CT scan of an ancient reptile skull reveals little evolutionary change over 22 million years
A CT scan of the skull of a long-necked plesiosaur shows the cranial architecture of these long-extinct marine reptiles didn't evolve much over 22 million years that they lived during the Cretaceous time.
That's very unusual, said SMU paleontologist Louis Jacobs, one of the world's foremost authorities on prehistoric creatures and co-author of a study published in PLOS One.
"Basically, in anything except living fossils, you don't go 22 million years without evolving," said Jacobs, professor emeritus of Earth Sciences at SMU and president of ISEM at SMU.
Elasmosaurid plesiosaurs, lookalikes of the mythical Loch Ness monster, were the largest of the long-necked plesiosaurs, growing as long as 43 feet with half of that length deriving from their small heads and very long necks. Paleontologists from SMU (Southern Methodist University), as part of an international team called Projecto PaleoAngola, based their findings on a CT scan of the 71.5 million year old skull from a species of elasmosaurid called Cardiocorax mukulu.
This detailed 3D model allowed the paleontologists to compare the well-preserved skull of C. mukulu found in Angola to that of other species of elasmosaurids. They found that C. mukulu looked nearly identical to skulls that came from much older elasmosaurids, including one found at Cedar Hill, Texas, in 1931, whose 93-million-years old remains can be found at SMU's Shuler Museum of Paleontology.
"The skull shape, organization of muscles, and the shape and arrangement of the teeth largely reflect how an animal acquired prey," said co-author Michael J. Polcyn, research associate and director of SMU's Digital Earth Sciences Laboratory "The interesting aspect of Cardiocorax mukulu is that it appears that this animal's predecessors adopted a particular feeding style early in their evolutionary history, and then maintained the same basic skull structure for the next 22 million years"
It will take more research to pinpoint why elasmosaurids might have been different from other reptiles in their evolutionary journey.
Elasmosaurids lived during the Cretaceous Period, which spanned from 145 million years ago to 66 million years ago. They were predators, thriving on fish and other marine life. Projecto PaleoAngola paleontologists first discovered C. mukulu in Angola in 2015.
The lead author of the CT scan study is Miguel P. Marx, who will be starting a Ph.D. program at Lund University in Sweden later this month and was a researcher in SMU's Earth Science department during this study. Other co-authors include Jacobs and Polcyn of SMU.; Octávio Mateus of Universidade Nova de Lisboa and Museu da Lourinhã, Portugal; Anne S. Schulp of the Naturalis Biodiversity Center and Utrecht University in the Netherlands; and A. Olímpio Gonçalves of the Universidade Agostinho Neto in Angola.
Skull found in the same area that yielded Smithsonian Museum exhibit
Mateus found the nearly complete cranium and jaw of C. mukulu, along with 12 associated teeth and other fossilized parts of the reptile's body in Bentiaba, Angola in 2017. That area is on the coast of Angola that Jacobs has called a "museum in the ground," because so many fossils have been found in the rocks there.
Many of those fossils are currently on display at the Smithsonian's National Museum of Natural History. The museum's "Sea Monsters Unearthed" exhibit, co-produced with SMU, features large marine reptiles from the Cretaceous Period—mosasaurs, turtles, and plesiosaurs.
Jacobs and Polcyn forged the Projecto PaleoAngola partnership with collaborators in Angola, Portugal, and the Netherlands to explore and excavate Angola's rich fossil history and began laying the groundwork for returning the fossils to the West African nation. Back in Dallas, Jacobs, Polcyn, and research associate Diana Vineyard went to work over a period of 13 years with a small army of SMU students to prepare the fossils excavated by Projecto PaleoAngola.
Like the Smithsonian exhibit, the discovery of the Cardiocorax mukulu remains were the result of that collaboration.
CT scan shows jaws and teeth of elasmosaurids didn't evolve much
Marx's computed tomography (CT) scan of the skull was designed to reveal parts of the skull that are otherwise difficult to see, such as the braincase. Only part of the skull was actually freed from the Angolan rock in which it was discovered because elasmosaurids skulls are so fragile. So the CT scan was taken largely through the rock that preserved the specimen.
However, "the good resolution of the resulting CT images allowed me to discriminate between the bone, the rock matrix, and the plaster jacket the skull was protected in," Marx said. "Thus, I could build a 3D model of the skull and be able to study the fragile parts of it, such as the braincase and palate, without touching it."
The team's conclusions about the cranial anatomy of C. mukulu were drawn from comparisons to the skull of Libonectes morgani, a much older elasmosaurid housed at SMU.
"The skull of L. morgani at SMU is so complete that the sutures between different bones can clearly be delineated," he said. "The skull of Libonectes morgani worked as a guide for me when making the skull model of Cardiocorax mukulu. This made the process of building the model much faster."
Marx and the PaleoAngola team also compared the 3D imaging to the skulls of Styxosaurus snowii and Thalassomedon haningtoni—all elasmosaurids from different time periods.
The similarity between the jaws, teeth and other skull anatomy of C. mukulu and its predecessors was a surprising discovery, Marx said.
For example, the skull of Cardiocorax mukulu and Libonectes morgani both exhibit a tall dorsal ramus of the maxilla, and the organization of the skull bones around the orbits is identical, Marx said. The skulls of these two species only differed in a couple of key aspects, including a slightly different number of teeth in the upper and lower tooth rows, the location of the premaxillary-parietal suture, and the presence or absence of the pterygoids contacting each other beneath the basioccipital bone.
"It appears that the skull of elasmosaurids did not undergo significant evolutionary change throughout their history, which is very cool," Marx said.