COLLABORATIVE RESEARCH: Integrative Approaches to the Turtle Body Plan: Evolutionary Origins of Structural Complexity in an Enigmatic Lineage

点击翻译按钮获取中文摘要

The turtle shell is truly a wonder of structural and evolutionary biology. It is a compound feature that includes a series of bones known only in turtles, but it also integrates a large number of vertebrae, ribs, and shoulder girdle elements. That the turtle scapula (shoulder blade) lies inside of the rib cage, whereas all other reptiles and mammals have the scapula on the outside of the ribs, is a clear indication that turtles represent a challenge to understand how such apparently dramatic evolutionary change occurs. Exactly how and why turtles acquired such a peculiar body plan is one of the longest standing problems in the history of comparative biology, and one that remains very much unresolved. The largest impediment to clarifying the origin of turtles has been the lack of transitional fossils linking turtles to other known reptile groups. This absence of direct evidence regarding turtle ancestry has resulted in strikingly disparate hypotheses as to where turtles sit within the reptile tree of life, what features of the modern turtle body plan evolved first, and what selective advantages these features provided turtle ancestors. This project will address these problems by coordinating novel anatomical data from recently discovered fossils, a more detailed understanding of the developmental mechanisms that regulate the production of turtle-specific traits, and the patterns of genetic sequences from a wide diversity of reptiles. The end product will be a highly refined model of turtle evolution that will not only facilitate our understanding of turtle origins but will serve as a guide for how to tackle difficult evolutionary problems across deep time. The broad appeal of turtles, and especially fossil turtles, also ensures that this project will deliver exciting educational opportunities for people of all ages and backgrounds through museum exhibits and outreach activities.The relative ease with which genomic data can now be acquired and analyzed provides unprecedented opportunities for inferring the evolution of developmental systems and their resultant phenotypes. This paradigm faces its greatest challenge where crown clades are separated by extremely long stem lineages, with one of the more famous examples being the origin of turtles and their shelled body plan. Our project builds on recent discoveries in development, systematics, and the fossil record to advance our understanding of early turtle evolution and its relevance for broad-based questions in comparative biology. The goals of the project can be summarized under four objectives: (1) use high-resolution imaging and evolutionary analyses to establish the identity, functionality, and evolutionary dynamics of character transformations in the skeletal phenotype of the turtle stem lineage based on the fossil record; (2) use densely sampled ontogenetic series of modern turtles and amniote outgroups, integrated with experimental approaches in development, to establish ancestral patterns of morphogenesis, regulation, and growth for the turtle crown and total group; (3) use diverse analytical approaches to address the phylogenetic position of Pan-Testudines among both extant and extinct amniotes and to articulate a time-calibrated evolutionary model that integrates morphological, developmental, and genetic data in describing the emergence of turtles and their characteristic features. In its pursuit to understand the basic principles that underlie the vertebrate body plan and the mechanisms through which large changes to this plan can occur, this project is highly congruent with one of NSF’s 10 Big Ideas — “Understanding the Rules of Life: Predicting Phenotype from Genotype.”This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.