Figure 1. Types of shark materials that are fossilized include (A) teeth (IUPC S-2138; shark tooth; Miocene epoch, Calvert Formation; Calvert Cliffs, MD), (B) spines (IUPC 13996, Ctenacanthus sp. spine in matrix (spine marked by the white outline), Mississippian period, Basal Haney Limestone; Crawford Co, IN), and (C) scales (IUPC S-764, Petrodus, Mississippian period, “Indian Springs Shale”; Shoals, IN). The grids in the pictures are 1 cm x 1cm.
Chondrichthyans are a group of jawed fish whose skeleton is primarily composed of cartilage and whose teeth and scales are made of dentin (the latter specifically called denticles). Chondrichthyans are divided into two major groups: the Elasmobranchii which includes the sharks, rays, skates, and sawfish, and the Holocephali which includes the chimeras, the only representatives living today and their closest relatives from the geologic past. Chondrichthyan species have a wide variety of feeding strategies including filter-feeding, shell-cracking, and microcarnivory, and many are generalists. Chondrichthyans range in size from a few centimeters to over several meters. They swim in shallow to deep marine open ocean habitats and are frequent visitors to coral reefs where they prey on reef fishes.
Because cartilage does not preserve well, the much harder teeth, spines, and denticles of sharks are typically the parts found in the fossil record. Sharks and their relatives possibly originated in the Late Ordovician period based on the presence of fossil scales. By the Devonian period, jawed fishes, including cartilaginous fish became more common, but the cartilaginous fish were mostly generalists and not very diverse. In contrast, another group of jawed fishes from this time, the placoderms, were much more diverse and were believed to have possibly limited cartilaginous fish diversity. After the Late Devonian Mass Extinction and the extinction of the placoderms, cartilaginous fish experienced the “Golden Age of Sharks” when they diversified into many species of different shapes and sizes and occupied new roles in ecosystems, including venturing into freshwater ecosystems. Throughout the Permian period, chondrichthyan biodiversity remained relatively stable until the mass extinction event that marked the end of the Paleozoic. Known as the Great Dying, this global extinction hit marine ecosystems especially hard, including chondrichthyans, with 94% of all marine species being wiped out. Fortunately, as seen on the graph below, surviving chondrichthyans were able to radiate back to previous levels.
The Triassic and Jurassic were periods of relative stability for sharks and their relatives, adapting to the emergence of marine reptiles like ichthyosaurs, pliosaurs, and plesiosaurs. In response to these new prey items, many shark genera evolved the larger, serrated teeth mostly associated with the sharks of the present. Fossils of marine reptiles with injuries caused by shark bites (some with teeth still in the bone) are relatively common finds.
Neoselachii, the Elasmobranchii subgroup encompassing all extant sharks and rays, would evolve in the Triassic but would not have been common until the Late Jurassic when the hypodont elasmobranchids declined. A rise in sea levels during the Early Cretaceous led to an explosion of chondrichthyan biodiversity to a degree not seen since the Carboniferous because of rising sea levels and the increased number of shallow marine environments.
The K-Pg extinction 66 million years resulted in the extinction of the marine reptiles, but chondrichthyans were not as heavily impacted as other marine classes, with genera like Carcharias surviving until the present day with the sand tiger shark.
Figure 2. Diversity of chondrichthyans over time based on number of genera per stage. Cm = Cambrian, O = Ordovician, S = Silurian, D = Devonian, C = Carboniferous, P = Permian, Tr = Triassic, J = Jurassic, K = Cretaceous, Pg = Paleogene. Notice the upticks in diversity during the Carboniferous and the Cretaceous. Chondrichthyan occurrences were downloaded from the Paleobiology Database on March 11, 2025, with the following parameters: time interval = Phanerozoic, environment = marine, taxon = Chondrichthyes
Figure 3. IUPC Carcharodon specimen from the Neogene period in Florida. While this specimen is labelled as Carcharodon in the older card on top, this is no longer a valid name for this specimen. When this occurs, collections will update the name as seen in the bottom card where the specimen is now referred to as Otodus.
After surviving the K-Pg extinction, the chondrichthyans were again able to rebound to previous levels in the Paleogene period. By the Eocene epoch (56-34 million years ago), representatives of all extant shark groups are present in the fossil record. The survivors were smaller generalist sharks that were able to adapt to rapid environmental changes by preying on a variety of creatures. A gradual global cooling in the Late Eocene pushed warmer waters towards the equator, explaining a lack of near-polar chondrichthyan fossils during that time. Otodus megalodon the largest chondrichthyan known in the fossil record, evolved during the Miocene, with fossils of many ancient whales bearing tooth marks from its powerful bite. O. megalodon has tooth fossils commonly found along most coastlines globally, but the species went extinct by the end of the Pliocene epoch. The Pleistocene witnessed an exodus of many sharks from the Atlantic to the warmer Pacific and Indian oceans. Onward into the Holocene and present day, chondrichthyans have occupied stable niches in the ocean’s ecosystem and continue to play vital roles from the gargantuan filter-feeding whale shark to the tiny parasitic cookie-cutter shark. However, 22% of all shark species are now listed as either endangered or critically endangered due to a changing ocean climate and overfishing.
The land we now call Indiana has not always been dry ground. Throughout much of the Paleozoic era, Indiana was a shallow marine environment teeming with underwater life. Alongside the chondrichthyans, marine animals like brachiopods, bivalves, trilobites, crinoids, and corals were common, creating a very biodiverse ecosystem. By the Pennsylvanian period, most of the shallow seas had seen heavy sediment deposition to create the bedrock which Hoosiers stand on today. Much of this is limestone, formed from the deposition of calcium carbonate shells of marine invertebrates. Limestone, siltstone, shale, and dolostone make up much of Indiana’s bedrock and are where most of the chondrichthyan fossils in our collections were deposited before they were discovered. Some fossils like teeth, spines, and denticles (shark scales) are found free lying in loose sediment, while others are found in the matrix (the sedimentary rock formed around the fossil upon deposition).
Figure 4. A map of the Midwest representing its paleoenvironments during the Mississippian period (358.9 to 323.2 million years ago). Much of southwestern Indiana would have been underwater in the Illinois Basin during this time, explaining the plethora of chondrichthyan fossils found in the sedimentary rock deposited in the yellow areas. Source: IGWS Map
Figure 8. A map of chondrichthyan fossil occurrences in Indiana. Each colored circle on this map of Indiana represents an entry into the Paleobiology Database (PBDB) of a chondrichthyan fossil. Although not a true representation of the quantity of fossils found within Indiana, it does convey the trend of locations where chondrichthyans generally lived within the state’s borders. Each yellow circle represents a fossil shark entry. Image source (FILTERS: Chondrichthyes (class), Paleozoic).
The sharks that once swam in Indiana when it was a marine environment often looked different to the great-white and tiger sharks of today. In the Mississippian period approximately 344 million years ago Cladodus, a ctenicanthid macropredator that likely fed on fish and other sharks, was a common find. The shell-crushing holocephalan Deltodus, was reminiscent of its present-day relatives in the group Holocephalii like the chimera. Another holocephalan, Petalodus, had different shaped teeth resembling the petals of a flower that were also used in crushing the shells of prey.
Total fish fossils (chondrichthyan and non-chondrichthyan) in the Paleontology Collection at Indiana University (IUPC) number up to 211 specimens. Chondrichthyan specimens are mainly represented by teeth, but some are represented by spines and denticles. The amount of non-chondrichthyan (any fish that is not a shark) specimens total to 50 with most specimens from the Mississippian period, and the Devonian period being a close second. Ninety-six fish fossils (88 of which are definitive chondrichthyans) in the IUPC come from the Mississippian period. The total number of cartilaginous and non-cartilaginous fish specimens from Indiana is 95 specimens.
In the IUPC, the majority of the Chondrichthyes and non-Chondrichthyes specimens are mostly from the Harrodsburg Formation (also known as the Harrodsburg Limestone) in Indiana during the Mississippian Period and were collected by A.C. Brookley. There are also some chondrichthyan specimens collected by Don E. Hattin from the Mississippian Period and some specimens collected by him from outside of Indiana from the Cretaceous Period. Other collectors of shark and fish fossils include James Jesse Galloway, Thomas G. Perry, and Clyde A. Malott who provided specimens from the Mississippian period.
Description of the video:
A 3-D scan of a cladodont shark tooth from the Mississippian Beaver Bend Limestone of Indiana from the IUPC. This scan was done using CBRC’s photogrammetry equipment.
Description of the video:
A 3-D scan of IUPC 17952 cf. Deltoptychius mandibular spine section from the Mississippian Salem Limestone of Indiana. This scan was done using CBRC’s photogrammetry equipment.
Undergraduate Co-authors: Abigail Smith and Matthew Sullivan
CBRC Research Assistant: CJ Salcido
CBRC Director: Claudia Johnson
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