Sauropods of the Mahajanga Formation, and Changing Lifestyles of the Middle Jurassic

Sauropod Teeth from the Middle Jurassic of Madagascar, and the Oldest Record of Titanosauriformes

Gabriele Bindellini and Cristiano Dal Sasso

Summarized by Reynolds Hansen. Reynolds Hansen is an undergraduate geography major / geology minor at the University of South Florida. With a lifelong passion for paleontology instilled from an early age, Reynolds always knew the academic path ahead had a singular destination. Along the way, he picked up equal affinities for history and geography, and by the time he was in college, he worried he might have to choose one over the others. With the help of the university’s esteemed academic professionals and resources, he shifted focus with the goal of becoming a science communicator, telling the story of our world from the formation of the earth to the modern day as an interconnected narrative. Reynolds is set to graduate in the spring of 2021, after which he wishes to seek a post-graduate degree in paleontology, and a career as an educator. His academic focus is utilizing GIS to research paleoecological phenomena.

Data used:  The 31 fossil sauropod (the group including long neck dinosaurs) teeth described in this work are all from the Mahajanga Formation of Madagascar. The teeth are often the only recoverable remains for these animals, as intense weathering typically reduces most other bones to a powder-like state.

Methods: Researchers took qualitative and quantitative measurements of the teeth to reconstruct the amount of physical wear and abrasion to record observations of physical wear or abrasion, and determine possible diet according to tooth features, positioning in relation to neighboring teeth, and evolutionary derivation. Measurements were obtained via the use of digital calipers on targeted areas of interest on the teeth, specifically, from four regions around the crown of the tooth. The results of these measurements grouped teeth into one of four categories: heart-shaped teeth, spatulate teeth, compressed cone-chisel teeth, and pencil-shaped teeth (or peg-like teeth; fig.1). Finally, these measurements were compared against known sauropod species tooth dimensions to come to conclusions on species taxonomy and ecology (fig. 2). 

Figure 1: The four tooth types outlined in the findings of the paper and labeled respectively. Notice the progression from left to right, where enamel wrinkling decreases and tooth becomes more peg-like. This spectrum is also representative of the progression in sauropod tooth morphology seen over time.

Results: With the methods described above, the researchers were able to categorize the 31 teeth into eight morphotypes, or broad shapes. They were then able to categorize these morphotypes further into four possible taxa (species), based on a number of factors including comparative analysis of tooth features, knowledge of local species (or lack thereof), and a combination of these two points against the total current knowledge of sauropod tooth shape. Most of the morphotypes are tentatively assigned to two possible species: Bothriospondylus madagascariensis, or Lapparentosaurus madagascariensis (taxa A and B). Scientists don’t yet understand the relationships of taxon A to other sauropods yet, but taxon B may be related to groups like brachiosaurs or titanosaurs. Two morphotypes (taxon C) are tentatively assigned to Archaeodontosaurus descouensi., which is classified as an ‘eusauropod’; a designation that remains somewhat flexible, but typically refers to animals making the transition from Triassic prosauropods and late Jurassic-Cretaceous ‘neosauropods’. The last remaining morphotype is set into a taxon of its own (D), although the identity is largely unknown, and only tentatively proposed to be a diplodocid or basal titanosaur of some kind. It has characteristics unlike any yet seen in this formation, having a mostly peg-like shape while all other teeth are stouter with some enamel wrinkling.

Figure 2: Sauropod teeth plotted chronologically and according to dimensions measured according to figure 1. Taxa proposed in this study are shown in larger colored points and highlighted at temporal location by light blue bar.

Why is this study important? The findings in this paper imply that titanosaurs may have been around just as long as other successful groups of sauropods, like brachiosaurs and diplodocids, and that the roots of titanosaurs extends into the middle Jurassic, a time where fossils are less commonly known. Discoveries like these from this time period provide valuable insight into the changing global climate that led to the late Jurassic boom in sauropod diversity. In this case, we see the partly bipedal Triassic prosauropods slowly evolving into the large, quadrupedal animals we are more familiar with. This change is highlighted here in the shifting tooth morphology: narrower, increasingly peg-like teeth are seen as an evolutionary deviation from the more multi-purpose, wrinkled teeth associated with prosauropods. Since most later sauropods have virtually pencil-shaped teeth associated with branch-stripping and gut-digesting rather than chewing and oral-processing, the appearance of traits leaning in this direction gives us evidence that these evolutionary changes may have been propelled by competition between species and niche-filling. Accessing higher vegetation borne from trees presents pioneering animals with an untapped resource. However, trees also present challenges in the form of tough branches and leaves that are harder to digest and also often less nutritious. The transition from wider, wrinkled teeth for processing and chewing, to peg-like teeth that strip leaves from branches, allows animals to acquire more food in less time and with higher efficiency. This also translates most or all of the digestion to the stomach, which passively grinds food with the aid of gastroliths (swallowed stones), further allowing for the animal to continually take-in food and remain on the move.

The Big Picture: Special attention should be awarded to those locales which are already underrepresented in the fossil record. If one could extract new taxonomic information from less than three dozen teeth, especially from a time without significant fossil representation, then it leads one to wonder what could be found with further excavation, and the insight of local knowledge, interest, and investment. That is not to say that every sparsely researched locale is a treasure trove awaiting plentiful fossil discovery, but the matter of the Mahajanga Formation is such that its potential for producing Middle Jurassic materials should certainly not be overlooked, and is mostly untapped. The teeth used in this study were not exclusively extracted for the research of this paper. Instead, they were found as a result of a field survey conducted in tandem by an Italian institution (Museo di Storia Naturale di Milano), and a couple of Malagasy geological ministries in the early 2000s. It is only recently, in 2019 that the teeth were pulled from archives for analysis. This means that this the geological deposits near Ambondromamy, Madagascar, likely have much yet to show us.   

Citation: Bindellini, G. and Dal Sasso, C. (2021), Sauropod teeth from the Middle Jurassic of Madagascar, and the oldest record of Titanosauriformes. Pap Palaeontol, 7: 137-161.

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