Ostafrikasaurus is a potentially dubious genus of theropod dinosaur from the Late Jurassic period of what is now Lindi Region, Tanzania. It is known only from fossil teeth discovered sometime between 1909 and 1912, during an expedition to the Tendaguru Formation by the Natural History Museum of Berlin. Eight teeth were originally attributed to the dubious dinosaur genus Labrosaurus, and later to Ceratosaurus, both known from the North American Morrison Formation. Subsequent studies attributed two of these teeth to a spinosaurid dinosaur, and in 2012, Ostafrikasaurus crassiserratus was named by French palaeontologist Eric Buffetaut, with one tooth as the holotype, and the other referred to the same species. The generic name comes from the German word for German East Africa, the former name of the colony in which the fossils were found, while the specific name comes from the Latin words for "thick" and "serrated", in reference to the form of the animal's teeth.
The holotype tooth is 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 adaptations. From comparisons with its later relatives, Ostafrikasaurus indicates that spinosaur teeth became more conical and lost their serrations throughout their evolution. This is possibly a result of becoming more specialized for a piscivorous (fish-eating) diet, as has been suggested for the family based on fossil evidence and the semiaquatic adaptations exhibited by many species. They are also known to have fed on pterosaurs and other dinosaurs. Ostafrikasaurus lived in a subtropical to tropical environment alongside many other dinosaurs, as well as pterosaurs, crocodyliforms, fish, mammals, and invertebrates. Due to its age and location, Ostafrikasaurus indicates spinosaurids may have been globally distributed prior to the breakup of Pangaea.
History of research
During the time of the German colonial empire, the Museum für Naturkunde (Natural History Museum) of Berlin arranged an expedition in German East Africa (now Tanzania) that took place from 1909 to 1912, and is now regarded by scientists as one of the largest expeditions in palaeontological history. Most of the excavations were situated in the southeastern Tendaguru Formation, a fossil-rich site part of the Mandawa Basin dated to the Late Jurassic period. Among the many dinosaur fossils retrieved from the dig sites were 230 specimens of theropod teeth. One of these was an isolated tooth catalogued as MB R 1084, found either nearby or atop Tendaguru Hill at the Upper Dinosaur Member. It was originally attributed to the species Labrosaurus? stechowi in 1920 by German palaeontologist Werner Janensch, based on comparable ornamentation to a tooth described as Labrosaurus sulcatus by Othniel Charles Marsh.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 detailed monograph by Janensch published in 1925 assigned MB R 1084, as well as eight teeth from the Middle Dinosaur Member to L.? stechowi and divided them into five morphotypes (from a to e).Janensch, W., 1925, "Die Coelurosaurier und Theropoden der Tendaguru-Schichten Deutsch-Ostafrikas", Palaeontographica Supplement 7: 1–99
In 2000, American palaeontologists James Madsen and Samuel Welles referred the L.? stechowi teeth to Ceratosaurus sp. (of uncertain species), because they resembled teeth from the premaxilla and dentary jaw bones of Ceratosaurus, a theropod from the North American Morrison Formation. In 2007, American palaeontologist Denver Fowler instead proposed that the teeth were those of a spinosaurid dinosaur similar to Baryonyx, which would make it among the oldest known spinosaurid fossils and thus some of the earliest evidence of the group. This analysis was maintained by French palaeontologist Eric Buffetaut, who examined the teeth that year, and in a 2008 paper referred specimen MB R 1084 to the Spinosauridae. Bufetaut found that this specimen differed from other teeth previously referred to L.? stechowi, and that another isolated tooth (MB R 1091) from the Middle Dinosaur Member might represent the same animal. He also questioned Janensch's provisional assignment of the teeth to the dubious genus Labrosaurus, which was based on scant remains from the Morrison Formation that were later attributed to Allosaurus. Furthermore, Buffetaut noted that the L. sulcatus tooth illustrated by Marsh is now regarded as belonging to Ceratosaurus. Similarly, L. stechowi has been relegated as a dubious ceratosaurian related to Ceratosaurus.Tykoski, Ronald S.; and Rowe, Timothy. (2004). "Ceratosauria", in The Dinosauria (2nd). 47–70.
In 2011, German palaeontologist Oliver Rauhut considered the Middle Dinosaur Member teeth ascribed to L.? stechowi as lacking diagnostic characters (unique derived traits), concurring that the species is a dubious name. Rauhut noted that they can still be differentiated from other theropod teeth from the Tendaguru Formation, based on their slight recurvature and sideways flattening of the tooth crown, and broad ridges on the lingual flank (which faced the inside of the mouth). He recognized L. sulcatus as a dubious name, since the tooth referred to it was shown only in a single illustration and not properly described in text. In addition, the original remains of Labrosaurus did not include teeth, and an additional species, Labrosaurus ferox (now considered synonymous with Allosaurus fragillis), was based on a dentary bone bearing teeth of different morphology to those of L. sulcatus. Thus, Rauhut concluded there was no basis for attributing the Tendaguru teeth to Labrosaurus. He tentatively referred all of them except MB R 1084 to Ceratosaurus (under the name Ceratosaurus? stechowi), on the basis of anatomical similarities with teeth from that genus. According to Rauhut, the different features between Janensch's type b, c, e, and d teeth did not represent distinct taxa, but rather variation along the tooth row in the animal's jaws. Rauhut also attributed another tooth (MB R 1093) described by Janensch, yet not referred to L.? stechowi, to the same taxon.
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 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 on the rear carina and 13 denticles per on the front carina.
In a 2012 paper, Buffetaut used MB R 1084 as the holotype specimen for the new genus and species Ostafrikasaurus crassiserratus, describing it as an early spinosaurid theropod. Its generic name is derived from the German name of the colony in which the fossils were found, Deutsch-Ostafrika, meaning "German East Africa", combined with the Greek (), meaning "lizard" or "reptile". The specific name comes from the Latin , meaning "thick"; and , meaning "serrated", in reference to the large serrations of its teeth. Due to similarities with MB R 1084, Buffetaut assigned MB R 1091 from the Middle Dinosaur Member to the same species. Both teeth have a curved front carina, no side to side curvature, and a comparable shape in cross section. Their main differences include MB R 1091 having five lengthwise ridges on its lingual side compared to MB R 1084's ten, the ridges on the former being less extensive. MB R 1091 also had less wrinkled tooth enamel. Buffetaut notes that these differences could be explained by individual variation within the taxon, but since both teeth originated from different members of the Tendaguru Formation, the referral is only tentative.
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.
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. Another study published in the same year argued that the fragmentary tooth taxa Ostafrikasaurus and Siamosaurus should be regarded as nomina dubia.
Description
The body size of Ostafrikasaurus cannot be reliably calculated to due the highly incomplete nature of current material.
The holotype tooth is thick, somewhat flattened sideways, and 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.
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 wide at most. None of the ridges on either side reach the tip of the crown. There is a 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 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.
Classification