Saturday, 22 August 2015

Isolated Theropod teeth from the Middle Jurassic of Niger.


Unlike Mammals, Dinosaurs replaced their teeth throughout their lives, continuously growing new teeth and shedding older, worn ones. This, combined with the hard enamel coating found on all vertebrate teeth, makes isolated teeth the most commonly encountered Dinosaur specimens, so being able to accurately assign teeth to specific Dinosaur groups has great importance for understanding the origins and distributions of such groups. Within the Theropods, some groups have  very distinctive teeth which allow confident assignment of teeth by simple examination, for example Spinosaurid teeth have deeply veined enamel and fluted ridges, as well as tiny denticles (toothlets) on the carinae (cutting blades on the back of the tooth), however most groups lack such obvious characteristics and therefore palaeontologists have developed methods based upon morphometric analysis (mathematical comparison of different measurements and angles on a tooth, bone or shell), which allow isolated teeth to be compared to teeth found in situ on more complete fossil specimens.

In a paper published in the journal Acta Palaeontologica Polonca on 1 July 2015,  Alejandro Serrano-Martínez of the Grupo de Biología Evolutiva at the Universidad Nacional de Educación a Distancia, the Departamento de Biología at the Universidad Autónoma de Madrid and the Departamento de Paleobiología at the Museo Nacional de Ciencias Naturales, Daniel Vidal, also of the Departamento de Biología at the Universidad Autónoma de Madrid, Lara Scisio of the Department of Geological Sciences at the University of Cape Town, Francisco Ortega, also of the Grupo de Biología Evolutiva at the Universidad Nacional de Educación a Distancia, and Fabien Knoll of the Departamento de Paleobiología at the Museo Nacional de Ciencias Naturales, the School of Earth, Atmospheric and Environmental Sciences at the University of Manchester and the School of Earth Sciences at the University of Bristol describe four isolated Theropod Dinosaur teeth that were found in association with a fossil Sauropod, Spinophorosaurus nigerensis, from the late Middle Jurassic Tiourarén Formation of Aderbissinat in Niger.

The first tooth, MUPE HB-142 is also the best preserved, lacking only its tip, estimated to have been 31 mm in height, laterally compressed (flattened sideways) and curved towards the tip. It has an unserrated carina on its upper portion, and large, closely spaced denticles, with a density of roughly 2.25 denticles per mm. The crown of the tooth has undulations (wrinkles) close to the carina, and a slightly braded enamel texture.

MUPE HB-142 in close-up (A1), labial (A2), distal (A3), lingual (A4), and basal (A5) views. Scale bars (A1) 5 mm, all others 1 cm. Serrano-Martínez et al. (2015).

Morphometric analysis of MUPE HB-142 suggests that it probably derived from a Megalosaurid, as it shows affinities with Megalosaurus, Dubreuillosaurus and Afrovenator. However it also shows some affinities with the basal Tetanuran Torvosaurus, (the Tetanurans are a wider group which include the Megalosaurs, Allosaurs, Spinosaurs and other groups, basal Tetanurans are Tetanurans that predate the splitting of the Tetanurans into these groups) and falls within the morphospace (complete range of size and shapes) of both Megalosaurus and Allosaurus, though it is closer to average values for Megalosaurus, so the possibility that this tooth came from an Allosaurid cannot be ruled out.

The second tooth examined, MUPE HB-118, is the largest tooth in the collection, with a total height of 37.47 mm. This is similar in shape to MUPE HB-142, with denticles on the top third portion of the tooth and a similar braided texture to the enamel. The carina has chisel-like denticles, with a density of 3 per mm.

MUPE HB-118 in lateral (B1), close-up (B2), lateral (B3), distal (B4), and basal (B5) views. Scale bars, (B2) 2 mm, all others 1 cm. Serrano-Martínez et al. (2015).

This tooth also shows affinities to the Megalosaurids Megalosaurus, Dubreuillosaurus and Afrovenator, as well as with the early Neotheropod Dilophosaurus (related to but not included in the early Tetanurans) and the basal Tetanuran  Piatnitzkysaurus. Like MUPE HB-142 it falls within the morphospace of both Megalosarus and Allosaurus, but while MUPE HB-142 was closer to average values for Megalosaurus, MUPE HB-118 falls closer to average values for Allosaurus.  However the similarity of MUPE HB-118 to the teeth of Dubreuillosaurus and Afrovenator, combined with its proximity to another tooth assigned to a Magalosaurid and the absence of known Allosaurid teeth from the Southern Hemisphere in the Middle Jurassic leads Serrano-Martínez et al. to conclude that this is also a Magalosaurid tooth.

The third tooth examined, MUPE HB-125, is the smallest in the sample, and has a broken apex, and an estimated crown height of 20.26 mm. It is laterally flattened with a curved tip, though less so than MUPE HB-142 and MUPE HB-118. It has a carina occupying ¾ of the length of the tooth, with convex denticles on its bottom part and chisel-like denticles towards the top. The enamel of the tooth is unwrinkled, and shows only weak texturing.

MUPE HB-125 in lateral (C1, C3) and distal (C2) views. Scale bar 1 cm. Serrano-Martínez et al. (2015).

This tooth shows affinities to Allosaurus the Megalosaurids Dubreuillosaurus and Afrovenator,  and the basal Tetanuran Erectopus, but does not fall within the morphospace of any known Dinosaur group, lying between the values of Allosaurids, Megalosaurids and Dromaeosaurids. Again the close proximity of this tooth to other Megalosaurid teeth, and absence of known Allosaurid teeth from the Southern Hemisphere in the Middle Jurassic leads Serrano-Martínez et al. to conclude that this is more likely to be a Megalosaurid than an Allosaurid tooth.

The final tooth examined, MUPE HB-87, is less curved than the other specimens and 23.82 mm in height. It has a deeply veined enamel texture, and week undulations on the labial (inner) side of its carina. The carina itself has tightly packed chisel shaped denticles towards its tip, with smaller more convex denticles lower down.

MUPE HB-87, from the Middle Jurassic Tegama Group, Niger; in distal (A), lingual (B), mesial (C), and labial (D) views; close-up view of the labial side (E). Note the deeply veined enamel surface texture and the shape and size of the distal denticles. Scale bars (A−D), 1 cm; (E), 1 mm. Serrano-Martínez et al. (2015).

This tooth shows affinities to Allosaurus the basal Tetanuran Berberosaurus, but falls within the morphospace of the Spinosaurid group. It shows the deeply veined enamel texture typical of Spinosaurids (and never top date recorded in any non-Spinosaurid Dinosaur), but lacks the fluted ridges also typical of the group. It shows reduction in the size of denticles on the carina, but not to the extent seen in later Spinosaurids, and has other features intermediate between Spinosaurids and other Tetanuran groups, suggesting that it is an early member of the Spinosaur group, probably close to the split between this group and the related Megalosaurids.

The Spinosaurs are divided into two groups, the Baryonychines from Laurasia (North America, Europe and Asia) and the Spinosaurines in Gondwana (Africa, South America, India, Madagascar, Antarctica, Australia and New Zealand, though the group is not known from all these areas). However while the closely related Megalosaurs are well established in the Middle Jurassic, almost all known Spinosaur specimens come from the Cretaceous, making the origins of the group somewhat obscure. The Spinosaurines are generally considered to be more derived than the Baryonychines, which led palaeontologists to theorize that the group originated in Laurasia and spread to Gondwana, however recent discoveries of possible Spinosaurid teeth from the Middle Jurassic of Tanzania and Niger, combined with the presence of the group in Australia, which split away from the rest of Gondwana early in the break-up of the supercontinent, leads Serrano-Martínez et al. to suggest an alternative view, that the Spinosaurs originated in Africa in the Middle Jurassic and from there spread to other areas of Gondwana, as well as to Laurasia through Iberia and Europe.

Generalized palaeogeographic locations of spinosaurids (yellow) and the specimen of HB site (red), through time from Bajocian/Bathonian, Tithonian, Barremian−Aptian, and Albian−Cenomanian. Serrano-Martínez et al. (2015).

See also…

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