field work results

[Ken Carpenter <KCarpenter@dmns.org>]

As an FYI to the list, I just got back from 2 weeks in the field. About 24 
volunteers and I worked Lorrie's Site, a bonebed of the polacanthid ankylosaur 
Gastonia located in the base of the Ruby Ranch Member of the Cedar Mountain 
Formation north of Arches National Monument (eastern Utah). 

The site has produced over 3000 bones, over 99% of which are ankylosaur and 
less than a dozen other bones of an unidentified iguanodontid, Utahraptor and 
an unidentified small theropod. Based on test pits and quarrying, the site is 
over 125 m (410 feet) long, and 60 m (200 feet) wide. Bone density is about a 
dozen per square meter. Best guess on the number of ankylosaur individuals in 
the bonebed ranges from 30 to up to several hundred ("Thundering herd of 
waddling ankylosaurs"). Further test pits are needed to verify the apparent 
wide distribution of bones. The bones are all disarticulated, although patches 
of armor indicate that some of the hide was present at the time of burial. 
These patches have the typical dinosaur pattern of rosettes (in this case, 
large central scute surrounded by smaller scutes). A poster will be presented 
by one of my volunteers at SVP.  

No complete skulls are known, although over a dozen braincases have been found. 
We hypothesis that a large sauropod (obviously a young male) went around 
stomping on the heads to watch them pop. ;-)

The cause of death for so many ankylosaurs is unknown, although drought kill 
has been suggested (but now doubted by me). There is agreement that the 
carcasses lay on the ground some time, were scavenged and the bones 
disarticulated. The bones were then buried by a crevasse splay, the result of a 
river at flood breaching its natural levee and spilling sediment and water onto 
the flood plain. There is definitely water orientation of the long bones 
(femur, radius, ribs, etc.) perpendicular (90 degrees) to the direction of 
water flow. Studies of modern bones in flowing water show that as water 
velocity and depth decreases (as flood waters drop), water currents are equal 
along the length of the bone. Interestingly, some bones at Lorrie's Site also 
show that the upstream side of the bone was lower in the sediment due to 
scouring of the sediments by flowing water. 

Despite the action of flowing water, we have recovered various sized and shape 
bones, indicating that water flow was not continuous enough to start winnowing 
the bones. Sometimes certain bones (e.g., vertebrae) are winnowed from a site 
because they are easily transported or rolled along by flowing water. That is 
one reason why many dinosaur skeletons are so incomplete. At Lorrie's Site, the 
apparent lack of winnowing indicates that the flood lasted long enough to 
orient the bones, but water level dropped rapidly so that little or no 
winnowing occurred. This might have happened as the flood spilled out onto a 
broad flood plain. Further support for this hypothesis comes from the apparent 
random orientation of the bones at the western end of the quarry.  Some of the 
shoulder blades here are also current unstable, meaning that they are lying in 
a manner that moving water would flip them over to a more stable position. All 
of this evidence indicates that the ankylosaur carcasses !
 lay close
 to the area we found the bones. The other non-ankylosaur bones were either 
brought in with the flood, or were bones already scattered on the flood plain.

The encasing sediments are predominantly sandstone nearest the river channel 
(eastern part of the quarry)  and become finer grained towards the western end. 
We also discovered the "nose" of the sediment lobe that the flood spilled out 
onto the flood plain. The bones here lay on a 30-35 degree slope that faces 
downstream (down flow direction). Masses of limestone (carbonate) around some 
of the bones (east end) or under the bones (west end) show the effects of 
ground water in a semi-arid environment.  Sheets of chert also occur among the 
bones and through some of the carbonate nodules. These cherts are also the 
result of silica deposited by ground water. 

Analysis of the bones indicate that the ankylosaur is Gastonia, but a species 
distinct from G. burgei from the underlying Yellow Cat Member.  For example, 
whereas the shoulder blade of G. burgei flairs outwards (i.e., the upper and 
lower margins diverge away from each other), the new specimens all show 
parallel upper and lower margins.  

I had hoped we could wrap up our work at the site this year, but discoveries on 
the last day (of course!) now makes that impossible. Further digging will take 
place later this year or sometime next year. 

Kenneth Carpenter, Ph.D.
Curator of Lower Vertebrate Paleontology &
Chief Preparator
Dept. of Earth Sciences
Denver Museum of Natural History 
2001 Colorado Blvd.
Denver, CO 80205

Phone: (303)370-6392
Fax: (303)331-6492
email: KCarpenter@DMNS.org

For fun:
 http://dino.lm.com/artists/display.php?name=Kcarpenter