A new species of dinosaur discovered in Argentina is challenging some current theories about dinosaur evolutionary lineages. The new dinosaur, Gualicho shinyae, sports a pair of tiny ineffectual arms similar to those of the Tyrannosaurus rex.
What makes this find more remarkable is the fact that the two species do not share a close evolutionary lineage, implying that these tiny forearms evolved independently in each species. In fact, the closest genetic relative of Tyrannosaurus rex and Gualicho shinyae had proportionately-sized forelimbs, making the tiny arm connection between the two species even more mysterious.
The paleontologists who discovered Gualicho shinyae have published their findings in PLOS One. According to the article, the tiny arms found on the new dinosaur and others such as the T-rex likely evolved independently of one another, for as yet unknown reasons:
[...] forelimb reduction in Gualicho likely occurred independent of other lineages with reduced forelimbs such as abelisauroids, alvarezsaurids, ornithomimosaurs, and especially didactyl tyrannosaurids.
This type of independent evolution of similar characteristics is known as convergent evolution. When multiple organisms evolve the same traits or structures independently of one another, it is generally assumed that those traits must be desirable for an organism's given environment. Why tiny forelimbs might be an evolutionarily-selected trait in some dinosaurs is still a mystery.
So far, the unique combination of physiological features in the new dinosaur has made it difficult to classify using existing classification systems. According to the published research, relationships among physiological traits found in different types of dinosaur classes must be examined in order to properly classify Gualicho shinyae:
This mosaic of synapomorphies from multiple, distantly related groups renders our understanding of the affinities of Gualicho highly uncertain. [...] A more robust evaluation of its affinities requires a more comprehensive review of theropod relationships that samples widely among ceratosaurians, allosauroids, and basal coelurosaurs.
It might simply be that forelimbs were not useful for many classes of carnivorous dinosaurs, or perhaps were even a hindrance or dangerous target for other carnivores to bite off. More study of dinosaur evolution is needed before these evolutionary adaptations are fully understood.