Day 174#: Eoabelisaurus mefi
Merry day three of Dino-December! Today's animal of the day is Eoabelisaurus mefi!
Image credit: Andrey Atuchin
This species of medium-sized theropod dinosaur lived during the Early Jurassic period in what is now Argentina. As its genus name suggests, it is one of the early ancestors of the Late Cretaceous Abelisaurus, and is a member of the Abelisaurid family, which also includes dinosaurs like Carnotaurus and Majungasaurus. Eoabelisaurus grew to around 19.7 to 21.3 ft in length, though the average size for the species seems to have been close to 19 ft. Like other Abelisaurids, it had a slightly rounded head and short arms, even compared to other theropod dinosaurs. However, their arms weren't yet quite as reduced in size as some of the later species of Abelisaurids were. Unlike some of its descendants from the Cretaceous period, Eoabelisaurus lacked any horns or other ornamentations on its skull, which were used by later Abelisaurids mostly for display purposes.
Image credit: cisiopurple on DeviantArt
Eoabelisaurus was first discovered in 2009 by paleontologist Diego Pol near the village of Cerro Cóndor in the Chubut Province of southern Argentina. Before this discovery, the oldest remains of Abelisaurids were a few bits of fossils from South America and Africa that dated back to the Early Cretaceous and were so fragmentary that they couldn't even be given a name. Eoabelisaurus helped show paleontologists that the Abelisaurid family dates back at least 40 million years later than previously thought! Eoabelisaurus also helped paleontologists better understand the evolution of Abelisaurid arms.
Image credit: Paleocolour
All known species of Abelisaurids have very tiny arms; in some genera, like Carnotaurus, these arms are thought to have been almost completely vestigial because they were so small. While these later Abelisaurids had both reduced arms and hands, Eoabelisaurus had only slightly reduced arm bones compared to more basal theropods, but greatly reduced hand bones. This shows that the reduction of Abelisaurid arms started with the fingers and hands, before eventually the rest of the arms began to shrink as well.