Ostafrikasaurus
Ostafrikasaurus | |
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Illustrated holotype tooth, with British penny for scale | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Dinosauria |
Clade: | Saurischia |
Clade: | Theropoda |
Family: | †Spinosauridae (?) |
Genus: | †Ostafrikasaurus Buffetaut, 2012
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Species: | †O. crassiserratus
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Binomial name | |
†Ostafrikasaurus crassiserratus Buffetaut, 2012
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Ostafrikasaurus is a
Ostafrikasaurus has been tentatively estimated at 8.4 metres (28 feet) long and weighing 1.15 tonnes (1.27 short tons; 1.13 long tons). The holotype tooth is 46 millimetres (1.8 inches) long, has a curved front edge, and is oval-shaped in cross section. The tooth shows serrations that—for spinosaur standards—are unusually large, more so than in any other known taxon. Both the front and back cutting edges are serrated, with two to four denticles per mm (0.04 in). The tooth also has longitudinal ridges on both sides, and the outermost enamel layer has a wrinkled texture in the regions between and without ridges.
Among the oldest known spinosaurid fossils, Ostafrikasaurus may be of importance in understanding the evolutionary origins of spinosaurids and their anatomical
History of research
During the time of the
In 2000, American palaeontologists James Madsen and Samuel Welles referred the L.? stechowi teeth to
In 2011, German palaeontologist Oliver Rauhut considered the Middle Dinosaur Member teeth ascribed to L.? stechowi as lacking diagnostic
Rauhut also found that Janensch's type a (MB R 1084) was distinct in form from the other eight teeth, and possibly represented a different taxon closely related to C.? stechowi. He listed some differences between it and the other teeth originally referred to L.? stechowi: MB R 1084 has more lingual ridges (up to eleven) and three ridges and grooves on the labial side, which faced the outside of the mouth. Moreover, some of MB R 1084's ridges are confined to the base of the crown, joined by longer ridges extending throughout almost the whole length of the crown, with a 5 mm (0.20 in) high region at the tip of the tooth lacking any ornamentation. Additionally, ridges are present over almost the entire front three-fifths of the crown, whereas the rear two-fifths are smooth. Towards the front of the tooth, the ridged part is separated from the carina (cutting edge) by an area that is slightly concave front to back. The only similarities between MB R 1084 and the Middle Dinosaur Member teeth lie in their general shape and density of their serrations, as all teeth have 10 denticles per 5 millimetres (0.20 in) on the rear carina and 13 denticles per 5 millimetres (0.20 in) on the front carina.[11]
In a
Buffetaut elaborated on the differences between the teeth of Ostafrikasaurus crassiserratus and Janensch's L.? stechowi morphotypes. Morphotype b (MB R 1083 and 1087) teeth had both front to back and side to side curvature, and a D-shaped cross section. Morphotype c (MB R 1090) was curved side to side but not front to back, was not flattened from side to side, had a rounded front with no carina, and bore five strong ridges on its lingual side, but none on its labial flank. Morphotype e (MB R 1092) resembled a typical theropod tooth. It is strongly flattened from side to side, curved from front to back, shows 3 denticles per millimetre (0.04 in), its front carina does not extend to the base of the crown, and there is no ornamentation aside from some weak furrowing of the crown, and two incipient ridges on the lingual side.[4]
In 2020, a study of teeth from the Tendaguru Formation and from the Late Jurassic of South America again suggested a ceratosaurid identification for Ostafrikasaurus.[12]
Description
In 2016, Spanish palaeontologists Molina-Pérez and Larramendi estimated Ostafrikasaurus at about 8.4 metres (28 feet) long, 2.1 m (6.9 ft) tall at the hips and weighing 1.15 tonnes (1.27 short tons; 1.13 long tons).[13] However, without more complete material, such as a skull or body fossil, the body size and weight of fragmentary spinosaur taxa, especially those known only from teeth, can not be reliably calculated. Thus estimates are only tentative.[14]
The holotype tooth is thick, somewhat flattened sideways, and 46 millimetres (1.8 inches) in length from top to bottom. Its tip has been rounded by erosion and the base is not fully preserved. The tooth crown has well-defined carinae (cutting edges), with the front carina being curved and the back carina almost straight. There is only mild side-to-side curvature. Both carinae are serrated, with rounded denticles perpendicular to the edge of the tooth. The serrations have no inderdenticle sulci, or grooves, in between them. They line the front carina from the base to its tip, and probably also did on the back carina, whose base has been largely eroded. Towards the tip of the tooth, these serrations are very worn down (especially on the front carina). On the front carina, there are two denticles per mm (0.04 in) near the tip of the tooth, and three to four per mm (0.04 in) as the denticles shrink towards the base of the crown. There are two denticles per mm (0.04 in) all along the back carina. The serrations are notably larger than in all other known spinosaurids.[4]
The enamel (outermost layer) of the tooth bears a series of ridges on its surface – 10 on the lingual side, and four fainter, less extensive ones on the labial side. The gaps between ridges are 1 mm (0.039 in) wide at most. None of the ridges on either side reach the tip of the crown. There is a 3 mm (0.12 in) wide region at the front of the tooth on both sides that lacks ridges; a similar area on the rear of the tooth side diminishes in width, from 8 to 4 mm (0.31 to 0.16 in) as it approaches the tip of the crown. On both sides of the tooth, between the ridges and ridge-less parts of the teeth, the enamel surface is finely wrinkled.[4]
Classification
Spinosaurids are usually separated into two
Rauhut doubted this interpretation in 2011, stating that MB R 1084 has more similarities than differences with Ceratosaurus? stechowi teeth, like the rounded cross section, only marginal curvature of the crown, a more convex lingual than labial side, and similar size and shape of the denticles. Thus, according to his analysis, only the ridge count and distribution were left as unique to MB R 1084. Rauhut noted that though baryonychines also have ridges on either side of their teeth, they are usually most developed at the rear of the tooth, whereas MB R 1084 lacks ridges on that side. He also asserts that the wrinkling Buffetaut observed in MB R 1084's enamel is very faint and largely restricted to the lingual side, compared to the more conspicuously grainy texture of Baryonyx teeth. According to Rauhut, though MB R 1084 is potentially spinosaurid in origin, it shares only generic resemblance to baryonychine teeth, and instead probably represents a close relative of Ceratosaurus? stechowi.[11] In Buffetaut's 2012 naming of Ostafrikasaurus, he placed it in the Spinosauridae, asserting instead that the tooth is very much akin to those of baryonychines including Baryonyx. Among their shared dental features he included the slight sideways-flattening of the crown, fine enamel wrinkles, and ridges on both sides that do not reach the tooth tip, and are stronger and more numerous on the lingual than labial face. If this identification is correct, Ostafrikasaurus represents some of the earliest known evidence of spinosaurids.[16]
The naming and distinction of new dinosaurs based solely on teeth has been frequently considered problematic by palaeontologists, such as with the debated identity of the Asian genus Siamosaurus.[14][17] Buffetaut stated that with thorough comparison and analysis of morphological features such as ornamentation, theropod teeth can be sufficiently diagnostic enough to raise new taxa.[16] Furthermore, spinosaurid teeth in particular share a unique morphology very divergent to those of other theropods.[16][18] Nevertheless, the precise identification of Ostafrikasaurus is still unclear, and researchers have pointed out similarities in its enamel ornamentation with Ceratosaurus, Paronychodon, Zapsalis, Acheroraptor, and Coelophysis.[18]
Evolution
Fowler in 2007 put forward the possibility of spinosaurids having evolved from ceratosaurian ancestors, given that baryonychine teeth have ridges on their crowns reminiscent to those seen on the premaxillary and dentary teeth of Ceratosaurus.[7] In 2008, Buffetaut rejected this proposal, citing that the D-shaped cross section of said Ceratosaurus teeth is not present in those of baryonychines.[8]
The main difference between MB R 1084 and all other known spinosaurid teeth, as Buffetaut noted, was in the large size of the denticles borne by the carinae. This led him to hypothesize in 2008 that spinosaurid dental evolution was largely characterized by the shrinking and eventual loss of serrations. In conjunction with their disappearing serrations, the cross sections of spinosaur teeth were initially blade-like and lenticular in earlier taxa like Ostafrikasaurus and Baryonyx, and became more circular in derived forms like Spinosaurus. The ornamentation of spinosaur tooth enamel, however, developed more irregularly than their serrations and cross section shape. Most non-avian theropods generally had smooth teeth with, at most, minimal wrinkles. Ostafrikasaurus teeth in contrast bore strong lengthwise ridges, a feature also seen in Baryonyx and its close relatives, though their prominence varies between taxa. Spinosaurus teeth, like those of typical theropods, were usually smooth, and Asian forms like Siamosaurus exhibited an increase in the amount of dental ridges. Furthermore, the tooth crown was wrinkled in Ostafrikasaurus, Baryonyx, and Suchomimus, and Asian spinosaurids, but smooth in Spinosaurus, with only some specimens of the latter showing fine wrinkles.[16]
In 2016, Spanish palaeontologist Aleandro Serrano-Martínez and colleagues described a possible spinosaurid tooth, catalogued as MUPE HB-87, from the Irhazer Shale of Niger. Found in association with a skeleton of the sauropod dinosaur Spinophorosaurus, the tooth likely dates to the Bathonian stage of the Middle Jurassic, 14 million years prior to Ostafrikasaurus. If this identification is correct, MUPE HB-87 represents the oldest known evidence of spinosaurids in the fossil record. Based on this specimen, the authors proposed a new evolutionary model for the transition between "normal" theropod teeth, to those of spinosaurids. The tooth is oval in cross section, is recurved, and bears small serrations, as well as strong fluting that does not reach the tip of the crown.[18] However, this specimen has been noted by other researchers to possess many non-spinosaurid features, these researchers instead favoring a megalosaurid identification.[19][12]
Though no skull material has been discovered for Ostafrikasaurus, it is known that spinosaurid skulls resembled those of crocodiles; they were long, low, narrow and expanded at their front ends into a terminal Palaeobiology
Palaeoenvironment and palaeobiogeography
The Upper Dinosaur Member of the Tendaguru Formation is composed mostly of
The Tendaguru Formation was home to a diverse abundance of organisms.
(D) marked on maps of those time spans.Other vertebrates that shared this environment included
In 2007, Fowler noted that a baryonychine identification for the L.? stechowi teeth would suit
References
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- ^ a b W. Janensch, 1920, "Ueber Elaphrosaurus bambergi und die Megalosaurier aus den Tendaguru-Schichten Deutsch-Ostafrikas", Sitzungsberichte der Gesellschaft Naturforschender Freunde zu Berlin 1920: 225–235
- ^ a b c d e f g h Buffetaut, Eric (2012). "An early spinosaurid dinosaur from the Late Jurassic of Tendaguru (Tanzania) and the evolution of the spinosaurid dentition" (PDF). Oryctos. 10: 1–8.
- ^ Janensch, W., 1925, "Die Coelurosaurier und Theropoden der Tendaguru-Schichten Deutsch-Ostafrikas", Palaeontographica Supplement 7: 1–99
- ^ a b Madsen, James H.; Welles, Samuel P. (2000). Ceratosaurus (Dinosauria, Theropoda), a Revised Osteology. Miscellaneous Publication, 00-2. Utah Geological Survey.
- ^ a b c d Fowler, D. W. (2007). "Recently rediscovered baryonychine teeth (Dinosauria: Theropoda): New morphologic data, range extension & similarity to Ceratosaurus". Journal of Vertebrate Paleontology. 27 (3): 3.
- ^ a b c d e Buffetaut, Eric (2008). "Spinosaurid teeth from the Late Jurassic of Tendaguru, Tanzania, with remarks on the evolutionary and biogeographical history of the Spinosauridae". Documents des Laboratoires de Géologie de Lyon. 164: 26–28.
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