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Aves: Struthio Camelus) with Implications for Interpreting Extinct Dinosaur Endocasts

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Aves: Struthio Camelus) with Implications for Interpreting Extinct Dinosaur Endocasts 1 ONTOGENY OF THE BRAIN ENDOCASTS OF OSTRICHES (AVES: STRUTHIO CAMELUS) WITH IMPLICATIONS FOR INTERPRETING EXTINCT DINOSAUR ENDOCASTS _______________ A Thesis Presented to The faculty of The College of Arts and Sciences Ohio University _______________ In Partial Fulfillment of the Requirements for Graduation with Honors in Biological Sciences _______________ By Cheyenne Ariel Romick April 2013 2 TABLE OF CONTENTS Abstract…………………………………………………………………………...……4 Introduction…………………………………………………………………………….5 Why Study Ostriches?.........................................................................................6 Endocasts as a Tool for Study………………………………………………….7 Function and Anatomy…………………………………....................................9 Overview of Current Knowledge……………………………………………..11 Holes in the Research…………………………………………………………16 Potential Benefits……………………………………………………………..17 Materials and Methods………………………………………......................................19 Materials………………………………………………………………………19 Table 1………………………………………………………………………...19 CT Scanning…………………………………………………………………..19 Generation of Digital Endocasts………………………...................................19 Results………………………………………………………………………………...20 Adult Endocast Introduction………………………………………………….20 Figure 1a……………………………………………………………………...21 Figure 1b……………………………………………………………………...22 Visible Qualitative Results……………………………………………………22 Figure 2……………………………………………………………………….25 3 Figure 3 and Figure 4…………………………………………………………26 Figure 5……………………………………………………………………….27 Quantified Relative Surface Area…………………………………………….27 Table 2………………………………………………………………………..29 Lugol’s Analysis………………………………………...................................29 Figure 6……………………………………………………………………….30 Discussion…………………………………………………………………………….30 Prioritization…………………………………………………………………..31 Ontogeny Recapitulates Phylogeny…………………………………………..32 Figure 7……………………………………………………………………….33 Implications for Ostrich Development………………………………………..33 Implications for Non-Avian Dinosaur Development…………………………35 Future Direction………………………………………....................................37 References…………………………………….….….……..…………………………39 Appendix A…………………………………………………………………………...44 4 Abstract The goal of this project is to document the ontogeny of the cranial (brain) endocast of Struthio camelus, the African ostrich. Comparison of size and shape via 3D reconstruction from CT scans provided the data needed to study the ostrich endocast in various stages of life. Endocasts of seven specimens were generated for analysis. The specimens consisted of two embryos, three juveniles of differing ages, and two adults. Comparison of the shape and size of the endocasts was done using relative and absolute scaling. Surface area comparisons between specimens were digitally performed to determine the changes in nine brain regions from embryo to adult. Results suggest a definite change in shape as the organism shifts from embryo to adult. The most notable change in surface area is seen in the Wulst region of the cerebrum, implying a prioritization of that region at little expense to the other regions. The Wulst is located dorsally on the telencephalon and is composed of two distinct bumps on either side of the dorsal cerebrum. Additional analysis of one of the specimens with the Lugol’s iodine staining method revealed that the flocculus of the endocast is mainly composed of venous rather than neural tissue. The Lugol’s analysis also revealed that the cerebellar foliation present in the ostrich brain is not detectable in the endocast, because a large venous sinus overlies that area. The inner ear changes mainly in size during development, with a fairly constant shape throughout life. My results suggest that the Wulst is eventually prominent in the ostrich yet is not present at hatching, perhaps reflecting a parallel between ontogeny and phylogeny in that the 5 Wulst also appears later in avian brain evolution. In addition, the fact that the inner ear maintains its shape is indicative of its importance throughout the history of non-avian dinosaurs and its immediate importance after hatching in a precocial species. Introduction The goal of this project is to create an informative ontogenetic series for the endocast of the brain of the African ostrich, Struthio camelus, and to apply this information to the inferences of non-avian dinosaur endocasts as well as extant taxa. In this text, non-avian dinosaur refers to those dinosaurs which are not members of Aves, or more simply, not birds. The study of the brain is important for multiple reasons. The brain acts as a control center and behavior generator. Its role as an information processing and motor control center can give us clues to the physical capabilities of extinct species in addition to their cognitive-behavioral faculties. The lack of soft-tissue preservation in most fossils relegates comparative studies to the endocast rather than to the actual brain. Little is known about the ontogeny of the endocast of non-avian dinosaurs (e.g., Evans, Ridgely, & Witmer, 2009), primarily because most species lack enough specimens to generate a growth series. This project can help advance the scientific understanding of the development of the endocasts of non-avian dinosaurs in addition to that of Struthio camelus. The ostrich was chosen as the study organism due to its relatively basal position on the phylogenetic tree of Aves (Burish, Kueh, & Wang, 2004), placing it evolutionarily closer to non-avian dinosaurs than most other extant avian taxa. A growth series of heads and skulls was readily available to be analyzed or had already been analyzed and rested in the specimen 6 freezer or in the Ohio University Vertebrate Collection. Two embryonic heads were available, along with one each of a 14-day old, a 2-month old, and a 4-month old, and several adult heads were accessible. Please note that the term “head” refers to a specimen that had not been skeletonized. Why Study Ostriches? Ostriches are members of the ratite clade, which is considered part of the earliest-branching clade of modern birds (Burish et al., 2004). Tinamous are the closest volant relatives to the ratites, and together, these form Palaeognathae. All other extant birds are called neognaths. Please note that in this paper, the term non-avian dinosaur is used preferentially to just dinosaur because birds are dinosaurs. Palaeognaths may have a similar shape of the brain to that of neognaths, but they have smaller brains compared to their body size (Iwaniuk & Nelson, 2003). Unfortunately, it is difficult to determine the significance of this difference, because most palaeognaths are so large, and the smallest, kiwis, are highly specialized (Corfield, Wild, Cowan, Parsons, & Kubke, 2008), which can confound conclusions (Iwaniuk & Nelson, 2003). All birds’ brains are fairly easy to study from skeletal remains, because birds, like mammals, fill most of their braincase with actual brain (Emery & Clayton, 2005), and so their endocasts tend to faithfully reflect basic brain structure. This is not true for most non-avian dinosaurs. Non-avian dinosaur brains are often a challenge to study because they usually do not fill their endocranial space with neural tissue, such that, aside from the theropod lineage (Evans, 2005), the endocast can have a rather ambiguous, amorphous shape. Much of the empty space in the skull 7 not occupied by the brain is instead home to what is called cerebrospinal fluid (Evans, 2005). This makes it difficult to infer what the actual brain looked like, whereas in birds and mammals, it is relatively straightforward. Non-avian theropods in general tend to have filled the endocranial volume with neural structure, which is consistent with the theory that this group gave rise to modern birds (Witmer, Chatterjee, Franzosa, & Rowe, 2003). Non-avian dinosaurs fall between more basal reptiles and extant avians on the phylogenetic tree, a situation referred to as the “extant phylogenetic bracket” (Witmer, 1995, p. 19) of birds and crocodilians. The bird brain is thought to be similar yet uniquely derived from the basal reptilian brain (Cragie, 1940). More exploration into both bird and crocodilian brains and endocasts would optimally help us understand non-avian dinosaurs. When studying the brain, it is important to remember the principle of proper mass (Jerison, 1973). This rule states that size indeed matters for the processing load of a given brain structure. The more space devoted to a region of the brain, the more work that structure is doing, which indicates that the function accomplished by this region must be more important. (Jerison, 1973). For example, the importance of sight in most birds is reflected by their enlarged optic tecta. Endocasts as a Tool for Study. Endocasts are the main scientific approach to understanding non-avian dinosaur brains. An endocast is the volume that can be discerned from the bony boundaries of the braincase in an organism. Scientists want to learn about the non-avian dinosaur brain, but we have only their endocasts to study. This obstacle is the reason for creating endocasts for the ostriches in the first place. If 8 we merely wished to study extant organisms, scientists would preferentially look at the actual brain, but the endocasts of dinosaurs are the only available indications of their brain anatomy. Therefore, when comparing birds to extinct dinosaurs, the endocast is actually more appropriate for comparative analysis. Obtaining endocasts in the past was achieved by creating rubber molds of the braincases of the skulls of specimens, a practice
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