Thescelosaurus
Name:
Thescelosaurus
(Wondrous lizard).
Phonetic: Fess-cul-o-sore-us.
Named By: Charles W. Gilmore - 1913.
Synonyms: Bugenasaura.
Classification: Chordata, Reptilia, Dinosauria,
Ornithischia, Thescelosauridae, Thescelosaurinae.
Species: T. neglectus, T. garbanii,
T. assiniboiensis.
Diet: Herbivore.
Size: Between 2.5 and 4 meters, depending
upon individuals/species. A particularly large specimen attributed to
T. garbanii has been identified as being 4.5
meters long.
Known locations: Canada, Alberta - Dinosaur
Park Formation, Oldman Formation, Scollard Formation,
Saskatchewan - Frenchman Formation, Ravenscrag Formation.
USA, Colorado - Laramie Formation, Montana - Hell Creek
Formation, Lance Formation, New Mexico - Fruitland Formation,
North Dakota - Hell Creek Formation, South Dakota - Hell
Creek Formation, Lance Formation, Wyoming - Lance Formation.
Time period: Maastrichtian of the Cretaceous.
Fossil representation: Many individuals in varying
states of completeness.
Thescelosaurus the Dinosaur
Thescelosaurus
seems to have been one of the main herbivorous dinosaurs roaming
around North America towards the end of the Cretaceous period. The
first fossils of Thescelosaurus began to be
recovered in the closing
decade of the nineteenth century, but were not described until
1913 by Charles W. Gilmore, who provided a much more detailed
description in 1915. For over a hundred years afterwards numerous
individuals of Thescelosaurus were recovered from
across the central
portion of North America, resulting in three distinct species being
named. One former genus called Bugenasaura has
also been discovered
to be a junior synonym to Thescelosaurus. Another
genus named
Parksosaurus
that in the past has been speculated to be synonymous with
Thescelosaurus, has in recent times been
re-affirmed as a distinct
genus.
Thescelosaurus
has often been described as a hypsilophodont, but modern analysis of
this dinosaur has found that Thescelosaurus is
actually quite different
from Hypsilophodon,
and therefore not likely related beyond both of
them being ornithopod dinosaurs. Like with most other ornithopod
dinosaurs, Thescelosaurus was a bipedal dinosaur
which had a body
counterbalanced by a long tail that accounted for much of the total
body length. Bizarrely for an ornithopod dinosaur, the upper leg of
Thescelosaurus was longer that the lower leg, the
complete opposite
to the arrangement seen in most other ornithopods. This meant that
Thescelosaurus would have actually been a relatively
slow runner.
This raises questions as to how Thescelosaurus
survived since there
are no other obvious defensive features on this dinosaur, yet we know
that Thescelosaurus became relatively widespread in
North America.
Scutes found in association with one Thescelosaurus
specimen may
indicate that at least one line of bony armour was present on the
body, however an alternative theory is that these scutes may have
come from a crocodile since these scutes have so far not been found
with any other individuals.
Numerous
individuals of Thescelosaurus are now known to us,
but there seems to
have been a wide variance in the sizes of adults which usually range
between two and half and four meters in length. Some of this
difference is likely down to a species level, with individuals of
T. garbanii often approaching four meters in
length, though one
specimen seems to have been at least four and a half meters long.
It’s possible though that Thescelosaurus may have
been sexually
dimorphic, meaning that there was a clear difference in how large
males and females grew.
Thescelosaurus
had pointed teeth at the front of the mouth and leaf shaped teeth
towards the back. Leaf shaped teeth like these are common in
herbivorous dinosaurs that feed upon softer plants as they easily slice
through leafy plant material. The pointed teeth are a little more
puzzling however, as they are not as well suited to an exclusive diet
of plants. This has led some to speculate that Thescelosaurus
might
have been omnivorous, either eating small animals like lizards or
occasionally scavenging carrion. Prominent ridges on the maxilla
bones of the skull and the observation that the leaf-shaped maxilla
teeth are set well inside the mouth support the idea that
Thescelosaurus had quite muscular cheeks to stop
food spilling out of
the sides of the mouth when processing food. Six pairs of small teeth
were also present in the pre-maxilla, though the tip of the
premaxilla was toothless to accommodate the horny beak that covered the
front of the mouth. This beak would have been the primary shearing
apparatus when cropping plants.
There
are two schools of thought concerning which kind of habitat
Thescelosaurus preferred. One is that Thescelosaurus
preferred
channel systems, feeding upon the banks of rivers and streams,
while another is that Thescelosaurus preferred
floodplains. It may
not sound like there is a lot of difference, and indeed the two
habitats are connected. Both have an abundance of plants that grow in
fertile soil, but channel systems feature a more regular growth of
plants where growth on floodplains can be more seasonal. Most
Thescelosaurus fossils have been dug out of
sandstone, which is more
a sign of a channel environment, whereas floodplains tend to produce
mudstone. There are also no mass bone beds of Thescelosaurus
individuals, again indicating a lack of flooding victims.
If
Thescelosaurus lived near water browsing upon softer
plants growing by
the water, then this may have also been an escape route for them when
they got attacked. As soon as a predator showed up they may have
entered and swum into and across the river, explaining why
Thescelosaurus did not retain leg proportions for
running, and also
how the genus came to be so widespread. Reduced limb proportions
would have also helped with locomotion through the water by not being
so easy to get tangled in underwater weeds and debris, and if the
latest reconstructions of Spinosaurus
are anything to go by, then
limb reduction in semi-aquatic dinosaurs may have been an easy thing to
achieve.
Thescelosaurus
lived during the latest stage of the Cretaceous and lived in the same
locations as hadrosaurs,
ceratopsians,
ankylosaurs,
pachycephalosaurs
and ornithomimids.
Predatory threats to
Thescelosaurus were many and included the famous tyrannosaurs
like
Tyrannosaurus,
to troodonts
like Troodon,
to one of the largest
dromaeosaurs,
Dakotaraptor.
Fossilised ‘heart’?
There
has been a lot of debate concerning a possible ‘heart’ believed to
have been fossilised in an individual Thescelosaurus
that was found in
1993. In a paper by Fisher et al published in 2000, they
described a fossilised structure found within a specimen of
Thescelosaurus that resembled a heart. Their
theory was that the
original heart muscle had been saponified (turned to soap) as a
result of anaerobic conditions and then through regular fossilisation
turned to geothite. The authors of this paper noted that the
‘heart’ was comparable with the heart of an ostrich with four
chambers, and that Thescelosaurus and probably
other dinosaurs had an
elevated metabolic rate closer to what we would term as warm-blooded.
It
was not long after that doubts about the interpretation of this
‘heart’ began to be publically voiced. The main problem that many
people had was that the ‘heart’ just did not look right with
anomalies in the basic structure. In a 2001 paper by Timothy et
al the ‘heart’ was re-interpreted as a simple concretion that had
simply built up an extra dense portion of sedimentary rock. It was
also noted that the ‘heart’ partially engulfed one of the ribs and
that a second similar concretion could be found behind the right leg.
Also observed was the part preserved as the aorta, the main blood
vessel that should carry oxygenated blood from the heart. Many noted
that this aorta actually became narrower where it actually joined the
‘heart’, something unprecedented in heart anatomy as this would
restrict blood flow. There were also no arteries branching off from
this aorta to take blood to other parts of the body. Despite these
observations, the original authors who described the ‘heart’
defended their earlier work, saying that the ‘heart’ was a
concretion, but one built up to the structure of the original heart
and aorta.
In
2011 a new study of the ‘heart’ by Cleland, Stoskopf and
Schweitzer used state of the art investigative methods including X-ray
diffraction, X-ray photoelectron spectroscopy, scanning electron
microscopy, histology and CT scanning to determine the truth of the
matter. Actually using computer modelling to ‘see inside’ the
‘heart’, they revealed that this structure did not have four
chambers like it was previously expected to. Instead they mapped out
three areas filled with less dense material than the surrounding
‘heart’ structure. Furthermore, these areas were completely
unconnected, which means in absolutely no way could they be heart
chambers, as heart chambers need to be connected so that blood can
flow through the heart.
The
2011 study also failed to find any of the common elements that are
found in fossilised remains of biological structures (i.e. carbon,
nitrogen, phosphorus). This means that this ‘heart’ was
composed of inorganic material that was not part of the original
dinosaur. There was however one patch where the impressions of soft
tissues identifiable at the cellular level were observed. These
impressions though do not reflect a heart structure, and were
recorded as simply being a case of a concretions pressing against soft
tissues and then preserving the form while hardening.
The
overall conclusion is that this structure is simply a concretion of
sand hardened to stone in such a way that at a glance it has become a
simulacrum of a heart, not the actual fossilised remains of one.
Further reading
- A new dinosaur from the Lance Formation of Wyoming. -Smithsonian
Miscellaneous Publications 61(5):1-5. - Charles W. Gilmore
- 1913.
- Osteology of Thescelosaurus, an orthopodus
dinosaur from the
Lance Formation of Wyoming. - Proceedings of the U.S. National
Museum 49 (2127): 591–616. - Charles W. Gilmore -
1915.
- Thescelosaurus warreni, a new species of
orthopodous dinosaur
from the Edmonton Formation of Alberta. - University of Toronto
Studies (Geological Series) 21: 1–42. - William A. Parks
- 1926.
- Classification of Thescelosaurus, with a
description of a new
species. - Geological Society of America Proceedings for 1936:
365. - Charles M. Sternberg - 1937.
- Thescelosaurus edmontonensis, n. sp., and
classification of
the Hypsilophodontidae. - Journal of Paleontology 14 (5):
481–494. - Charles M. Sternberg - 1940.
- Notes on Thescelosaurus, a conservative
ornithopod dinosaur from
the Upper Cretaceous of North America, with comments on ornithopod
classification. - Journal of Paleontology 48 (5):
1048–1067. - Peter M. Galton - 1974.
- Dinosaurs of the Lance Formation in eastern Wyoming, by Kraig
Derstler - In, The Dinosaurs of Wyoming. Wyoming Geological
Association Guidebook, 44th Annual Field Conference. Wyoming
Geological Association. pp. 127–146. - Gerald E. Nelson
(ed.) 1994.
- The species of the basal hypsilophodontid dinosaur Thescelosaurus
Gilmore (Ornithischia: Ornithopoda) from the Late Cretaceous of
North America. - Neues Jahrbuch f�ur Geologie und Pal�ontologie
Abhandlungen 198 (3): 297–311. - Peter M. Galton -
1995.
- Cranial anatomy of the basal hypsilophodontid dinosaur
Thescelosaurus neglectus Gilmore (Ornithischia; Ornithopoda) from
the Upper Cretaceous of North America. - Revue Pal�obiologie,
Gen�ve 16 (1): 231–258. - Peter M. Galton - 1997.
- Cranial anatomy of the hypsilophodont dinosaur Bugenasaura
infernalis (Ornithischia: Ornithopoda) from the Upper Cretaceous
of North America. - Revue Pal�obiologie, Gen�ve 18 (2):
517–534. - Peter M. Galton - 1999.
- Technical comment: dinosaur with a heart of stone. - Science
291 (5505): 783a. - Timothy R. Rowe, Earle F. McBride
& Paul C. Sereno - 2001.
- Reply: dinosaur with a heart of stone. - Science 291
(5505): 783a. - Dale A. Russel, Paul E. Fisher, Reese
F. Barrick & Michael K. Stoskopf - 2001.
- Taxonomic revision of the basal neornithischian taxa Thescelosaurus
and Bugenasaura. - Journal of Vertebrate
Paleontology 29
(3): 758–770. - Clint A. Boyd, Caleb M. Brown,
Rodney D. Scheetz & Julia A. Clarke - 2009.
- Histological, chemical, and morphological reexamination of the
‘heart’ of a small Late Cretaceous Thescelosaurus.
-
Naturwissenschaften 98 (3): 203–211. - Timothy P.
Cleland, Michael K. Stoskopf & Mary H. Schweitzer -
2011.
- A new basal ornithopod dinosaur (Frenchman Formation,
Saskatchewan, Canada), and implications for late Maastrichtian
ornithischian diversity in North America. - Zoological Journal of
the Linnean Society 163 (4): 1157–1198. - Caleb M,
Brown, Clint A. Boyd & Anthony P. Russel - 2011.
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