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Cretodus crassidens Shark remains (FHSM VP-17575) from the Blue Hill Shale of Mitchell County, Kansas Copyright © 2010 by Mike Everhart Adjunct Curator of Paleontology, Sternberg Museum of Natural History Created 06/09/2010; Last updated 08/26/2010
LEFT: Teeth of the Cretodus crassidens (FHSM VP-17575) specimen in lingual view, collected in 2010 from the Blue Hill Shale of southwestern Mitchell County, Kansas. |
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The original description by Frederick Dixon , 1850, p 367: Genus
Oxyrhina, Agassiz.
Oxyrhina crassidens, new, R. 3. (Tab. XXXI. figs. 13 &
13".) This
large and thick tooth I discovered at Houghton; it differs from O.
Mantelli by
being shorter and more obtuse. The shoulders are deeply corrugated,
almost to give the appearance
of the lateral denticles of an Otodus. |
In March, 2010, I returned to the site where I had collected fragments of a large plesiosaur rib (Everhart, 2010) from the Blue Hill Shale in southwestern Mitchell County, Kansas. Accompanying me were Gail Pearson (who in 2008 discovered the only known skull of the turtle Desmatochelys lowii from Kansas) and Fred Smith, editor of the local newspaper. While Gail and I were collecting additional rib fragments, Fred went further upslope to the east and found what he thought was a piece of petrified wood in a concretion. We went over to see what he had discovered and I immediately recognized the circular objects with the concentric growth rings as shark vertebrae....A really, REALLY nice discovery for Fred's first time collecting in the Blue Hill Shale. Based on their large size, I suspected that they were from a large lamniform shark called Cretodus, but without a tooth to confirm the identification, there was no way to be certain. The shale was too wet that day to do any exploratory digging for other remains, so we decided to mark the site and wait for drier weather. As often happens in Kansas winters, a blizzard swept through north central Kansas the next day and we had to wait until April. (CLICK ON THE PICTURES TO ENLARGE)
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LEFT: This is an upper view of the concretion that Fred
Smith discovered in March. The narrow end was poking out of the weathered
shale at about a 30 degree angle.
RIGHT: Lower view of the same concretion. Although only 5 centra are visible on the outside, it is likely that the concretion is filled with them. |
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LEFT: This is the end that Fred saw first. The round
shape and concentric rings are suggestive of a tree branch, but
fortunately, this fossil is much more important than a piece of petrified
wood. Note that there are parts of three vertebral centra visible
here.
RIGHT: Two other centra on exposed on the other end of the concretion. |
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The return trip in April was rained out when an overnight thunderstorm dumped several inches of rain on Mitchell County and made the trail into the site impassible. Due other other commitments, I had to postpone the next opportunity until early June. We were very fortunate to get a break in the warmer weather and have cool, cloudy day. Loading up in our 6-wheeler all terrain vehicle, we drove up to the site and found that things looked a bit different from when we were there three months earlier. Several big rains and the verdant growth of the prairie had markedly changed the appearance of the area. However, we quickly found our marker and started digging....
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LEFT: Here Gail (left) and I are removing shale from
around a small concretion. As it turned out, we wasted about
an hour digging at this spot and found nothing. It was cloudy and cool at
this point, but the sun was starting break though. (Photo by Fred Smith)
As we were getting ready to assume the worst and walk away, Fred went up the hill and found another stone marker. Once we saw it, Gail and I confirmed that it was in fact the marker he had left on the site last March. At that point, we realized our mistake and started to remove the shale at the new spot. Afterwards, I realized that the first marker was one that I had left there in 2008 on the site where I had collected the plesiosaur rib fragments. |
It didn't take very long to discover new shark remains, right where we had hoped to find them... (CLICK ON THE PICTURES TO ENLARGE)
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LEFT: First we discovered several vertebral centra that were preserved outside of a concretion. I suspect that they were scattered far enough apart prior to being fossilized that concretions could not form around them. That said, they were invaded by roots and badly fractured by the slumping shale as it weathered. RIGHT: Here Gail is removing some of the shale over the shark remains as we explored the area around concretions.
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LEFT: Because we are concentrating on what is in the
ground, it is nearly impossible to get a good picture of a person's face
while they are working on a dig.... but relatively easy to get photos of
the other end.
RIGHT: Soon after we discovered the centra, I found the first shark tooth. Definitely an anterior from a large Cretodus. Now we knew for certain what we were collecting. |
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LEFT: Here's a general view of the dig site, with Gail
Pearson on the left and Fred Smith in the middle. The outlines show the
general location of the remains that we collected on June 6 and also the
concretion that Fred discovered on March 18. The little Polaris
six-wheeler in the background is a lifesaver!
RIGHT: Another view of the site with Gail and Fred down in the classic "paleo-position" working on the shark. In the background, you can see a good view of the "Blue Hills" of Kansas. |
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LEFT: An overhead view the area we were working with
notations for the locations of the concretions, shark centra and teeth
that were collected. Note that the "neck" on left side of
the largest concretion is an articulated series of centra that head off
into the undisturbed shale. We are hopeful that the head of the shark, or
at least a large number of teeth will be found there.
RIGHT: A closer view of the largest concretion that we collected. |
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LEFT: Another view of the largest concretion with my hand
included for scale. The last tooth we found on June 6 was located roughly
under my hand, along with another large vertebral centra. (Photo by Fred
Smith)
RIGHT: When found, the shark teeth were about the same color as the rest of the shale, and were usually visible only by their shape. (Photo by Fred Smith) |
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LEFT: The three phosphatic concretions that we collected
on June 6. The two on the right contain the smallest centra that we
found, and suggest that they were part of the tail or at least the rear
part of the shark.
RIGHT: The smallest concretion that we collected with at least 9 visible centra. This one was located adjacent to the second largest concretion, but not solidly connected to it. It was actually partially exposed on the surface when we found it, hidden in the grass near my knee. |
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LEFT: These are the Cretodus teeth in lingual view
that we collected on June 6. Once cleaned with water and a soft brush,
they look pretty impressive! The arrangement is completely artificial,
based only on decreasing size toward the back of the shark's mouth, but
the fact that several of the smaller teeth are leaning different
directions suggests that we have teeth from opposing jaws (upper / lower
or right / left). A complete upper and lower
jaw set would look something like this for Cretoxyrhina mantelli.
RIGHT: A close-up of one of the larger anterior teeth from this specimen. This shark would have been comparable in size and feeding behavior to a modern Great white, or to a Cretoxyrhina mantelli from the Smoky Hill Chalk. |
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Shimada (2006) noted that "The
teeth of Cretodus crassidens are large
and robust. … Each tooth possesses a central cusp with a pair of triangular
lateral cusplets, and a bilobed root. The central cusp shows many minute,
wrinkle-like striations along the base. Lateral cusplets have a broad base and a
triangular shape, but are much smaller than the central cusp. The root is
massive and the basal concavity varies from narrow to broad, presumably
depending on tooth positions. Cretodus
crassidens is known only by its teeth, which are common
in Turonian marine deposits of
Dig Number 2 - June 22-23, 2010
Kenshu Shimada joined us on June 22-23 to continue the excavation of the shark remains. We recovered about 80 more teeth as we enlarged the pit area to the east and north. We kept remarking that we hadn't found any more vertebrae (most were collected in the concretions during the first dig), but late in the afternoon I ran into a large vertebra that was basically in line and north of where the others had occurred. A second and a third followed quickly... and then three more on Day 2 when we enlarged the pit again. It appears that the shark remains have been sorted somewhat by currents / tidal action before final burial.. With the last last three vertebrae that I removed on Thursday, we are still looking for the north and west 'edges" of the dig.... and of course are going upslope and deeper into the hill. The south edge is downslope and the east edge apparently ends in a shallow gully. The good thing is that as we further below the layer of top soil, we should encounter fewer roots (and root damage).
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LEFT: On Day 1 of the second dig, Kenshu Shimada and I
started off by mapping the location of the previously collected specimens
(concretions containing vertebrae, other vertebrae and teeth at lower left
) on a large shell of clear vinyl. Here Kenshu, Gail and Fred map some of
the many teeth that we discovered as we enlarged the pit. We suspended
digging periodically to map in the teeth that we found so we could remove
them for safe storage.
RIGHT: A closer view of the vinyl sheet showing the outlines of the concretions, vertebrae and teeth. I wish I could show a picture of all the teeth we collected (90+)... but that will have to wait until we get them cleaned up. |
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LEFT: A very nice crown of a big anterior replacement
tooth... the outline of the incomplete root and accessory cusplets can be
seen as a thin film on the shale. The roots are from some of the
native grasses (mostly Big Bluestem - Andropogon gerardii) that
make up much of the original stand of prairie on this ranch. This land has
been grazed by cattle (and buffalo for thousands of years before them)...
but never broken to the plow.
RIGHT: This tooth was badly weathered and fractured by the time Gail discovered it. We had to give it a coating of PaleoBond just to be able to get it out of the shale. |
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LEFT: More mapping on Day 2 of the second dig.
RIGHT: The dig crew, left to right: Fred Smith, Gail Pearson, Kenshu Shimada and Mike Everhart. Note that the roll of Duct Tape in the foreground of the picture was used to patch the plastic sheets together... not the fossils! It's a busy time of the year, so we temporarily closed the dig for another day. Kenshu is heading back to Chicago and I have to run a field trip out of the Sternberg Museum... Fred and Gail are in the middle of the 2010 wheat harvest. We had a great two days with near perfect weather for June in Kansas. More to come. |
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REPLACEMENT TEETH IN SHARKS - For those of you not
familar with how teeth are formed in sharks, here's a short
explanation....
LEFT: The lower jaw of a modern Bull Shark (Carcharinus leucas) on exhibit in the Fryxell Geology Museum at Augustana College, Rock Island, IL. This jaw is about 30 cm (12 in.) across and nicely illustrates the process by which shark teeth are formed in rows or 'families' of 7 or more teeth on the insides of the lower and upper jaws. The crown of the tooth is formed first, with the root to be added as the crown moves along the "assembly line" toward edge of the jaw. As the tooth matures, it rotates almost 180 degrees and moves over the edge of the jaw, replacing an older tooth which has been shed. The picture also illustrates the differences in size and shape of the teeth from front to back in the same jaw (note that the shape of the teeth, especially the shape of root, can also differ between the upper and lower jaws). Teeth are replaced at intervals from a few weeks to several months, depending on the age of the individual shark and the species. A single shark may shed thousands of teeth during its lifetime. |
Suggested references:
Cappetta, H. and Case, G.R. 1987. Chondrichthyes II - Mesozoic and Cenozoic Elasmobranchii. Gustav Fischer Verlag, Stuttgart and New York. 193 p., 148 fig.
Cappetta, H. and Case, G.R. 1999. Additions aux faunes de sélaciens du Crétacé du Texas (Albien superier-Campanian). Paleo Ichthyologica 9:5-111, 8 fig. 1 table, 30 pls.
Cicimurri, D.J. 2004. Late Cretaceous chondrichthyans from the Carlile Shale (Middle Turonian to Early Coniacian) of the Black Hills region, South Dakota and Wyoming. The Mountain Geologist 41(1):1-16.
Dixon, F. 1850. Geology and fossils of the Tertiary and Cretaceous formations of Sussex. pp. i-xvi; 1-422.
Everhart, M.J. 2009. Probable plesiosaur remains from the Blue
Hill Shale (Carlile Formation; Middle Turonian) of north central
Everhart, M.J., Everhart, P., Manning, E.M. and Hattin, D.E. 2003. A Middle Turonian marine fish fauna from the Upper Blue Hill Shale Member, Carlile Shale, of north central Kansas. Journal of Vertebrate Paleontology, Volume 23 (Supplement to Number 3): 49A.(Abstract)
Schwimmer,
D.R., Hooks, G.E., III and Johnson, B. 2002. Revised taxonomy,
age, and geographic range of
the large lamniform shark Cretodus semiplicatus.
Journal of Vertebrate
Paleontology 22:704–707.
Shimada, K. 2006. Marine vertebrates from the Blue Hill Shale Member of the Carlile Shale (Upper Cretaceous: Middle Turonian) in Kansas. In Lucas, S.G. and Sullivan, R.M. (eds.), Late Cretaceous vertebrates from the Western Interior. New Mexico Museum of Natural History and Science Bulletin 35, p. 165-175.
Sokolov, M. 1965. Teeth evolution of some genera of Cretaceous sharks and reconstruction of their dentition. Moskovkoe Obshchestvo Ispytatelie Prirody, Biulleten Otodel Geologicheskii 40:133-134.
Stewart, J.D. and Martin, J.E. 1993. Late Cretaceous selachians and associated marine vertebrates from the Dakota Rose Quarry, Grant County, South Dakota. Proc. South Dakota Academy of Science 72:241-248.
Welton, B.J. and Farish, R.F. 1993. The collectors guide to fossil sharks and rays from the Cretaceous of Texas. Horton Printing Company, Dallas, 204 pp.
