Cranial fossils belonging to two extinct species of monkey – Caipora bambuiorum and Cartelles coimbrafilhoi – were examined by computed tomography (CT) scan and reconstructed with three-dimensional imaging by a group of scientists from various countries.
The fossils were found almost 30 years ago in a cave complex in Bahia, Brazil, located in the Caatinga, a semiarid biome that occupies part of Brazil’s Northeast Region.
The images were compared with those of craniums from 14 extant Central and South American primate species, enabling the researchers to identify adaptations and infer previously unknown relationships between the extinct and extant species.
“This is the first ever study of endocranial morphology involving fossils of New World monkeys, or platyrrhines,” said André Menezes Strauss, a professor at the University of São Paulo’s Archeology and Ethnology Museum (MAE) and an associate researcher affiliated with the Laboratory of Archeology and Environmental/Evolutionary Anthropology (LAAAE) at the university’s Institute of Biosciences (IB-USP) in Brazil.
Using a cast taken from the inside of the cranium (braincase), paleoanthropologists analyzed endocranial morphology to estimate the shape and size of the brains of the fossil primates.
The results of the study, which was supported by São Paulo Research Foundation – FAPESP, are published in the American Journal of Physical Anthropology.
The researchers described cranial and endocranial shape variations in 14 species belonging to the four extant genera in the family Atelidae – Alouatta (howler monkeys), Ateles (spider monkeys), Brachyteles (woolly spider monkeys or muriquis), and Lagothrix(woolly monkeys) – as well as the extinct species C. bambuiorum and C. coimbrafilhoi. There are approximately 350 primate species in the world today. More than 200 are platyrrhines.
The study was led by Ivan Perez, an anthropologist at Argentina’s La Plata Museum. His collaborators included Brazilian scientists affiliated with the University of São Paulo (USP) and the University of Campinas (UNICAMP), as well as researchers at institutions in Belgium, France, Germany and the United States. Cástor Cartelle, a paleontologist at the Pontifical Catholic University of Minas Gerais state (PUC-MG) for whom Cartelles coimbrafilhoi is named, was also a member of the research team.
In addition to FAPESP, Brazil’s National Council for Scientific and Technological Development (CNPq) and Argentina’s Scientific and Technological Research Fund (FONCYT) and National Scientific and Technical Research Council (CONICET) also supported the study.
The fossil specimens of C. bambuiorum and C. coimbrafilhoi are deposited at PUC-MG’s Natural History Museum in Belo Horizonte, Minas Gerais. The 14 crania of the extant platyrrhines came from collections held by Argentina’s La Plata Museum, Brazil’s National Museum in Rio de Janeiro, the Argentinian Natural Science Museum in Buenos Aires, and the US National Museum of Natural History in Washington, DC.
“All 16 specimens were digitized using a medical CT scanner. A virtual 3D model of the endocranium was generated for each sample, and the 3D models of the cranial surfaces were extracted from the CT scan data,” Strauss told.
The fossil specimens were damaged, particularly in the region of the zygomatic arches (cheekbones), so the researchers opted for two strategies to analyze them.
According to Strauss, in the case of C. bambuiorum, the right zygomatic arch was absent, but the left arch was intact.
“We reflected the undamaged arch to the damaged side in the 3D model, taking advantage of bilateral symmetry, and by means of this virtual repair, obtained a complete specimen,” he said.
“In C. coimbrafilhoi, both sides were absent, so we used an imputation method to estimate the positions of the missing parts.”
Perez digitized 26 anatomical landmarks and 373 semilandmarks along the curves and surfaces of each endocranium, as well as 64 landmarks and 196 semilandmarks on each cranium. In geometrical morphometrics, a landmark is a 2D or 3D point of evolutionary significance. Semilandmarks are defined by locations relative to other landmarks, e.g., midway between landmarks X and Y.
“The data served as a basis for multivariate analysis to compare all the characteristics of the 16 specimens and to look for similarities and differences that indicated morphological [and hence] adaptive patterns,”
Strauss said. In other words, because the specimens of extant species of Atelidae, the largest New World monkeys, included crania of Alouatta,