“I see dead sharks" - how fossil shark research makes us better shark researchers
Fossil sharks are cool and fascinating. Some species are very similar to what we see in the modern ocean but the others are so different and bizzare. Helicoprion* is only known from fossils of its distinctive tooth whorl (have you seen the photo? It’s so bizarre), Ptychodus has weird button-like teeth with wavy ornamentation on the surface (how were you using such teeth, mate?) and Squalicorax has intricate and beautiful heart-shaped serrated teeth (similar to modern tiger shark teeth).
I was lucky enough to do research on fossils during my undergraduate studies; I was at a Geology and Paleontology Lab at my third and fourth year of university in Tokyo, where I was working on shark fossil research for my graduation thesis. My supervisor is one of the most famous plesiosaur researchers in the world and the lab specialises in research on vertebrate palaeontology, which literally is the study of fossil or ancient vertebrates, such as fish, turtles, mosasaurs, crocodiles, dinosaurs, birds and seals (all of these are studied in our lab). There is a wide range of study topics in palaeontological research, such as taxonomy, biomechanics, taphonomy (the study of fossilisation, the process of how fossils were made in palaeo-environment), histology and reconstruction of palaeo-environment and palaeo-climate.
Most shark fossil specimens are isolated teeth because prehistoric shark skeletons were also made of cartilage that is easily decomposed and rarely mineralised and fossilised, unlike dinosaurs and teleost fish that had calcified bones. Except for some rare cases, where vertebrae, jaws and fins were luckily preserved, mineralised teeth are the only clue for us to reveal the mystery of extinct shark species. In my research (and probably in the most cases of palaeontological research), morphology, cladistics, and biomechanics are the central part of study. For shark fossils, we are often looking at tiny fragmented teeth (not always, if your specimen is megalodon!), and eventually you realise that you are studying TEETH rather than SHARKS (from my experience, and my supervisor said “you are on the right track”).
The followings are basic steps I took in my shark fossil research:
Take photos of all specimens to make a specimen catalog, and sketch each specimen to see the details
Read research papers that reported the specimen of the same or closely related groups (at species or genus level) to check diagnostic features of holotypes, or to compare my specimens with previously known specimens. Finding diagnostic notes is the key; the very first papers describing a new species (holotype specimens are used to formally describe and document a new species) are often very old, like 1800’s. Hopefully they are written in English, but in many cases the papers are in German, French, or Russian. Finding and reading the first record of new species diagnosis and the holotype information is often the hardest part.
If possible, go on field trips for data collection for both for geological data (settings, matrix, location) and additional fossil specimens (both sharks and associated fossils to determine ages and to guess palaeo-environment) (Fig. 1).
Do fossil preparation using air scribes and grinders. The difficulty of fossil preparation depends on the fossil’s shape and size, and the texture of sediment.
Explore museums to find stored fossil specimens for comparison with my samples, took pictures and collected data (size, age, texture, overall arrangement of teeth if there is a series of teeth etc.) for further comparison.
Meanwhile, keep reading reports and papers again and again (this is never ending!)
Search for discussion topics i.e. reading research papers and reports for discussion. In my case, I was: (1) reviewing literature on one group that I was interested in and I thought a review of the occurrence/reports would be required, and (2) comparing fossil thresher shark teeth with modern thresher shark teeth.
Fig. 1: Field data collection at the outcrop. My research collaborators are looking for fossils from the deposit (light colour conglomerate) while I was taking a picture
I will introduce you my actual work published back in 2017. It was from my thesis and about the occurrence of Sphenodus in Hokkaido, northmost part of Japan from the Late Cretaceous deposit (70-80 million years ago, mya) and their possible extinction scenario. Sphenodus was present between Jurassic and Paleogene (190~56 mya). They had weird teeth with a very broad root with a fang-like, tall cusp (Fig. 2). Because of its shape, the teeth are very fragile, and few complete specimens have been found across the world; most specimens are either only the crown or root only as they are readily broken at the crown-root junction. This tooth was supposed to function as a “fork” (a piercing type tooth), which is good at stabbing a relatively small-sized food, probably a similar function to modern mako shark teeth.
Fig. 2 Tooth of Sphenodus (Kanno et al. 2017)
Our research reported the first complete specimen from Circum-Pacific regions and reviewed the global occurrence of Sphenodus. The distribution of Sphenodus seemed to be concentrated in the mid- to high palaeo-latitude (note: Earth’s magnetism changed through time and latitude and longitude in the past were also different from those in the modern world, called palaeo-latitude/paleo-longitude) (Fig. 3). Such distribution patterns might indicate Sphenodus’s cold water preference, which may also explain why this group went extinct during the Paleocene-Eocene boundary as mid- and high palaeo-latitudinal regions experienced ocean warming around that time.
Fig.3: Palaeogeographical distribution of Sphenodus (revised from Kanno et al. (2017))
Shark fossil research is quite different from my current (ecology-based) PhD project that I am currently working on at the F&F Lab. You need knowledge of both geology and biology, particularly morphology and cladistics. I really appreciate the opportunities to have learnt different types of knowledge, skills and research for my shark research, as I can see (palaeo-)ecological, morphological perspectives with a proper cladistic background applying to all kinds of modern shark research. It is really nice to have multiple research projects from different disciplines so that you can widen your knowledge and interests. While I am currently working with living sharks and stingays for my PhD, I am also collaborating with my supervisor and other researchers for another project on fossil sharks. Stay tuned!
My publication is found here:
Kanno S., Nakajima Y., Hikida Y. and Sato T. (2017). Sphenodus (Chondrichthyes, Neoselachii) from the Upper Cretaceous in Nakagawa Town, Hokkaido, Japan. Paleontological Research 21(2): 122-130
Blog post cover photo - Helicoprion* whorl (wikkicomons) - James St. John, CC BY 2.0 <https://creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
* DID YOU KNOW . . . . Helicoprion is a Chondrichthyan but it is technically not a "shark" as this genus is now assigned to Eugenodontiformes, which is different from the modern shark group Neoselachii.