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Revealing the skeleton of the polar dinosaur Leaellynasaura amicagraphica using synchrotron computed tomography

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Abstract

Leaellynasaura amicagraphica was a small, bipedal, herbivorous Australian dinosaur that lived about 106 million years ago within the Antarctic Circle of that time. Although three cranial elements of Leaellynasaura exist, they are too delicate to be removed from the rock in which they are embedded. Synchrotron computed tomography (CT) provides a uniquely non-destructive means of creating digital models, allowing the skull to be virtually removed and rearranged to reveal the Leaellynasaura skull and brain anatomy. Known aspects of the brain of this dinosaur suggest that it had enhanced visual ability enabling it to see under the low light conditions that prevailed during the polar winter. On the under-side of the parietal and frontal bones is a natural mould of the top of the brain. However, the anterior part of the endocast cannot be seen without preparation. The digital reconstruction of these bones reveals that the olfactory bulbs are also relatively large. Reconstruction of the similarly delicate post-cranial elements also allows for further study and display. The partial post-cranial skeleton of Leaellynasaura consists of limb bones, various vertebrae with ossified tendons, and the vertebrae of the pelvic region and the entire tail. It is clear that Leaellynasaura had an incredibly long tail, more than twice the length of the body anterior to the pelvis. Knowing the size of the skull and the partial hind limb bone of this dinosaur, its body size can be estimated by comparison with other known skeletons of the same family. The synchrotron scans and resulting 3D reconstruction of the skull and post-cranial skeleton provide a unique understanding of the morphology, not possible using any other known technique, and can be 3D printed for display.
Revealing the skeleton of the polar dinosaur Leaellynasaura amicagraphica using
synchrotron computed tomography
Alana C. Sharp1,2, Omar R. Regalado-Fernandez1, Karen Siu2, Thomas H. Rich3
1University College London, UK; 2Monash University, Melbourne, Australia; 3Museum Victoria, Melbourne, Australia
Introduction
Leaellynasaura amicagraphica was a small, bipedal, herbivorous dinosaur from the Early Cretaceous (~106 million years ago) of Australia within the Antarctic Circle of that time. Evidence for permafrost suggests that these
animals lived when mean annual temperatures ranged between -2°Cand 5°C, based on oxygen isotope studies (Gregory et al.,1989). Known aspects of the brain and cranial anatomy suggest that it had enhanced visual
ability enabling it to see under the low light conditions that prevailed during the polar winter.
Synchrotron scans, and the resulting 3D reconstruction of the skull and post-cranial skeleton, provide a unique view of the morphology of Leaellynasaura,and allows this material to be 3D printed for display.
Material & Methods
The cranial and post-cranial elements of Leaellynasaura are too delicate to be
removed from the rock in which they are embedded. Synchrotron computed
tomography (CT) provides a uniquely non-destructive means of creating digital
models, allowing the bones to be virtually removed and rearranged to reveal the
anatomy for further study and display.
On the inferior surface (B) of the parietal
and frontal bones is a mould of the top of the
brain. However, the anterior part of the
endocast cannot be seen without removing the
matrix. The digital reconstruction (B) of these
bones reveals a more complete view of the
brain including the olfactory and optic lobes.
The skull roof (MV P185990) was originally
considered to be from the same individual as
the holotype cheek fragment (below) based on
its size and the location it was discovered
(Rich et al., 2010). The size of the two
specimens is comparable, however, due to
distortion the joint between the lacrimal and
frontal does not align accurately (Herne, 2014).
fem
tib
fib
References
Herne, M.C. (2014) Anatomy, systematics and phylogenetic relationships of the Early Cretaceous ornithopod dinosaurs of the Australian-Antarctic rift
system. Doctoral Thesis. University of Queensalnd.
Rich, T.H, Galton, P.M. & Vickers-Rich, P. (2010)The holotype individual of the ornithopod dinosaur Leaellynasaura amicagraphica Rich & Rich, 1989 (late
Early Cretaceous, Victoria, Australia). Alcheringa 34(3): 385-396.
Gregory, R.T., Douthitt, C.B., Duddy, I.R., Rich, P.V. & Rich, T.H. (1989) Oxygen isotopic composition of carbonate concretions from the lower Cretaceous
of Victoria, Australia: implications for the evolution of meteoric waters on the Australian continent in a paleopolar environment. Earth Planet. Sc. Lett.
92:27-42.
Leaellynasaura had an
incredibly long tail, more than
twice the length of the body and
approximately 13 times the
femoral length. This is unlike any
other member of the same family
and is perhaps one of the
longest tails relative to body
length of all dinosaurs.
The caudal vertebrae series for
this individual is approx. 850
mm, and the entire tail may have
been up to 980 mm.
ilium
A
B
olf
opl
lac
lac
pt
quad
quad
jugal
ept
max
10 mm
10 mm
The bones of the pelvic region are mostly hidden by the matrix. Segmentation and
reconstruction of these bones revealed a near complete ilium (previously unknown for
this species), the head of the left and right pubis, the head of the left and right ischium,
and possible sacral vertebrae.
The ambiens process of the pubis, dorsal crest of ilium, ischial shaft and pubic blade are
missing from this block.
head of femur
pubis ischium
100 mm
Complete specimens (photo courtesy of Jon Augier, Museum Victoria)
From Herne (2014)
The post-cranial material (MV P185992 and MV P 185993) are
from a single individual, and include the entire tail. Over 70 caudal
vertebrae are preserved in articulation with three distinct regions
(Rich et al.2010, Herne, 2014).
The holotype
(MV P185991) is in
two parts (1) the left
maxilla with dentition,
partial jugal, quadrate
and pterygoid; (2)
lacrimal, palatal and
mediocaudal maxillary
region.
Acknowledgements
We would like to thank Anton Maksimenko (Australian Synchrotron) for the scanning, David Pickering and Lesley Kool for fossil preparation and all the
volunteers of the Dinosaur Dreaming Project who are responsible for the discovery and excavation of many of the specimens used in this study.
©Julio Lacerda
Thoracic vertebrae with
ossified epaxial tendons
The cranial material (MV P185990 cranial table; MV P185991 holotype partial maxilla and dentition) include the nasals, frontals, parietals, left
maxilla with dentition and partial jugal, quadrate, lacrimal and pterygoid. The post-cranial material (MV P185992 &MV P185993 18 blocks)
include the hind limb bones, vertebrae of the pelvic region with ossified tendons, a partial pelvis and the entire tail with over 70 vertebrae (Rich et al.
2010).
The material was scanned at the Imaging and Medical Beamline (IMBL) of the Australian Synchrotron. The cranial pieces were scanned using a
voxel size of 10 microns to achieve the highest resolution possible, and the larger post-cranial pieces were scanned at approximately 40 micron
voxels. Segmentation and reconstruction was performed in Avizo 9.3.
mt II
mt I
mt III
pd II-1
pd I-1
pd IV-1
mt IV
ptar
Thoracic
ribs
tib
MV P185990 The vertebral bodies become progressively more elongate along the caudal series.
The cranial-most caudal vertebral region have spinal processes that expand distally and
vertebral bodies with an oval section. Haemal arches are evident from Ca1 to Ca29 and
become boot shaped more distally. The distal caudal vertebrae are elongate and
dorsoventrally compressed.
Although there are ossified tendons in the thoracic region, none are present along the
caudal vertebrae.
The proportions of the hind limb segments, and the extended tail with elongate vertebral
bodies, suggest that L. amicagraphica may have been a cursorial ornithischian and
possibly very agile.
Thoracic vertebrae Caudal vertebrae
Silhouette edited from Herne (2014)
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