Header image courtesy of the study’s authors. Credit: Julio Lacerda

It’s harder to find spinosaurs than you’d think. We understand they had a strong relationship with aquatic environments, yet their fossil record is poor. As we’ve seen with other fossil deposits, species endemic to marine and freshwater environments typically are more well-represented. This leaves paleontologists scratching at the few clues they have. Are we sure these predators lived amphibious lifestyles? We’re lacking the necessary data to answer some of the most crucial questions about the lives of spinosaurs.

Getting a better understanding of spinosaurs is important for both understanding them as a species and Cretaceous environments as a whole. As sea levels rise today as a result of global climate change, these dinosaurs can help inform us about how ecosystems and organisms adapted with those changes. A new study of a partial spinosaur tibia from northeast Brazil is helping us to do just that.

Where Do We Find Spinosaurs?

This is a critical question to understand when talking about spinosaurs. Where they’re found informs our understanding of the greater paleobiogeographic dynamics of those areas, as well as greater insights into their behavior. While the most well-known species of Spinosauridae (Spinosaurus aegyptiacus, Baryonyx walkeri, Suchomimus tenerensis, etc.) come from northern Africa and Europe, South America–particularly Brazil–is a key location for finding their remains.

During the Cretaceous, South America and Africa split apart and sea levels rose. This “flooded the margins” of the continents, causing landmasses to shrink and coastal environments (lagoons, shallow seas, etc.) to appear.

Evidence of these shifts can be seen in the Romualdo Formation of northeast Brazil. During the Cretaceous, Romualdo was the site of a coastal lagoon that experienced “transgressive-regressive marine cycles” and varying “freshwater influence”. Sea levels rose and fell, expanding or contracting aquatic habitats. Meanwhile, freshwater from rivers and estuaries entered these lagoons, impacting the local flora and fauna.

Romualdo is mostly known for fishes, pterosaurs, crocodyliforms, and other aquatic Cretaceous fauna. However, spinosaurs have been found, with two distinct species–Irritator challengeri and Angaturama limai–so far identified (Oxalaia quilombensis, another spinosaur species, has been found in the nearby Alâcantara Formation). This makes sense; as Africa and South America drifted apart and habitats they favored grew, conditions may have favored their spread and diversification. 

Studying the Bone Fragment

In the new study–“Semi-aquatic adaptations in a spinosaur from the Lower Cretaceous of Brazil”—Tito Aureliano et al. describe a left tibia fragment from an unidentified Brazilian spinosaur. While the specimen’s exact origin is unknown (the material was donated from a private collection), its style of preservation suggests it’s likely from Romualdo. The material, like many fossils from Romualdo, was preserved in 3D and had calcite infilling–a phenomenon associated with marine sedimentary environments. 

To analyze the material, the research team conducted CT scans and histological analysis. The scans were used to created digital models for visualizing segmenting images of the bone. Additionally, the scans aided in bone density analysis. To conduct the histology–studying the microscopic structure of tissues–the researchers examined thin sections of the bone. They then photographed those sections with a petrographic microscope (a fancy microscope for studying rocks).

Results

From this small specimen, the researchers gathered some incredible data. First and foremost, the researchers determined the specimen’s fibular crest–a raised feature of the tibia that fuses to the fibula–resembled tetanuran theropods. And, because the crest is “low and distally broad” in comparison to other tetanurans, they concluded it resembles that of S. aegyptiacus. Much like S. aegyptiacus, the specimen also has an extremely thick-walled and dense outer bone surface. This does not appear in other non-avian theropods.

Looking at the bone histology, researchers also noticed the specimen exhibited Lines of Arrested Growth (LAGs). Seen in many species–both extant and extinct–LAGs are a sort of record of an organism’s growth and development into adulthood and sexual maturity. Evaluating the specimen’s LAGs, the team came to an intriguing hypothesis: this species of spinosaur may have been larger than its cousins from the same formation. In fact, the subadult spinosaur from which the material came may have been larger than a full-grown A. limai when it died–over 10 meters (33 feet) in length!

Little Bone, Lots of Data

There are so many inferences and insights buried within this study. While I could go into depth about all of them, I’ll try to just touch two main points.

More Weight to Semi-Aquatic Lifestyles

First, as we’ve already established, the spinosaur this specimen comes from appears to share a strong relationship with S. aegyptiacus. But, with the age of the Romualdo Formation (deposited during the Aptian-Albian stages of the Lower Cretaceous), the specimen’s high bone density, or osteosclerosis, actually predates S. aegyptiacus by around 10 million years or so. This may indicate dense bones were more prevalent in spinosaurines (the clade featuring spinosaurid taxa more similar to S. aegyptiacus than B. walkeri) than we previously thought.

While this has a number of implications, this most importantly adds weight to the hypothesis of spinosaurs’ semi-aquatic lifestyles. As observed in other species–mammals and marine reptiles in particular–bones become more dense as the animals adapt to shallow water environments. This increased density, or “bone ballast,” is a specialization that helps the animal reduce buoyancy. Its presence in this newly described specimen further nudges the needle towards establishing near-amphibious lifestyles in spinosaurs–especially given the fact that the majority of their theropod cousins possess hollow bones.

Better Understanding of Size

The size of this Brazilian spinosaur also brings up a number of interesting ideas. Because the researcher’s estimates put the animal, fully-grown, close to the size of S. aegyptiacus, size may have played a role in the clade’s early success and eventual spread and diversification. In the context of other paleogeographic data, their size might have been a response to competition and space.

The paleobiogeography of Romualdo suggests that surrounding area could not support a wide variety of tetrapods. In all likelihood–shown by the plant and herbivore fossils records–it was an arid environment with low floral and faunal productivity. Therefore, these spinosaurs may have spent a majority of their time hunting and feeding within the lagoon. In this dynamic, size can be influenced by competition.

Limited resources due to the terrestrial environment would have forced a taxing of the trophic web. The crocodilyforms, pterosaurs, and large fishes occupying predator roles in these ecosystems likely competed for resources. However, these groups never grew large enough to present major challenges to spinosaurs. Likely, they instead fed off small prey animals. Spinosaurs, meanwhile, may have grown in size to exploit ranges of food items too large for other predators. 

The Spinosaur Puzzle Continues

While this new study helps to build more of our understanding of how spinosaurs lived, it still leaves doors open for a lot of unanswered questions. How do these shifts from terrestrial to aquatic environments begin? What traits and adaptations make this transition more successful? How might the environmental changes spinosaurs were responding to similarly impact our modern ecosystems? Furthermore, understanding spinosaurs for their own sake is a noble pursuit in itself. The mysteries that surround these unique theropods still are begging to be explored. There’s a lot more spinosaur material to be found, and a lot more research to do. 

Interested in learning more? Check out the video the researchers did on their find! (Note: you may need to turn English subtitles on):

Special thanks to Dr. Aline Ghilardi of the Laboratório de Paleoecologia e Paleoicnologia, Departamento de Ecologia e Biologia Evolutiva, Universidade Federal de São Carlos, Brazil. One of the co-authors of the study, Dr. Ghilardi was kind enough to take time out of her busy schedule to help me out with this blog post (only for me to delay it for two months…). Go follow her on Twitter! @alinemghilardi  

References:

Aureliano, Tito, Aline M. Ghilardi, Pedro V. Buck, Matteo Fabbri, Adun Samathi, Rafael Delcourt, Marcelo A. Fernandes, and Martin Sander. “Semi-aquatic Adaptations in a Spinosaur from the Lower Cretaceous of Brazil.” Cretaceous Research 90 (May 2018): 283-95. Accessed July 11, 2018. doi:10.1016/j.cretres.2018.04.024.