Bone Histology in Synapsids and Archosaurs

WHY study the microstructure of fossil and recent bone?

ALTHOUGH the fossil record of the synapsid forebears of mammals and early archosauromorphs reveals the morphological adaptations of some of the world’s first terrestrial athletes, surprisingly little is known about the diversity of growth patterns and physiology of early tetrapods outside of the dinosaurian clade. The microscopic study of bone and other fossilized hard tissues (or “paleohistology”) is an extremely useful tool for reconstructing the biology of extinct tetrapods and other organisms, as variation in the accretion of hard skeletal tissues provides insights into growth, physiology, ecology, and past environmental conditions. In behaving this way, the fine structure of bone is like a black box into past environments and ecosystems, as well as the ecology and physiology of the organisms under study. Below are some highlights of recent Paleohistology Projects:

Growth in the ancestors of mammals before and after the Permo-Triassic mass extinction

This work, the subject of my PhD research (2008-2013), examines the relationship between major environmental changes in Earth’s history and growth patterns recorded in the bones of the therocephalian and cynodont predecessors of mammals (members of a larger group called ‘therapsids’). The goal is to better understand the long term effects of environmental instability on the evolution of vertebrate life histories. Continued work with Jennifer Botha-Brink (National Museum, Bloemfontein) aims to elucidate broader scale patterns shared between therocephalians and cynodonts, as well as other therapsids and archosauromorphs, in order to identify whether certain growth tactics may have conferred an advantage on some clades in the harsh, unpredictable conditions of the earliest Triassic following the extinction (~252 million years ago). Collaborator: Jennifer Botha-Brink, National Museum, Bloemfontein.


Linearly polarized light (LPL) microscopy of the humerus of the earliest Triassic therocephalian Tetracynodon, showing bright streaks of parallel-fibered bone and an outer growth mark indicating yearly cessation of growth. (from Sigurdsen, Huttenlocker & al., 2012, J. of Vert. Paleont.)

Comparative osteohistology and skeletochronology in Permo-Carboniferous pelycosaur-grade synapsids

At first glance, early pelycosaur-grade synapsids appear fundamentally ‘reptilian’ in their overall skeletal organization. However, whereas early surveys of their bone microstructure suggested a prevalence of slow, cyclic growth, more recent histological sampling hints at a hidden diversity of life habits, skeletal mechanics, and growth strategies, including asymptotic growth in at least some ‘pelycosaurs.’ As an extension of my Master’s thesis work (2006-2008), this research explores the hidden histological diversity of one of the first amniote groups that dominated terrestrial communities during the late Paleozoic (ca. 300 Mya), and eventually gave rise to mammals. I am currently pursuing projects on the limb bone histology of the crested Sphenacodon and varanopid synapsids from the Permian of Oklahoma. Collaborators: Elizabeth Rega, Western University of Health Sciences; Christen Shelton, University of Cape Town; Stuart Sumida, California State University, San Bernardino.

Histology of the sail-supporting ‘spines’ in Dimetrodon and other fossil tetrapods

Sail-backed tetrapods such as the synapsid Dimetrodon and the famous dinosaur Spinosaurus are iconic symbols of vertebrate prehistory. But how did sails arise in such disparate groups, and at different times in Earth’s history? While their functions remain controversial (e.g., temperature regulation, sexual display, or species recognition), their structural mechanics, development, and evolutionary histories are revealed through their bone microstructure. This research, the subject of my Master’s work with Beth Rega and Stuart Sumida, explores the development of hyperelongate neural spines in sail-backed vertebrates, their soft-tissue correlates, and emphasizes how distantly related animals have solved similar biomechanical problems through different microarchitectural solutions. Collaborators: Elizabeth Rega, Western University of Health Sciences; Stuart Sumida, California State University, San Bernardino; Sterling Nesbitt, Virginia Tech.

Selected Publications on Paleohistology:

Huttenlocker, A. K., and C. G. Farmer. 2016 (for 2017). Bone microvasculature tracks red blood cell size diminution in Triassic mammal and dinosaur forerunners. For Current Biology 27. doi:10.1016/j.cub.2016.10.012

Botha-Brink, J., D. Codron, A. K. Huttenlocker, K. D. Angielczyk, and M. Ruta. 2016. Breeding young as a survival strategy during Earth’s greatest mass extinction. Scientific Reports 6:srep24053. doi:10.1038/srep24053

Huttenlocker, A. K., and J. Botha-Brink. 2014. Bone microstructure and the evolution of growth patterns in Permo-Triassic therocephalians (Amniota, Therapsida) of South Africa. PeerJ 2:e325. doi:10.7717/peerj.325

Huttenlocker, A. K. 2014. Body size reductions in nonmammalian eutheriodont therapsids (Synapsida) during the end-Permian mass extinction. PLOS ONE 9:e87553. doi: 10.1371/journal.pone.0087553

Huttenlocker, A. K., and J. Botha-Brink. 2013. Body size and growth patterns in the therocephalian Moschorhinus (Therapsida) before and after the end-Permian extinction in South Africa. Paleobiology 39:253–277. doi:10.1666/12020

Huttenlocker, A. K., H. N. Woodward, and B. K. Hall. 2013. Chapter 1: Biology of Bone. In Bone Histology of Fossil Tetrapods: Issues, Methods, and Databases – Eds., K. Padian and E.-T. Lamm. University of California Press.

Lee, A. H., Huttenlocker, A. K., K. Padian, and H. N. Woodward. 2013. Chapter 8: Analysis of Growth Rates. In Bone Histology of Fossil Tetrapods: Issues, Methods, and Databases – Eds., K. Padian and E.-T. Lamm. University of California Press.

Rega, E. K. Noriega, S. S. Sumida, A. Huttenlocker, A. Lee, and Brett Kennedy. 2012. Healed fractures in the neural spines of an associated skeleton of Dimetrodon: Implications for dorsal sail morphology and function. Fieldiana: Life and Earth Sciences (2012):104–111.

Sigurdsen, T., A. K. Huttenlocker, S. P. Modesto, T. Rowe, and R. Damiani. 2012. Reassessment of the morphology and paleobiology of the therocephalian Tetracynodon darti (Therapsida), and the phylogenetic relationships of Baurioidea. Journal of Vertebrate Paleontology 32:1113–1134. doi:10.1080/02724634.2012.688693

Huttenlocker, A. K. & E. Rega. 2012. Chapter 4: The paleobiology and bone microstructure of pelycosaurian-grade synapsids. Pp. 90-119 in A. Chinsamy-Turan (ed.) The Forerunners of Mammals: Radiation, Histology and Biology. Indiana University Press.

Huttenlocker, A. K., D. Mazierski, & R. Reisz. 2011. Comparative osteohistology of hyperelongate neural spines in the Edaphosauridae (Amniota: Synapsida). Palaeontology 54:573-590.

Huttenlocker, A. K., E. Rega, & S. Sumida. 2010. Comparative anatomy and osteohistology of hyperelongate neural spines in the sphenacodontids Sphenacodon and Dimetrodon (Amniota: Synapsida). Journal of Morphology 271:1407-1421.


The success of dinosaurs in the Triassic is often credited to their specialized cardiopulmonary system and high growth rates. However, my research emphasizes the equally sophisticated vascular systems and rapid, attenuated growth durations of coeval Permian-Triassic therapsids (the forebears of mammals) using bone histologic techniques. My lab is expanding fossil data sets in other groups (e.g., dinosaurs and earlier archosauromorphs) to apply large-scale phylogenetic comparative methods, allowing paleontologists to develop models explaining whether histologic traits predict selective advantage in some groups (e.g. Scientific Reports, 2016)