Morphological study of the ancient shark Cretodus crassidens

Morphology and paleobiology of the Late Cretaceous large-sized shark Cretodus crassidens (Dixon, 1850) (Neoselachii; Lamniformes)

Summarized by Mila Carter, a senior at the University of South Florida, majoring in Geology with a minor in Geographic Information Systems. After graduation, Mila plans to attend grad school to study climate science and would like to eventually work as a research technician. Some of Mila’s hobbies include going to the beach, hiking outdoors, reading, and spending time with friends and family. 

What was the hypothesis being tested (if no hypothesis, what was the question or point of the paper)?: The point of this paper was to learn more about the growth and lifestyle of newly-discovered ancient shark species Cretodus crassidens.

What data were used?: The data that was used consisted of a partial skeleton (including 101 teeth, 86 vertebral centra, and pieces of mineralized skull cartilage) found in an 85 million year old rock formation in Veneto, Italy called the Scaglia Rossa (a limestone rock formation from the Late Cretaceous), and a deformed tooth set, one vertebral centrum, a disarticulate set of teeth, and many single teeth found in a rock formation in southeast England called the Chalk Group (a limestone formation from the Upper Cretaceous).

Methods: Researchers took measurements of the specimens using photographs imported into a software called ImageJ. To estimate the original number of vertebral centra the shark had, a mathematical analysis method called a ‘least square linear regression’ was used. The size of the teeth was used as a parameter to determine the total length of the shark. The growth pattern of the shark was assessed using a growth function that models average length of the specimen versus its age, known as the von Bertalanffy growth function. The vertebrae were cut in half to observe the growth rings of the shark (approximately one ring of growth per year) to determine the shark’s age at death, which was 23 years (the average lifespan of this shark species was 64 years).

Results: The shark was determined to be larger in size (9 to 11 meters in length) and macropredatory, meaning it was a relatively large carnivore. Based on its length, this species of shark falls into a category of gigantic fish, as it is more than 6 meters in length. Its calculated adult size of 9 to 11 meters makes it more similar in size to Otodus (an extinct giant mackerel shark from the Cretaceous). The fossils suggest that the shark had a wide mouth and head along with a stout body. The shark also likely swam at moderate speeds and lived near the shore based on its teeth measurements (figure 1). We know this as scientists have found that there is a correlation between the tooth measurements of existing fish and their tendencies to be fast or slow swimming and their tendencies to live nearshore or offshore. This same principle can be applied to extinct fish to learn about their lifestyle, as it was in this case. Based on the fossils found in Veneto, Italy, the shark was found to have fed on large marine turtles. Along with the teeth and vertebrare of this shark being preserved in life position, there were also broken pieces of turtle shell deposited in a cluster next to the aligned vertebrae (where the shark’s stomach once was), leading the researchers to assume that the turtle was the last meal of the shark and was in its stomach when it died.

This graph shows mean ridge distance of shark teeth (measured in micrometers) plotted on the x-axis on a scale of 0 to 135 versus mean scale crown width of shark teeth (measured in micrometers) plotted on the y-axis on a scale of 0 to 675. Known shark species are plotted as points on this graph based on their tooth measurements. These points are grouped into two categories, fast swimming pelagic taxa and moderate swimming and nearshore taxa. The shark species points that fall toward the left side of the graph are grouped into the fast-swimming pelagic taxa category, and the shark species that fall to the right side of the graph are grouped into the moderate swimming and nearshore taxa category. Cretodus crassidens falls into the moderate swimming and nearshore taxa based on the data analysis conducted in this study.
Figure 1: This is a graph showing the mean ridge distance of shark teeth versus the mean scale crown width of shark teeth. The mean ridge distance refers to the average length of a particular feature on the tooth called the ridge, and the mean scale crown width refers to the average width from one point to another of a particular part of the tooth called the crown. Different shark species are plotted on this graph based on their tooth measurements. The light gray group is labeled as ‘fast swimming pelagic taxa’, meaning any shark in this group is known to be fast swimming and lived in the open ocean. The dark gray group is labeled as ‘moderate swimming and nearshore taxa’, meaning any shark in this group is known to swim at a moderate speed and live near the shore. Based on the measurements taken of the Cretodus crassidens teeth, this species was determined to fall into the ‘moderate swimming and nearshore taxa’ category.

Why is this study important?: Prior to this study, the information that scientists had about the genus Cretodus was mostly based on isolated tooth fossils (fossil shark teeth that were found on their own with no other accompanying remains from the shark). Within the genus Cretodus, there are five species, and among these five species, only one (Cretodus houghtonorum) has evidence of a partial skeleton. For an almost complete skeleton to be found for Cretodus crassidens is a great discovery for scientists wishing to find out more about this genus and this species.

Broader implications beyond this study: This study helps us to more thoroughly understand evolutionary patterns as we now know more about the morphology and lifestyle of an extinct species of shark. From this information we can perhaps derive why it became extinct and draw connections to the environmental and ecological conditions in the oceans at that time. Understanding the evolutionary history of large oceanic predators is critical today, especially, because many sharks are threatened due to human activity today.  

Citation: Amalfitano, J., Dalla Vecchia, F. M., Carnevale, G., Fornaciari, E., Roghi, G., & Giusberti, L. (2022). Morphology and paleobiology of the late cretaceous large-sized shark Cretodus Crassidens (Dixon, 1850) (Neoselachii; Lamniformes). Journal of Paleontology, 96, 1166–1188.