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Introduction
Plesiosaurs (Sauropterygia) are found in the fossil record
from the Late Triassic through the end of the Cretaceous. They
evolved into many diverse forms, including the giant
pliosaurids, Liopleurodon and Kronosaurus,and the extreme-
ly long-necked elasmosaurids, Thalassomedon,Styxosaurus
and Elasmosaurus. Plesiosaurs, in general, survived several
near extinctions, the latest occurring in the Early Cretaceous
(Bakker, 1993). The two groups of plesiosaurs remaining dur-
ing the last 25 million years of the Cretaceous were the ‘long-
necked,’ small-headed elasmosaurids and the ‘short-necked’,
large-headed polycotylids. In the Late Cretaceous Western
Interior Sea, elasmosaurids such as Elasmosaurus platyurus
Cope 1868 and Thalassomedon haningtoni Welles 1943
reached lengths of 14 m (45 ft), whereas the much smaller
polycotylids such as Dolichorhynchops osborni Williston 1902
grew only as large as 3-6 m (10-20 ft). Both groups fed pre-
dominantly on small, nektonic fish and cephalopods (Brown,
1904; Massare, 1987; Cicimurri and Everhart, 2001), and
would have been relatively harmless to larger fish and other
marine reptiles.
Mosasaurs were the top predators in the oceans of the Late
Cretaceous, with some species growing to lengths of 17 m (55
ft), and were well adapted for eating large prey. A highly flex-
ible, kinetic skull and an additional joint in the lower jaw
(Russell, 1967) allowed mosasaurs to swallow their prey
whole, very much like the feeding adaptations in snakes.
Williston (1898, p. 213) suggested that while “a saurian of the
largest size might swallow something as large as a two-year-
old calf,” he believed that a mosasaur’s “food was evidently the
numerous small fishes that swarmed the seas, with perhaps an
occasional animal of their own kind.” More recently, Lee, Bell
and Caldwell (1999) proposed that the “highly flexible lower
jaw” evolved in mosasaurs “for accommodating large prey.”
From preserved gut contents associated with mosasaur
remains, these marine reptiles are known to have fed on a vari-
ety of prey, including fish, marine birds, turtles, and other
mosasaurs. Williston (1898) and Camp (1942) reported fish
scales and bones within specimens of Tylosaurus and
Plotosaurus,respectively. Williston (1899) also described the
stomach contents of a well-preserved Platecarpus mosasaur
that contained the remains of 1.2 m (4 ft) fish (Cimolichthys).
The partially digested remains of a shark (teeth of
Cretolamna?), a teleost fish (Bananogmius), a marine bird
(Hesperornis), and a small mosasaur (Clidastes) were reported
by Martin and Bjork (1987) as the preserved stomach contents
of an adult Tylosaurus proriger from the Pierre Shale (Middle
Campanian) of South Dakota. Dollo (1887) identified turtle
remains associated with a specimen of the giant tylosaurine,
Hainosaurus bernardi,from the Late Maastrichtian deposits in
Belgium. From the evidence in the fossil record, it is readily
apparent that mosasaurs, especially the larger individuals,
were capable of swallowing large prey.
Varricchio (2001, p. 405) indicated that “extant
archosaurs, crocodilians and birds, typically ingest prey with
little mastication and can fully digest bones.” It is likely that
mosasaurs were also able to completely digest large prey.
In 1918, Charles H. Sternberg and his sons (George F. and
Levi) recovered a nearly complete skeleton of a 8.8 m, adult
Tylosaurus proriger that included the partially digested bones
of a plesiosaur (Sternberg, 1922). Sternberg reported the
41The Mosasaur, 7: 41-46
Plesiosaurs as the Food of Mosasaurs; New Data on the Stomach
Contents of a Tylosaurus proriger (Squamata; Mosasauridae) from the
Niobrara Formation of Western Kansas
Michael J. Everhart
Sternberg Museum of Natural History • meverhar@fhsu.edu
Fort Hays State University, Hays, Kansas, 67601
ABSTRACT - Although plesiosaurs and mosasaurs co-existed for about 25 million years at the end of the Cretaceous, the
fossil record was mute regarding interactions between these two groups of marine reptiles until a discovery made in 1918.
At that time, Charles H. Sternberg uncovered the partially digested bones of a plesiosaur as stomach contents in an adult
(8.8 m) Tylosaurus proriger skeleton in the Smoky Hill Chalk Member (Early Campanian) of the Niobrara Formation near
Twin Butte Creek in Logan County, Kansas. Sternberg reported his discovery at the annual meeting of the Kansas
Academy of Science in 1919 and indicated that the material had been sent to the United States National Museum. Due to
unusual circumstances regarding the publication of his brief paper in the Transactions of the Kansas Academy of Science,
however, the association of the two specimens went largely unnoticed until 2001. This association demonstrates conclu-
sively that mosasaurs fed on plesiosaurs and provides additional data about the ecology of the Western Interior Sea. Here
the remains are re-examined and discussed in light of related information that has become available in the more than
eighty years since their original discovery.
unique specimen at the 51st annual meeting of the Kansas
Academy of Science in 1919, and stated that the specimen had
been sent to the United States National Museum. His brief
paper, however, was not published until 1922 (Volume 30,
including the years 1919-1921) because of problems between
the Kansas Academy of Science and the State Printer (Skelton,
1998). From the lack of recognition of this discovery in more
recent literature, it is evident that the single paragraph in the
brief, two-page paper drew little attention outside that of the
membership attending the meeting in 1919.
The mosasaur and plesiosaur remains were received by
the National Museum in 1919 and curated separately. The
mosasaur (USNM 8898) became a permanent exhibit at the
museum where it is today, whereas the plesiosaur bones
(USNM 9468) were stored in the main collection. The USNM
records for both specimens, however, do specifically record
their association. Sternberg’s hand-written field tag is included
with the plesiosaur remains. It reads; “Half digested plesiosaur
bones between the ribs of No. 17. Also pebbles from the stom-
ach of plesiosaur. C. H. Sternberg.” Although the Tylosaurus
proriger skeleton was discussed by Russell (1967), its connec-
tion with the plesiosaur was not mentioned. In any case, pre-
dation or scavenging by mosasaurs on plesiosaurs is not docu-
mented in recent publications.
Abbreviations
AMNH – American Museum of Natural History, New
York, NY; FHSM – Fort Hays State University Sternberg
Museum of Natural History, Hays, KS; KUVP – University of
Kansas Museum of Natural History, Lawrence, KS; LACMNH
– Los Angeles County Museum of Natural History, Los
Angeles, CA; MCZ – Museum of Comparative Zoology,
Harvard University, Cambridge, MA; USNM – United States
National Museum, Washington, D. C.
Locality and Stratigraphy
The USNM records state that the remains were discovered
on the south side of [Twin] Butte Creek, 15 miles southeast of
Russell Springs, in Logan County, Kansas. The locality data,
and the chalky matrix remaining on the plesiosaur bones, con-
firm that the specimen came from the Smoky Hill Chalk
Member of the Niobrara Chalk Formation. Both Hattin (1982)
and Bennett (2000) consider this general area to be in the
upper one-third of the chalk, probably in the vicinity of
Hattin’s marker unit 15 or 16. The locality data suggests that
the age of this specimen is probably Late Santonian or Early
Campanian. Stewart (1990) noted that the “nearly all the par-
tial skeletons of Niobrara polycotylids have been collected
from the upper horizons of the Smoky Hill Chalk Member.” At
the time of deposition, the locality would what been roughly
300 km from the nearest (eastern) shore of the Western Interior
Sea (Russell, 1967).
According to Storrs (1999), both elasmosaurid and poly-
cotylid plesiosaurs have been reported from the upper Smoky
Hill Chalk Member; however, the available records indicate
that polycotylid remains are the more common. The type spec-
imen of Polycotylus latipinnis Cope 1869 (AMNH
1735/USNM 244534) was discovered in Logan County by
William. E. Webb (Cope, 1870). The type specimen of
Dolichorhynchops osborni Williston 1902 (KUVP-1300) at
the University of Kansas Museum of Natural History, the
mounted specimen of D. osborni (FHSM VP-404) at the
Sternberg Museum of Natural History, and the juvenile D.
osborni remains (MCZ-1064) at the Harvard Museum of
Comparative Zoology were also from the upper chalk of Logan
County.
Although plesiosaur remains are very rare in the lower
chalk, J. D. Stewart (pers. comm., 2002) has collected a juve-
nile propodial from Sand Creek in southern Rooks County
near the contact of the Smoky Hill Chalk Member with the
Fort Hays Limestone (Late Coniacian). In 1990, I observed J.
D. Stewart collect a sub-adult Dolichorhynchops propodial and
associated epipodials (LACMNH 148920) in the Early
Santonian chalk of Lane County. I have also collected 5 sets of
fragmentary plesiosaur remains (FHSM VP-13962 to 13966),
including fragments of a juvenile Dolichorhynchops skull and
lower jaws, from the Late Coniacian and Early Santonian
lower chalk of Gove County (Everhart, 2003).
The first Cretaceous reptile reported from Kansas,
Elasmosaurus platyurus Cope 1868, was collected from the
Sharon Springs Member of the Pierre Shale Formation in
Logan County, northeast of Fort Wallace, Kansas, by Dr.
Theophilus H. Turner (Cope, 1868) in 1867. Although thou-
sands of mosasaur remains have been recovered from the
Smoky Hill Chalk Member (Everhart, 2001) since Tylosaurus
proriger (Cope) was first described in 1869, only a few dozen
plesiosaur specimens have been discovered there. Thirty years
after the first discovery of a plesiosaur in Kansas, Williston
THE MOSASAUR — THE JOURNAL OF THE DVPS42 Vol. 7
Figure 1. Unidentified plesiosaur bone fragment showing loss
of surface layer and deep pitting due to attack by stomach
acids.
(1897) interpreted the upper chalk as being deposited in shal-
low water nearer to shore than the lower chalk. The rarity of
plesiosaurs suggests that they were only occasional inhabitants
of the central portion of the Western Interior Sea over what is
now Kansas, and possibly were only migrating across that
large body of open water to reach feeding grounds closer to
either the eastern and western shores.
Discussion
The well-preserved Tylosaurus proriger specimen report-
ed by Sternberg (1922) included an articulated skull, front
limbs, ribs, pelvic bones and both femora, and all but the last
1.9 m (6 ft) of the tail. In between the ribs was “a large part of
a huge plesiosaur with many half digested bones, including the
large humeri, part of the coracoscapula, phalanges, vertebrae,
and, strangest of all, the stomach stones, showing that this
huge tylosaur … had swallowed this plesiosaur in large enough
chunks to include the stomach (Sternberg, 1922, p.120).”
Sternberg, however, did not comment as to whether he
believed the plesiosaur had been live prey or a scavenged car-
cass.
In November, 2001, the plesiosaur specimen (USNM
9468) was examined at the United States National Museum.
Although the bones show extensive surface erosion and repre-
sent only a small portion of the entire skeleton, they appear to
be those of a polycotylid, Dolichorhynchops osborni. The
remains are consistent with the description provided by
Sternberg (1922), except that it is not a “huge” individual.
Based on measurements of the two reasonably complete
humeri, the plesiosaur was somewhat smaller than either the
type specimen (KUVP-1300) or the mounted specimen in the
Sternberg Museum (FHSM VP-401). All of the bones show
evidence of some dissolution of the surface layer by stomach
acids (Fig. 1). The damage is similar to the surface erosion
described on hadrosaur bones identified as gut contents of a
tyrannosaurid. Varricchio (2001, p. 401-402) noted that sub-
surface bone structures such as the “vascular canals and inter-
trabecular spaces have been enlarged, giving the bone a spongy
appearance clearly distinct from fresh bone.” None of the ple-
siosaur bones appear to have been weathered and /or sun-
bleached (as opposed to being corroded by stomach acids), and
from Sternberg’s description (1922), it is likely that the ple-
siosaur remains were still completely embedded in the chalk
matrix when discovered.
Some of the plesiosaur bones appear to be heavily corrod-
ed on one side and relatively undamaged on the other, a possi-
ble indication that they were partially protected from attack by
stomach acids by attached muscle, other undigested tissue or
adjacent remains. Although no tooth marks are evident on the
bones, the presence of numerous small fragments argues that
some of the larger bones in the pectoral or pelvic girdles were
crushed or broken when the plesiosaur was bitten as it was
being swallowed by the mosasaur. Bite marks that were pres-
ent originally may also have been removed by the digestive
process. In addition to corrosion by stomach acids, portions of
the relatively thin, right coracoid appear to have been damaged
during recovery and / or storage. Many bone fragments are suf-
ficiently corroded as to make identification difficult, including
some relatively thick pieces that are probably parts of the miss-
ing femora.
The cartilaginous ends of the humeri are severely eroded
(Fig. 2), whereas the dense perichondral bone on the shafts
appears to have been more resistant to the stomach acid
PLESIOSAURS AS THE FOOD OF MOSASAUR FOOD — EVERHART 43May 2004
Figure 2. Dorsal and ventral views of the left humerus, show-
ing extensive damage to the cartilaginous end of the bone and
relatively little erosion of the denser perichondral bone of the
shaft.
Figure 3. Four partially digested caudal vertebrae and three
caudal ribs. A small gastrolith (arrow) remains attached to one
of the ribs.
(O’Keefe, pers. comm., 2002). They measure 26 and 27 cm in
length and, taking into account the damage to both ends, were
probably no longer than 30 cm in life. This compares favorably
with the length of the right humerus (33.6 cm) of the 3 m spec-
imen of Dolichorhynchops osborni (FHSM VP-404; Bonner,
1964) in the Sternberg Museum collection, indicating that the
plesiosaur was somewhat less than 3 m (10 ft) in length. Three
mesopodials / metapodials are also included in the limb ele-
ments. Twenty-seven podials (phalanges) were also recovered
from the remains, far fewer than the number (70+) normally
present in a single paddle of Dolichorhynchops. There are only
four, small caudal vertebrae (22-27 mm diameter) associated
with the specimen and all appeared to have been corroded by
acid. The small size of the vertebrae and the presence of sev-
eral, small, caudal ribs (Fig. 3) indicates that they were from
near the end of the tail.
Sternberg (1922) reported that stomach stones (gas-
troliths) were collected with the specimen. Although there was
no notation of gastroliths indicated in the USNM records, 12
small (up to 28 mm in length), mostly siliceous stones were
included with the remains along with two small concretionary
masses that appeared to contain smaller grains of quartz sand
(Fig. 4). Quartz or quartzite sand and gravel is not a normal
component of the limestone or chalk beds that make up the
Niobrara Formation (Mudge, 1877; Hattin, 1982) and the near-
est sources were located hundreds of miles away (Williston,
1893). With the likely exception of a group of 12 rounded and
polished stones (FHSM VP-13919), from the lower Smoky
Hill Chalk Member (Late Coniacian) in Gove County, gas-
troliths from the Niobrara are always associated with verte-
brate remains. As noted by Sternberg (1922), the presence of
gastroliths suggests that the plesiosaur’s gut was intact when it
was swallowed by the Tylosaurus.
The largest USNM 9468 gastrolith measures 28 x 14 x 8
mm. Two of the smaller gastroliths (8-9 mm maximum length)
were still embedded in the chalk matrix and closely associated
with partially digested bone fragments. Another small stone is
pressed tightly against a caudal rib. In the Late Cretaceous
Western Interior Sea, gastroliths are most often associated with
the remains of long-necked elasmosaurid plesiosaurs, such as
Styxosaurus and Thalassomedon (Cicimurri and Everhart,
2001), and are only rarely documented from the short-necked,
polycotylid group (see Martin and Kennedy, 1988). Although
12 gastroliths is a small number compared to the hundreds that
have been discovered with the remains of other plesiosaurs
(Everhart, 2000), they have never been reported from a
mosasaur. It is most likely that the gastroliths were from the
plesiosaur and not from the larger mosasaur that ate it.
A single, un-corroded Squalicorax falcatus tooth is also
included with the plesiosaur remains and was likely shed dur-
ing post-mortem scavenging of the Tylosaurus carcass by these
sharks. Squalicorax bite marks have often been observed on
vertebrate remains in the Smoky Hill Chalk Formation
(Schwimmer, Stewart and Williams, 1997). Scavenging of the
unprotected and readily removable viscera of the mosasaur by
these sharks (Cicimurri and Everhart, 2001) could also explain
why more of the plesiosaur remains were not recovered with
the Tylosaurus skeleton.
In addition, half of a large (antero-posterior length = 47
mm; diameter = 66 mm) and partially digested teleost vertebra
(Xiphactinus audax) was also included with the plesiosaur
remains (USNM 9468). Although the corroded appearance of
the vertebra is similar to the condition of the plesiosaur bones,
it also appears to have been weathered, and may not have been
collected with the specimen. It was not mentioned in
Sternberg’s (1922) report.
A method which measures the amount of water displaced
by a scale was used to estimate the weights of the mosasaur
and the plesiosaur model (Alexander, 1989). This approach
assumes that the density of the animal is approximately the
same as water (1 gm/cc). An 8.8 m Tylosaurus proriger would
have weighed about 1100 kg while a 2.5 m pliosaur would
have weighed between 60-70 kg. While this pliosaur would
have certainly been a very large meal for the mosasaur to swal-
low, the evidence appears to support that it did occur.
The acid-etched condition of the plesiosaur remains,
whether it was killed by the mosasaur or scavenged, suggests
that the plesiosaur had been consumed several hours prior to
the death of the Tylosaurus. There is no indication of what may
have killed the mosasaur and postmortem scavenging by
sharks appears to be have been limited to the viscera, the miss-
ing rear paddles and the distal caudal elements. The relative
completeness of the mosasaur skeleton as described by
Sternberg (1922) suggests that the remains reached the sea bot-
tom shortly after death. It is likely that the intact ribcage of the
much larger mosasaur may have prevented this relatively small
portion of the plesiosaur’s remains from being carried away by
the scavengers.
THE MOSASAUR — THE JOURNAL OF THE DVPS44 Vol. 7
Figure 4. Gastroliths and two larger masses (left) containing
sand grains recovered from the plesiosaur remains.
Conclusion
Partially digested plesiosaur bones were discovered with-
in the rib cage of an articulated, adult Tylosaurus proriger by
Charles Sternberg in 1918, and reported by him at the 1919
meeting of the Kansas Academy of Science. His brief note
describing the remains was published in 1922 and remained
largely unnoticed since that time. This specimen provides the
first evidence that Tylosaurus occasionally fed on marine rep-
tiles other than smaller mosasaurs. That an 8.8 m mosasaur
(skull length = 1.2 m) was able to swallow all, or a large por-
tion of, a 2.5 m plesiosaur in mid-ocean, provides additional
data as to the predatory / scavenging habits and feeding capa-
bilities of these large marine animals. Although predation or
scavenging by mosasaurs on plesiosaurs should not be unex-
pected, the rarity of preservation of stomach contents in the
fossil record and an unusual combination of modern circum-
stances have delayed prior recognition of Charles H.
Sternberg’s important discovery.
Acknowledgements
I am obliged to Robert Purdy and Michael Brett-Surman,
(United States National Museum, Washington, D.C.), for their
assistance in providing data on the USNM collection and
access to this specimen. Larry Martin and Desui Miao,
(University of Kansas, Lawrence, KS), provided access to rel-
evant specimens in the collection in the Museum of Natural
History. F. Robin O’Keefe, (New York College of Osteopathic
Medicine, Old Westbury, NY) shared age, condition and iden-
tification information from his earlier examination of the mate-
rial. In addition to the data provided by J. D. Stewart, (Los
Angeles County Museum of Natural History, Los Angeles,
CA), about Niobrara plesiosaurs, I have greatly benefited from
our many hours of discussions in regard to the biostratigraphy
of the Smoky Hill Chalk Member. I also thank Earl Manning
(Tulane University, New Orleans) and Richard Zakrzewski
(Sternberg Museum, Fort Hays State University) for their
reviews and comments on the manuscript.
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