Platecarpus tympaniticus

  Shark scavenged mosasaur remains from the Smoky Hill Chalk

Copyright 2000-2010 by Mike Everhart

Last revised 10/13/2010

 

 

LEFT: "Scavenging sharks" adapted from a painting by Vladimir Krb, courtesy of and copyright by the Royal Tyrrell Museum of Palaeontology.

IMPORTANT NOTE:  The recent publication of an article in the JVP by Takuya Konishi and Michael Caldwell clarifying the identification and relationships of the various species of Platecarpus will necessitate some major changes in some of my web pages. Please note that Platecarpus planifrons Cope (1874) is now identified as the most common species of Platecarpus in the lower chalk (late Coniacian to middle Santonian), and P. ictericus (Cope, 1871) is the most common species of this genera in upper chalk (middle Santonian through early Campanian). P. coryphaeus (Cope, 1872) is a junior synonym of P. ictericus.  The name Platecarpus tympaniticus (Cope, 1869) is now limited to a single specimen (holotype) from Mississippi. The species that I had previously identified as Platecarpus planifrons (above) is now "unidentified" and possibly a new genus / species which we are working to identify / describe.  I consider this paper to be a major improvement in mosasaur phylogeny. The citation is:

Konishi, T. and Caldwell, M. W. 2007. New specimens of Platecarpus planifrons (Cope, 1874) (Squamata: Mosasauridae) and a revised taxonomy of the genus: Journal of Vertebrate Paleontology 27(1): 59-72.

Further changes:  Konishi, Caldwell and Bell (2010) have further clarified the phylogeny of Platecarpus, making P. tympaniticus Cope 1869 the senior synonym:

Konishi, T., Caldwell , M.J. and Bell , G.L., Jr. 2010. Redescription of the holotype of Platecarpus tympaniticus Cope 1869 (Mosasauridae: Plioplatecarpinae), and its implications for the alpha taxonomy of the genus. Journal of Vertebrate Paleontology 30(5):1410-1421. 

In early April of 1996, our friend, Pete Bussen, called and said that he’d found some mosasaur vertebrae eroding out of a chalk gully on a ranch west of Russell Springs, KS. Pete said he’d already picked up some of the vertebrae that had broken loose and thought that they were from a large Platecarpus. He had also noticed that the several of the ribs had shark bite marks on them.

Then he said, with a grin in his voice, that the remains were coming out from under a foot and a half of chalk overburden and wanted to know if we were interested in digging on them. Pete had told us (and demonstrated) several times in the past that he’d rather watch other people dig up the big fossils he found than dig them up himself.

platecar.jpg (71048 bytes) A wall mount of a nearly complete Platecarpus tympaniticus mosasaur from Western Kansas (About 20 feet long) in the Johnson Museum of Geology at Emporia State University, Emporia, Kansas.

Without much other information to go on, we told him we’d be out the following week-end, weather permitting. The locality that Pete had mentioned was quite a bit higher in the chalk (a couple of million years younger) than we normally collected in and the fauna was somewhat different. We’d never collected from that area before and we are always interested in seeing a new site.

Besides, conventional "paleo-wisdom" said that, although fossils were generally rarer in the upper part of the Smoky Hill Chalk, the ones you did find were usually more complete. That’s supposedly why Sternberg, Cope, Marsh and a whole host of paleontologists had collected higher in the chalk..... to get the best, most complete fossils. In this case, a good string of vertebrae could lead to a complete specimen, including a skull.

On the other hand, we’d had pretty good luck over the years in the low chalk and had found several things that the early paleontologists had overlooked. Mosasaurs like Platecarpus, however, are relatively rare in the low chalk and we were still looking for our first complete skull there.

site-1a.jpg (5466 bytes) Saturday, April 13, started out as a warm, sunny spring day. We followed Pete to a ranch west of Russell Springs and then past a gate for about a mile across a pasture to the site. We were able to park at the edge of the exposure, about 100 feet or so west of where Pete said the fossil was coming out. We walked down the hill and into the gully where he had found the vertebrae.
site-3a.jpg (4746 bytes) About 4 feet up the sidewall were the easily recognizable impressions of a dozen or so, two-inch vertebrae. Several ribs still protruded from the gray chalk. The remaining vertebrae disappeared into the gully wall. The other end of the impressions disappeared into thin air. Those vertebrae had eroded out long ago and were gone.
site-2a.jpg (4248 bytes) In mosasaurs, each vertebra has a cupped end (anterior) and a rounded end (posterior) ...... the rounded end of one fits into the cupped end of the next vertebra. The rounded end always points away from the skull. A quick examination of the remaining vertebrae in the chalk showed that they were aligned in such a way as to be coming from the skull. This meant we at least had a chance to find the skull, instead of the tail....a much more interesting proposition if you are going to be moving a couple of tons of rock.

We policed up a few more bone fragments at the base of the gully wall and then I went to get the primary working tools......a heavy pick and a shovel. There were about 18 inches of chalk over the fossil that would have to be removed.....and if we were reading the size and direction of the bones correctly, we would have to clear an area about 4 feet wide and 8 feet long. That turned out to be about twice as much area as we really needed but we didn’t discover the mistake until we’d cut chalk down to the working level.

The first surprise was the toughness of the chalk. Chalk is usually laid down in thin layers and breaks up easily along the multiple bedding planes. In our case, it turned out that we had three 6 inch thick layers of bioturbulated chalk (It had been reworked and mixed by the activity of lots of little invertebrates while it was being laid down) that refused to be broken easily, or even with lots of pounding.

About that time, Pete started reminding me about how glad he is that he gave me the fossil, and I was wondering what I got myself into. Finally, we managed to split the layers apart and then break them vertically in to large chunks. Then the hundred pound chunks were levered to the edge of the gully and pushed over the side.

By about 10 AM, we had a working area cleared and started working the chalk with a small pick and brush, trying to locate the level where the rest of the bones were without damaging them in the process. Following the direction of the remaining vertebrae as they pointed into the hill, we tried to locate the skull where it should have been at the end of the neck.

Surprise number two was that the skull was not where it was "supposed" to be. Your heart sort of drops into your left boot when you realize that you have probably done a lot of digging for not very much return. It happens frequently in the chalk when you are working on the remains of some critter’s discarded meal. We already knew that this specimen had been scavenged by sharks. What else was missing? There’s a whole lot more fish tails out there than complete fish!

Not giving up, we went back to the buried vertebrae and started tracing them out. Surprise number three followed shortly when we discovered that the neck of the animal did an almost 90 degree turn...... and bingo!....I lifted a small piece of chalk and underneath was the shape of a pterygoid bone (hard palate of the mosasaurs mouth), laying upside down, teeth and all. We had located the skull, about 3 feet away from where we had expected it to be. Later examination showed that the skull had been dislocated from the cervical vertebrae. An injury in a fight with a larger mosasaur that caused the death of this mosasaur? .....or the post-mortem damage that occurred while the carcass was being ripped apart by the feeding frenzy of sharks? We’ll probably never know.

skull-1a.jpg (4551 bytes) The fourth surprise is another bad one. The nose of the skull went back under the 18" of chalk at the edge of the excavation. More digging! The first two layers of chalk come off fairly easily, but the last one is wedged in tightly on three sides, and sitting on part of the skull. It took almost a half hour to loosen it and then there were some scary moments as we levered it over the vertebrae and away from the skull.
skull-2a.jpg (3800 bytes) Finally, we were ready to work on the skull. Within a few minutes we located the edges of the skull and found that it was laying upside down, apparently still articulated. The lower jaws were laying off to the side, one on top of the other.  Scale is 6 inches long.
skull-3a.jpg (3719 bytes) The skull was about 2 feet long and since in Platecarpus the skull is about 10 percent of the body length, this meant we were working with an animal that was about 20 feet long in life. From the looks of the teeth that outlined the margins of the skull, it certainly wasn’t an animal I’d want to share my ocean with!

While taking a break to stretch legs and unkink backs after working on our knees for far too long on the broken chalk, we took a good look around at the site and decided we’d better locate the stratigraphic marker units so that we would know what elevation the fossil was coming out from. This would provide us with the approximate age of the specimen and it's relationship to other material in the area. A walk up the exposure to the south soon located a tell-tail, one inch thick red bentonite that had to be Marker Unit 16 (There are more than 20 such 'marker units' in the chalk). The fossil was coming out about two feet below the Marker Unit ...too easy.... but by then I was ready for something to be easy! That task completed, it was back to the dig.

skull-4a.jpg (4192 bytes) Then began the tedious work of clearing around the fossil so that it could be removed from the chalk. It was early afternoon and our plan was to take the skull out in a block of chalk, without plastering or other protective wrappings. The chalk was solid enough that it would, in fact, be the "jacket" that supported and protected the skull. Before that could be done, however, the vertebrae and remaining ribs were exposed, coated with preservative, photographed and removed. Only then could the final work on the skull be started.
skull-5a.jpg (4193 bytes) The cervical or neck vertebrae of the mosasaur.  Although these vertebrae were still articulated and unbroken, the skull itself was no longer attached and was laying at an angle from the line of vertebrae.
skull-6a.jpg (3662 bytes) The underside of the mosasaurs muzzle or snout.  The skull was upside down and both rows of teeth had been laid out nicely along the outside edge of the jaws.
skull-7a.jpg (3953 bytes) It was immediately apparent that the bone was soft and pithy, so extra care was necessary to keep from damaging it during this stage of preparation. Once the skull was exposed enough to accurately show the outer boundaries, we began to trench downward around and between the skull and lower jaws.
skull-8a.jpg (3621 bytes) Finally, about 4 PM, the skull was given a final coating of preservative, and photographed one last time. As happens frequently in Kansas, the weather was changing rapidly. The sky to the west was getting darker by the minute. The temperature was dropping and the wind was picking up. What had started as a nice spring day was about to turn into something else......Surprise number 5....a late spring snowstorm.
skulla.jpg (37256 bytes) Fortunately for us, there was a good separation layer under the skull and jaws, and both came up easily. The heavy blocks of chalk were carried carefully back to our field vehicle and packed securely for the rough trip back to the highway. Finally, all the tools were loaded up and we made the slow drive across the pasture. We hurriedly said good-bye to Pete at the gate just as it started to rain and got on the road for our 5 hour trip home. That night, while the site was sitting under fresh snow, the Platecarpus skull was sitting in our prep room and the real work was about to begin.

LEFT: A dorsal view of the prepared skull (Now FHSM VP-17017)

POSTSCRIPT: The skull was identified as Platecarpus tympaniticus, a fairly common mosasaur in the upper chalk. Damage to the skull indicated that the mosasaur may have died as the result of a fight with a larger mosasaur that resulted in a crushed skull and / or a broken neck. There was ample evidence of scavenging by sharks on the ribs and on some elements of the skull and lower jaws. The scavenger was positively identified as Squalicorax falcatus / kaupi on the basis of the serrations that were readily visible in the teeth marks on the mosasaur bone. A Squalicorax kaupi tooth was collected nearby but not associated with the remains. What killed this mosasaur will probably never be known for certain, but we do know that the carcass had been pretty well stripped by sharks before it was buried on the sea bottom.         

More recently, this specimen was donated in 2007 to the Sternberg Museum of Natural History and is curated there as FHSM VP-17017.

This dorsal view of the skull of Platecarpus (ictericus) tympaniticus skull was adapted from Systematics and Morphology of American Mosasaurs, by Dale A. Russell, Peabody Museum of Natural History, Yale University Bulletin 23, 1967.

For more pictures of the prepared skull, go HERE.