Dinosaur paleohistology: review, trends and new avenues of investigation

In the mid-19th century, the discovery that bone microstructure in fossils could be preserved with fidelity provided a new avenue for understanding the evolution, function, and physiology of long extinct organisms. This resulted in the establishment of paleohistology as a subdiscipline of vertebrate paleontology, which has contributed greatly to our current understanding of dinosaurs as living organisms. Dinosaurs are part of a larger group of reptiles, the Archosauria, of which there are only two surviving lineages, crocodilians and birds. The goal of this review is to document progress in the field of archosaur paleohistology, focusing in particular on the Dinosauria. We briefly review the “growth age” of dinosaur histology, which has encompassed new and varied directions since its emergence in the 1950s, resulting in a shift in the scientific perception of non-avian dinosaurs from “sluggish” reptiles to fast-growing animals with relatively high metabolic rates. However, fundamental changes in growth occurred within the sister clade Aves, and we discuss this major evolutionary transition as elucidated by histology. We then review recent innovations in the field, demonstrating how paleohistology has changed and expanded to address a diversity of non-growth related questions. For example, dinosaur skull histology has elucidated the formation of curious cranial tissues (e.g., “metaplastic” tissues), and helped to clarify the evolution and function of oral adaptations, such as the dental batteries of duck-billed dinosaurs. Lastly, we discuss the development of novel techniques with which to investigate not only the skeletal tissues of dinosaurs, but also less-studied soft-tissues, through molecular paleontology and paleohistochemistry—recently developed branches of paleohistology—and the future potential of these methods to further explore fossilized tissues. We suggest that the combination of histological and molecular methods holds great potential for examining the preserved tissues of dinosaurs, basal birds, and their extant relatives. This review demonstrates the importance of traditional bone paleohistology, but also highlights the need for innovation and new analytical directions to improve and broaden the utility of paleohistology, in the pursuit of more diverse, highly specific, and sensitive methods with which to further investigate important paleontological questions.

Molecular Paleontology and Biochemical Evolution

Carl Woese’s drive for a unified system of biological classification didn’t just open the microbial world to exploration: it reshuffled the entire taxonomic system and revolutionized the way that biologists study evolution, reigniting interest in preanimal evolution. Studies of evolution from the mid-nineteenth through most of the twentieth century relied on the comparison of forms in living and fossil organisms and were limited to the complex multicellular organisms that developed over the past 550 million years. In other words, much was known about the evolution of animals and land plants that left distinctive hard fossils, and very little was known about the unicellular algae and microorganisms that occupied the seas for most of the earth’s history. Woese’s Tree of Life, derived from nucleic acid sequences in ribosomal RNA, has revealed ancestral relationships that form and function don’t even hint at, allowing biologists to look beyond the rise of multicellular life and link it with less differentiated, more primal forms—which was precisely Woese’s intention. But evolution is a history, not just a family tree of relationships. If the information stored in the genes of extant organisms is to provide true insight into that history, it needs to be anchored in time, linked to extinct organisms and to past environments. Ultimately, we must look to the record in the rocks and sediments, just as paleontologists and biologists have been doing for the past two centuries. In Darwin’s time, that record comprised rocks from the past 550 million years, a span of time that geologists now call the Phanerozoic eon, based on Greek words meaning visible or evident life. The eon began with the rocks of the Cambrian period, in which nineteenth- and early-twentieth-century paleontologists discovered a fabulous assortment of fossils—traces of trilobites, anemones, shrimp, and other multicellular animals that were completely missing from any of the earlier strata. Thousands of new animals and plants, including representatives of almost all contemporary groups, as well as hundreds of now-extinct ones, appeared so suddenly between 542 and 530 million years ago that paleontologists refer to the phenomenon as the Cambrian “explosion.”