ''Haplocanthosaurus'' the official name (ICZN Opinion #1633).

Originally described as a " cetiosaurid", José Bonaparte decided in 1999 that ''Haplocanthosaurus'' differed enough from other sauropods to warrant its own family, the Haplocanthosauridae.Bonaparte, J. F. (1999). "An armoured sauropod from the Aptian of northern Patagonia, Argentina." In Tomida, Y., Rich, T. H. & Vickers-Rich, P. (eds.), 1999.'' Proceedings of the Second Gondwanan Dinosaur Symposium'', National Science Museum Monographs #15, Tokyo: 1-12.

Phylogenetic studies have failed to clarify the exact relationships of ''Haplocanthosaurus'' with any certainty. Studies have variously found it to be more primitive than the neosauropods, a primitive macronarian (related to the ancestor of more advanced forms such as ''Camarasaurus'' and the brachiosaurids), or a very primitive diplodocoid, more closely related to ''Diplodocus'' than to titanosaurs, but more primitive than rebbachisaurids.

In 2005, Darren Naish and Mike Taylor reviewed the various proposed positions of ''Haplocanthosaurus'' in their study of diplodocoid phylogeny. They found it could be a non-neosauropod eusauropod, a basal macronarian, or a basal diplodocoid. In 2011, an analysis by Whitlock recovered Haplocanthosaurus as the basalmost member of the Diplodocoidea, the third potentiality of Taylor & Naish. In 2015, a specimen-level phylogenetic analysis was published, finding ''Haplocanthosaurus'' to be a confirmed diplodocoid, either very basal, or more derived than rebbachisaurids. Their implied weighting cladogram is shown below.

References

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Diplodocoids
Late Jurassic dinosaurs of North America
Dinosaurs of the Morrison Formation
Paleontology in Colorado
Taxa named by John Bell Hatcher
Fossil taxa described in 1903