Fossil of the month: Saivodus sp.
This month’s fossil is the tooth of an extinct shark called Saivodus. Because shark skeletons are composed of cartilage, rather than bone, their teeth are often the only hard parts which remain as fossils. Recently, however, rare soft parts of this shark were discovered in Mammoth Cave!
Description. Saivodus is a genus of fossil shark teeth, and also the name of the ancient shark with those teeth. Saivodus is a relatively new fossil name. The genus is one of several genera that have been split from the shark tooth genus Cladodus (Duffin and Ginter, 2006). Teeth genera based on Cladodus are termed cladodont teeth. Cladodont teeth have multiple prongs (called cusps) on a broad basal platform. To those not familiar with fossil shark teeth, the individual cusps may look like separate teeth, but all of the cusps are part of a single, multi-pronged tooth.
Different views of Saivodus striatus tooth from the Poppin Rock Member of the Slade Formation in south-central Kentucky. Specimen no. KGS1003 from the KGS paleontological collection. Different-sized arrows on tooth views represent larger cusps and smaller cusplets.
Saivodus teeth have a long (as much as 6 cm), slim median cusp on the inner (lingual, mouth) side of a relatively thin, broad platform with many, much smaller cusps. The main central cusp is somewhat S-shaped (sigmoidal) in side view, and ornamented (at least on its lower part) with long thin lines (called cristae). The tooth has smaller lateral cusps on the outer edge of its tooth base (on either side of the medial cusp), and many smaller intermediate cusplets between the main median cusp and the lateral cusps. The tooth base itself has a trapezoidal outline when viewed from above or below. The front (labial side) of the tooth base is sharp. The broader part of the base, is on the back (inner mouth, oral, lingual) side. The bottom surface of the tooth base usually has two small ridges, or thickenings, separated by a depression beneath the medial cusp,, which are distinctive for the genus (Duffin and Ginter, 2006).
Several different types of cladodont teeth have been found in Kentucky and could be confused with Saivodus, especially when cusps on the teeth are broken or the teeth are only partially exposed in rock. Cladodus (revised, limited definition), Glikmanius, and Stethacanthus, all have similar ornamentation on their medial cusp. Saivodus can be distinguished from these other types of cladodont teeth by its relatively long, slightly tubular, medial cusp, and the large number of intermediate cusplets on either side of the medial cusp. The long medial cusp is rounded outward (convex) on both the inner and outer side (termed biconvex). Many cladodont teeth are only convex on the inner side and are flattened on the outer side. Details of its basal platform also differ from other cladodonts (Duffin and Ginter, 2006). Stenacanthus is perhaps most similar in appearance to Saivodus, because it also has a tall, slender, biconvex medial cusp but Stenacanthus has far fewer lateral cusps and no cusplets, and different features on its tooth base(Lund, 1985; Duffin and Ginter, 2006).
Comparison of some cladodont teeth genera reported from Kentucky. See Duffin and Ginter (Table 2, 2006) for more detailed comparisons. Cladodus and Saivodus tooth illustrations based on images in Duffin and Ginter (2006); Ctenacanthus from images in Shaffer and Williams (1977); Glikmanius from images in Ginter and others (2005); Stethacanthus from images in Lund (1985).
Saivodus striatus is the only current species of the genus Saivodus. At least 13 previous species of Cladodus, including Cladodus striatus, are now placed in Saivodus striatus. Most of the specimens listed as Cladodus sp. or Cladodus grandis in the KGS collection, appear to be Saivodus striatus or Saivodus? (possible or likely Saivodus, but not definitively Saivodus). Cladodus remains a valid genus, and at least one species reported in Kentucky remains in Cladodus, C. bellifer.
Examples of fossil Saivodus from the Poppin Rock Member of the Slade Formation in south-central Kentucky, KGS paleontological collection. The back side of the basal platform is broken in KGS 1002, but the tall medial cusp, cristae, and rounded edge of the front of the basal platform suggest it is possibly Saivodus. The other examples all have some lateral cusps and cusplets, and a pair of ridges on the underside of the basal platform distinctive for the species Saivodus striatus. The circular features on the underside of specimen 1005B are small Orbiculoidea brachiopods. They must have attached to the tooth when it was lying on the sea floor, before they were both buried.
Recent finds at Mammoth Cave
In 2019, a series of discoveries, including one in the wall of one hard-to-reach passage in Mammoth Cave, uncovered rare soft parts of a large fossil shark. While several shark teeth and spines have been reported from Mammoth Cave, the fossil is still being studied and results to date are tentative. The National Park Service reports that the fossil is part of a Saivodus striatus shark. The remains include part of the lower jaw cartilage. The lower jaw appears to be at least 2.5 ft long, which would make this a sizeable shark, perhaps more than 15 ft in length. In addition, teeth of several smaller sharks and an eel-like fish were found with the specimen, suggesting these other fish were feeding on the carcass of the larger shark. More information about the Mammoth Cave discoveries, a 3-D image of the fossil in the cave wall, and a beautiful painting of the Saivodus shark on the sea floor with other sharks and creatures are available on the Mammoth Cave discoveries website.Recently, the park also released some details of another cladodont shark fossil, Glikmanius sp., which also had soft parts preserved!
Saivodus striatus occurs in upper Middle to Upper Mississippian strata in North America, Europe, and northern Africa (Duffin and Ginter, 2006). In Kentucky, Saivodus striatus has been reported from the Ste. Genevieve Limestone and the Poppin Rock Member of the Slade Formation (a Glen Dean Limestone equivalent). These are upper Middle to Upper Mississippian (Chesterian series) units. The species has been reported from the slightly older, Middle Mississippian Salem Limestone in Indiana (Duffin and Ginter, 2006). The Ste. Genevieve Limestone occurs throughout the Mississippian Plateaus region of Kentucky in at least 24 counties. It is the rock unit in which many of Kentucky’s better-known caves are formed. The Ste. Genevieve Limestone is approximately 333-334 million years old. The Glen Dean Limestone also occurs in the Mississippian Plateaus region but it occurs closer to the boundary with both of Kentucky’s coal fields. The equivalent Poppin Rock Member of the Slade Formation occurs in south-central Kentucky (where it was previously called the Bangor Limestone) and along the Cumberland Escarpment from Greenup to Wayne counties (Ettensohn and others, 1984). The Poppin Rock Member occurs in at least 14 counties. It is slightly younger than the Ste. Genevieve Limestone, and is 329-330 million years old.
Examples of Saivodus striatus in which the tall, median cusp is broken. In some specimens, (1) lateral cusps are still visible or (2) a f thickened ridge or shelf separated by a depression beneath the medial cusp is visible on the underside of the basal platform, which can still be used to identify the species. Specimens are from the KGS paleontological collection and were collected from the Poppin Rock Member in south-central Kentucky.
During much of the Mississippian Period, Kentucky was covered by shallow seas. During the Late Mississippian the area that is now Kentucky was closer to the equator than it is today (e.g., Ettensohn and others, 1984; Sable and Dever, 1990). Both the Ste. Genevieve Limestone and Glen Dean Limestone were deposited in shallow tropical seas. The Ste. Genevieve and Glen Dean Limestones both contain many different kinds of marine fossils, including fenestrate bryozoans, crinoids, blastoids, and brachiopods. Saivodus shared the seas with these creatures.
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Text, photographs, and illustrations by Stephen Greb