The great vertebrate radiations are characterized by distinctive suites of morphological specializations, some of which appear to be key innovations that helped to drive their success. I document the deep history of these characters and illuminate the developmental mechanisms underlying their origins. To accomplish these goals, I employ a straightforward but powerful research program:
An increasing number of paleontological research groups use steps 1 and 2 in their work, but step 3 is much less common as it requires core skills and facilities for developmental biology, and step 4, which completes the sequence and closes the circle, is thus far unique to my work.
The origin of the bird skull: My dissertation focuses on the evolution and development of the unique cranial characteristics of birds, notably the enlarged toothless premaxilla that forms the beak, the reduction of the maxilla, and the enlarged braincase. The first part of this work is published in Nature (Bhullar et al., 2012) and posits that many of the changes toward the avian lineage owe to the global developmental mechanism of progenetic paedomorphosis, a process by which, through a truncation of growth, descendants resemble the juveniles of their ancestors.
The second two parts of this work, for which all major data have been collected and which are being finalized for submission, successfully posit and test molecular mechanisms for the evolution of the avian beak and for the reduction of the maxillary region, using the sequence outlined above. Outgroup taxa used for gene expression analysis include alligators and lizards.
Another ongoing project that uses a variety of taxa and approaches has elucidated a mechanism for tooth loss and the transformation of tooth-bearing skeletal elements in birds. It is supplemented by exclusive access to high-resolution CT scans of important stem-group toothed birds. Finally, I have begun a joint morphological and developmental analysis of the unique transformations of the braincase in birds that has yielded some startling results with implications for brain-bone interaction, developmental lability, and homology in the cranial vault.
The origin of the mammal skull and the amphibian/amniote split: In collaboration with Fuzz Crompton at Harvard, Zhe-Xi Luo of the University of Chicago, and Timothy Rowe of The University of Texas at Austin, I have begun to investigate two major mammalian innovations – the braincase wall and the unique mammalian middle ear. The morphological component of this work includes an intensive CT-based examination of braincase and middle ear morphology and its association with brain enlargement in exceptionally preserved early mammals, including many specimens from the Museum of Comparative Zoology at Harvard. Molecular testing of hypotheses will follow. Additionally, the MCZ holds the best record of the steps leading to the unique architectures of the amphibian and amniote skulls, which have profound effects on the diversification of these major tetrapod clades. I have begun a collaboration with members of the Hanken lab to examine the amphibian/amniote split and plan to follow up with molecular analyses of early facial patterning.
Evolutionary and developmental origins of major functional innovations: The last frontier in understanding macroevolutionary patterns of character evolution is to integrate transitions in functional morphology with their structural and developmental underpinnings. For several years I have been leading a project on locomotor evolution at the origin of the crocodile lineage, centered on three-dimensional reconstruction of the first complete early stem-group crocodile (“sphenosuchian”) skeleton ever found, an undescribed specimen from the Yale collections. Also included in the analysis are crucial taxa from higher in the crocodilian stem, and data from extant alligators and crocodiles. This project is in close collaboration with John Hutchinson at the Royal Veterinary College Structure & Motion Lab, and with his student Julia Molnar, who performed segmentation and imaging for recently published work on the locomotion of the early stem tetrapod Ichthyostega. Several limb specializations of crocodilians represent important correlated anatomical features, and our lab already has many genes cloned from Alligator that can be used to analyze the development of these specializations.
The fossil record at the origin of major radiations: My purely paleontological work increasingly focuses on evolution and phylogeny at the base of vertebrate radiations. In particular, I am interested in character evolution, macroevolutionary patterns, and reconciling morphological and molecular phylogenetic signal. I have published on turtle origins, including the first morphological support for the phylogenetic position of turtles as sister to archosaurs as suggested by molecules (Bhullar and Bever, 2009). My investigation of squamate abdominal musculature as a non-traditional source of morphological characters also resulted in some support for the otherwise counterintuitive molecular squamate phylogeny (Bhullar, 2009). Recently, I worked with Yale coauthors Nick Longrich and Jacques Gauthier to describe the most basal known stem-group snake, which presents a transitional morphology including a snake-like body and a partially lizard-like skull (Longrich, Bhullar, and Gauthier, 2012, Nature). Our team later published an extensive study that used an interdisciplinary approach to elucidate the radiation of lizards and snakes before and after the end-Cretaceous extinction, demonstrating the dramatic effects that extinction had on squamate diversity and on the progenitors of the extant squamate fauna (Longrich, Bhullar, and Gauthier, 2012, PNAS). The detailed phylogenetic analysis using fragmentary remains built in part on prior work I had done on phylogenetic signal in fragmentary small vertebrate fossils (Bhullar and Smith, 2008; Smith et al., 2008) and on the radiation of Xenosauridae, a smaller lizard clade with a deep fossil history crossing the end-Cretaceous extinction (Bhullar, 2011; Bhullar, 2010).
Using the fossil record to address ontogeny, variation, and homoplasy: In keeping with my interest in development and paleontology I emphasized in a Journal of Experimental Zoology paper (Bhullar, 2012) the importance of a phylogenetic perspective bracketed by extant taxa when attempting to describe ontogenetic transformations in the fossil record, using as an example two unusually small fossil platynotan squamate specimens, one of which I showed to be a juvenile and the other a proportionally dwarfed adult.
I am also leading an effort, in collaboration with Gabe Bever of NYCOM, Tyler Lyson of the Smithsonian, and Günter Wagner and Jacques Gauthier of Yale, to trace the influence of variation and variability on concentrated homoplasy at the base of major clades. We have collections that appear to bridge the gap between population-level intraspecies variation and macroevolutionary change in key characters at the origin of major vertebrate clades.
New fossil discoveries targeted at the origins of major radiations: Two collaborative field programs of mine target exceptionally preserved small vertebrate material that captures major transitions in vertebrate evolution. The first is a program that I lead in collaboration with my colleague Gabe Bever in the Jurassic of the western United States. We have recovered exquisitely preserved small vertebrate material pertinent to sphenodontian evolution, the earliest squamate radiations, amphibian evolution at the base of the crown-group clades, early crocodile-lineage evolution, and mammalian evolution. This material is included in some of the morphological and developmental analyses described previously.
The second, spearheaded by my colleague Patrick Lewis of Sam Houston State University, has recovered exquisitely preserved small vertebrate fossils from South Africa. They date to just after the Permo-Triassic extinction, when the stem lineages of the major amniote and amphibian clades were beginning to radiate. The site has produced early archosaurian and possible lissamphibian material. Also among these sediments may be the earliest relatives of turtles,
I have been lucky enough to teach Evolutionary Human Anatomy and Physiology with George Lauder, Andy Biewener, and Dan Lieberman, and, alongside fellow teaching fellow Liz Sefton, to help develop and teach the lab for the new course Vertebrate Evolution and Development with Farish Jenkins and Arhat Abzhanov. Teaching is a great joy of mine and I was honored to receive the Harvard Derek Bok Award for Excellence in Graduate Student Teaching of Undergraduates, one of five awarded across the university, in 2010.
In addition to my teaching, I have mentored several undergraduate and graduate students during various research projects. Alicia Kennedy, with whom I have coauthored several talks and papers, is one of these students. Fernando Racimo, a Harvard undergraduate, is third author on the 2012 Nature paper on bird skulls.