By Peter Makovicky, Field Museum
Peter Makovicky (Curator, Earth Sciences) coauthored a paper with Field Museum Research Associates Terry “Bucky” Gates (lead author) and Eric Gorscak on the chondrichthyan (sharks and rays) fauna of the SUE locality in the Journal of Paleontology. Based on fossil teeth and fin spines, the authors showed that at least one species of ray and three species of shark lived in SUE’s environment. One of these sharks represents a new species of carpet shark, the group that includes bottom dwelling species like wobbegongs and bamboo sharks, but also the pelagic whale shark.
The new discovery owes much to the foresight of Bill Simpson, the Museum’s Collection Manager of Fossil Vertebrates and leader of the team that prepared SUE, who saved the sediment that accumulated during the preparation process. About a decade later, Terry “Bucky” Gates joined the Makovicky lab as a research associate and began a project to screen the sediments to look for microvertebrate fossils – teeth, scales, and bones of small animals – to build a better picture of what lived in the environment with SUE. This involved suspending the sediment in water and then running the slurry through a set of connected sieves, each with a finer mesh than the preceding one. Once the sediment was screened and the sand and silt washed away, the remaining concentrate was carefully picked through. Karen Nordquist, a steadfast and dedicated Field Museum volunteer for over three decades, spent hours peering through a microscope meticulously picking tiny fossils out of screen-washed sediments, including more than two dozen minute teeth that turned out to be a new species of carpet shark. The teeth are tiny – only a millimeter across and have an unusual shape with three unequal points and a wide apron at the root perforated by small canal. Some of the teeth bear an uncanny resemblance to the spaceship in the 80’s arcade game Galaga, which inspired the genus name, while Karen’s crucial role in this discovery is honored by having the species named for her: Galagadon nordquistae. Galagadon would likely have been a very small, bottom-dwelling shark between one and two feet long with barbels by its mouth and a diet of small invertebrates. Any interactions with T. rex were likely incidental and more likely than not, the two species would have been largely oblivious of each other.
The classification of fossil shark teeth has long been dominated by traditional taxonomic practices (i.e. expert opinion). The authors took a more modern approach to deciphering the relationships of Galagadon by expanding a cladistic study of living carpet sharks to include several Cretaceous species. Depending on which phylogenetic methodology was employed, Galagadon is either a member of the wobbegongs, or an archaic relative of living bamboo sharks, although the latter result is more consistent with molecular results which find living wobbegongs to represent a more recent radiation.
All living carpet sharks are marine and most, including wobbegongs and bamboo sharks, live in the Indopacific region. So finding a carpet shark in the SUE quarry shows not only that ancient carpet sharks could live at least intermittently in fresh water habitats, but that they also had a different geographic distribution in the Mesozoic. The authors used current model-based methods to examine how the evolutionary history of carpet sharks may explain biogeographic distribution of Galagadon and other fossil and living carpet sharks. While the most favored results suggest that random, long range dispersals could account for the patchy distribution of these sharks through space and time (i.e. a ‘Sharknado’ effect), this runs counter to paleogeography and aspects of the shark fossil record. Rather, as supported by the next-most favored results, carpet sharks were likely globally widespread when warm, shallow seas covered parts of many continents in the hot-house world of the Cretaceous and Paleogene, and their present day range represents a relict of that range. This is a wonderful case study of how fossils can provide a fuller picture of how the evolutionary history of living lineages has changed dramatically over the course of Earth history.
A tooth of Galagadon is on display in the new SUE hall, but bring a loupe – it’s the smallest fossil there. And if you see Karen, please congratulate her on the species named for her and thank her for all that she does for the Museum!
To learn more:
Fossil shark teeth discovered in leftover rock that contained SUE the T. rex
Prehistoric Shark Named After Videogame (and volunteer)