EEB498Y1Y Project Description
In birds and mammals, the enlarged brain approximately fills the bony endocranial cavity. As such, measurements of endocranial volume in fossil taxa, which do not preserve the soft nueral tissues themselves, can be used to approximate brain size/morphology for the study of brain evolution. In extant non-avian reptiles, the brain fills a much small proportion of the endocranium, the remainder of which is occupied by connective tissue, venous sinuses, and cerebrospinal fluid. For studies on brain size evolution that includes fossil reptiles, an estimate that the brain fills 50% the endocranial space is commonly used. However, when this value is traced back in the literature, we find that it is based only on three data points (Sphenodon, Alligator, and Iguana). This project will be investigating the extent to which the brains of non-avian reptiles fill the endocranial cavity space to confirm whether this assumption is valid. This is particiluarly important in studying the origin of the avian condition from the plesiomorphic reptile condition, notably through non-avian dinosaurs.
Two main methods to measure the brain/endocranial relationship of reptile specimens will be employed:
- I will use CT data from the DigiMorph database and render 3-D volumetric models of the endocranial cavity for those specimens whose body mass and brain mass recorded in Hurlburt (1996).
- I will dissect frozen/preserved specimens manually and measure the brain mass, and then make silicone endocasts from the braincase. The body masses of these specimens and other important measurements will be taken.
Upon collecting data, I will (i) investigate the volumes occupied by the brain in their respective endocranial cavities and test whether the assumption of 50% brain volume is valid across a large sample size of reptiles; (ii) perform a regression analysis to compare lizards and birds (bird data available in literature from Iwaniuk 1992); and (iii) investigate whether dinosaur data are intermediate between the reptile and avian regression lines. With this, we may be able to infer whether the birds have evolved a larger brain mass-body mass ratio by simply “filling up” the endocranial cavity with their brains or whether their endocranial cavity volumes also enlarged.