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= Nemegtomaia =
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Introduction
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'Nemegtomaia' is a genus of oviraptorid dinosaur from what is now
Mongolia that lived in the Late Cretaceous Period, about 70million
years ago. The first specimen was found in 1996, and became the basis
of the new genus and species 'N. barsboldi' in 2004. The original
genus name was 'Nemegtia', but this was changed to 'Nemegtomaia' in
2005, as the former name was preoccupied. The first part of the
generic name refers to the Nemegt Basin, where the animal was found,
and the second part means "good mother", in reference to the fact that
oviraptorids are known to have brooded their eggs. The specific name
honours the palaeontologist Rinchen Barsbold. Two more specimens were
found in 2007, one of which was found on top of a nest with eggs, but
the dinosaur had received its genus name before it was found
associated with eggs.
'Nemegtomaia' is estimated to have been around 2 m (7 ft) in length,
and to have weighed 40 kg (85 lb). As an oviraptorosaur, it would have
been feathered. It had a deep, narrow, and short skull, with an arched
crest. It was toothless, had a short snout with a parrot-like beak,
and a pair of tooth-like projections on its palate. It had three
fingers; the first was largest and bore a strong claw. 'Nemegtomaia'
is classified as a member of the oviraptorid subfamily Heyuanninae,
and is the only known member of this group with a cranial crest.
Though 'Nemegtomaia' has been used to suggest that oviraptorosaurs
were flightless birds, the clade is generally considered a group of
non-avian dinosaurs.
The nesting 'Nemegtomaia' specimen was placed on top of what was
probably a ring of eggs, with its arms folded across them. None of the
eggs are complete, but they are estimated to have been 5to 6 cm (2to
2.3in) wide and 14to 16 cm (5to 6in) long when intact. The specimen
was found in a stratigraphic area that indicates 'Nemegtomaia'
preferred nesting near streams that would provide soft, sandy
substrate and food. 'Nemegtomaia' may have protected its eggs by
covering them with its tail and wing feathers. The skeleton of the
nesting specimen has damage that indicates it was scavenged by skin
beetles. The diet of oviraptorids is uncertain, but their skulls are
most similar to other animals that are known or thought to have been
herbivorous. 'Nemegtomaia' is known from the Nemegt and Baruungoyot
formations, which are thought to represent humid and arid environments
that coexisted in the same area.
History of discovery
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In 1996 the Japanese palaeontologist Yoshitsugu Kobayashi (as part of
the "Mongolian Highland International Dinosaur Project" team) found an
incomplete skeleton of an oviraptorid dinosaur in the Nemegt Formation
of the Gobi Desert in southwestern Mongolia. The specimen
(MPC-D100/2112 at the Mongolian Palaeontological Center, formerly PC
and GIN100/2112), consists of a nearly complete skull and a partial
skeleton, including cervical, dorsal, sacral, and caudal vertebrae, a
left scapula, the lower ends of both humeri, the right radius, both
ilia, the upper ends of both pubic bones, both ischia, and the upper
end of a femur. The specimen was described as a new specimen of the
genus 'Ingenia' (referred to as 'Ingenia' sp.; of uncertain species)
by the Chinese palaeontologist Lü Junchang and colleagues in 2002, and
used to highlight the similarities between oviraptorosaurs and birds.
In 2004 Lü and colleagues determined that the skeleton belonged to a
new, distinct taxon, and made it the holotype specimen of 'Nemegtia
barsboldi'. The genus name refers to the Nemegt Basin, and the
specific name honours the Mongolian palaeontologist Rinchen Barsbold,
the leader of the team that found the specimen. In 2005 the describers
discovered (after being notified by a biologist) that the name
'Nemegtia' had already been used for a genus of freshwater seed shrimp
(Ostracoda) from the same formation in 1978, and was therefore
preoccupied. They proposed instead the new genus name 'Nemegtomaia'
("'maia'" means "good mother" in Greek, and the full name means "good
mother of the Nemegt"), making reference to the then-recent discovery
that oviraptorids brooded eggs rather than stealing them, though no
trace of a nest or eggs had yet been found associated with
'Nemegtomaia' itself. The first known member of the oviraptorid family
was found with a nest of eggs originally thought to have belonged to
the ceratopsian genus 'Protoceratops', and was therefore named
'Oviraptor' in 1924; this name means "egg-seizer". In the 1990s more
oviraptorid specimens were discovered associated with nests and eggs,
wherein oviraptorid embryos were found, thereby proving that the eggs
belonged to the oviraptorids themselves. 'Ingenia' was similarly
renamed as 'Ajancingenia' in 2013, since the former genus name was
preoccupied by a roundworm (Nematoda).
In 2007 two new specimens of 'Nemegtomaia' were found by the
"Dinosaurs of the Gobi" expedition, and were described by the Italian
palaeontologist Federico Fanti and colleagues in 2013. The first
specimen, MPC-D107/15, was found by Fanti (who nicknamed it "Mary") in
the Baruungoyot Formation, and consists of a nest with the presumed
parent on top. As the fourth genus of oviraptorid found on top of a
nest (after 'Oviraptor', 'Citipati', and cf. 'Machairasaurus'),
'Nemegtomaia' had therefore received a genus name referring to this
feature before it was itself found associated with eggs. The specimen
was excavated from a vertical cliff under "difficult circumstances",
including heavy rain and collapsing sandstone blocks.
The nesting skeleton preserves parts of the skull, both scapulae, the
left arm and hand, the right humerus, the pubic bones, the ischia, the
femora, the tibiae, fibulae, and the lower portions of both feet. This
specimen was found less than 500 m (1640 ft) from the holotype, and
was of the same size; it was assigned to 'Nemegtomaia' due to its
similar anatomical features and geographical proximity. It was
collected in a single block so that the spatial relationship of the
bones and eggs would be preserved. The second specimen, MPC-D107/16,
was found by the American palaeontologist Nicholas R. Longrich in the
Nemegt Formation, and consists of the hands, a partial left ulna and
radius, ribs, a partial pelvis, and both femora. This specimen was 35%
smaller than the others, and was assigned to 'Nemegtomaia' due to its
hands having the same characteristics as those of specimen
MPC-D107/15. It is possible that the hands may have belonged to a
different individual, as they were not found articulated with the rest
of the skeleton (other oviraptorids are known from quarries with
multiple skeletons), but this cannot be confirmed.
Description
======================================================================
'Nemegtomaia' is estimated to have been around 2 m (7 ft) in length,
and to have weighed 40 kg (85 lb), a size extrapolated from more
completely known relatives. As an oviraptorosaur, it would have been
feathered. The neural spines of the neck (cervical) vertebrae were
short, and the neural arches had an x-shaped appearance. The middle
three of these vertebrae were the largest. The scapula (shoulder
blade) appears to have been 185 mm (7 in) in total length. The humerus
(upper arm bone) had a fossa (depression) in a position similar to
modern birds, but atypical among oviraptorosaurs, and appears to have
been 152 mm (6 in) long. The radius of the lower arm was straight,
oval in cross-section, and may have been 144 mm (5 in) long. The first
finger was relatively large and had a strong ungual (claw bone), and
was more massive than the two other fingers. The second finger was
slightly longer than the first, and the third finger was the smallest.
The upper margin of the ilium of the pelvis was straight, and though
both ilia were close to each other, they were not fused together. The
pubic shaft was turned backwards. The femur (thigh bone) is estimated
to have been 286 mm (11 in) long, and the tibia of the lower leg 317
mm long (12 in).
The skull of 'Nemegtomaia' was deep, narrow, and short (compared to
the rest of the body), and reached 179 mm (7 in) in length. It had a
well-developed crest, formed by the nasal and premaxilla bones (mainly
the latter) of the snout. The nearly vertical front margin of the
holotype's crest formed an almost 90degree angle with the upper margin
of the skull. Compared to other oviraptorids, the nasal processes
(projections) of the premaxillae were barely visible when viewed from
above (where they connected with the nasal bones on the highest points
of the crest). The crest extended hindwards and down, forming a round
arch at the highest point. The diameter of the orbit (eye opening) was
52 mm (2 in); the eyes looked large due to the shortness of the skull.
The antorbital cavity in front of the eye consisted of two fenestrae
(openings); a large antorbital fenestra at the back, and a small
maxillary fenestra at the front. 'Nemegtomaia' was distinct from other
oviraptorids in that the frontal bone on the midline of the skull was
about 25% the length of the parietal bone from front to back. The
nares (external nostrils) were relatively small and placed high on the
skull.
The jaws of 'Nemegtomaia' were toothless, and like other oviraptorid
dinosaurs, it had a short snout with a deep, robust, and somewhat
parrot-like beak. It had a hard palate formed by the premaxillae,
vomers, and maxillae, like other oviraptorids. The palate was strongly
concave (downwards-projecting), and had a cleft on the central part.
As in other oviraptorids, it had a pair of tooth-like projections on
the palate that were directed downwards (a feature that has been
called "pseudo-teeth"). 'Nemegtomaia' had small foramina (openings) on
the sides of the suture (joint) between the premaxillae at the front
of the snout, which may have been nutrient openings (and which
indicate the presence of a keratinous bill). The lower jaw was short
and deep, with a convex lower surface, and reached 153 mm (6 in) in
length. The dentary bone of the lower jaw reached 50 mm (2 in) at its
highest point. The mandibular symphysis (where the two-halves of the
lower jaw connected) was short, deep, and very pneumatised (with
air-spaces). The mandibular fenestra was large and was located at the
front part of the lower jaw. As in most other oviraptorids, the front
of the lower jaw was down-turned.
Though 'Nemegtomaia' does not possess any single feature that
distinguishes it from other oviraptorids (autapomorphies), the
combination of a crest, an enlarged first finger, and a high number of
sacral vertebrae (eight), is unique to this taxon.
Classification
======================================================================
In their 2004 phylogenetic analysis, Lü and colleagues classified
'Nemegtomaia' as a derived (or "advanced") oviraptorosaur, and found
it to be most closely related to the genus 'Citipati'. In 2010
Longrich and colleagues determined that 'Nemegtomaia' belonged in the
family Oviraptoridae, as part of the subfamily Ingeniinae, making it
the only member of the latter group with a prominent crest. Members of
the other recognised subfamily, Oviraptorinae, all possess crests.
Members of this subfamily were distinguished by smaller size, short
and robust forelimbs with weakly curved claws, the number of vertebrae
in the synsacrum, as well as certain features of the feet and pelvis.
Longrich and colleagues suggested that the presence of a crest on
'Nemegtomaia' makes it possible that this feature evolved or
disappeared several times among oviraptorids, or that the animal may
not have been a ingeniine. In 2010, the American palaeontologist
Gregory S. Paul suggested that crestless oviraptorids were either the
juveniles or females of otherwise crested species, and that the number
of genera in the group was therefore exaggerated. He listed
'Nemegtomaia' as "'Citipati' (='Nemegtomaia') 'barsboldi', considering
it very similar to that genus, but in 2016, he instead listed it as
"'Conchoraptor' (='Nemegtomaia') 'barsboldi'".
In 2012 Fanti and colleagues also found 'Nemegtomaia' to be part of
Ingeniinae as a derived member, closest to 'Heyuannia', due to the
proportions of the hands of the two new specimens (relatively short
with a robust first finger). They stated that, though the presence of
crests is generally associated with oviraptorines rather than
ingeniines, the feature may be correlated with size and maturity. They
pointed out that the nasal and frontal bones of the ingeniine
'Conchoraptor' were pneumatic and could potentially have grown into a
crest as the animal matured, though all known skeletons of that genus
are of the same small size (and one specimen appears to have been
fully grown). The subfamily name Ingeniinae has since been replaced by
the name Heyuanninae (since 'Ingenia' was preoccupied). The cladogram
below shows the placement of 'Nemegtomaia' within Oviraptoridae,
according to Fanti etal., 2012:
{{clade| style=font-size:90%;line-height:90%
|label1=Oviraptoridae
|1={{clade
|1='Oviraptor'
|label2=unnamed
|2=
|label3=unnamed
|3={{clade
|1='Khaan'
|label2=unnamed
|2={{clade
|1='Conchoraptor'
|label2=unnamed
|2={{clade
|1='Machairasaurus'
|label2= unnamed
|2= }} }} }} }} }}
Evolution
===========
The clade Oviraptorosauria is generally regarded as a group of
non-avian (or non-bird) theropod dinosaurs, and their similarity to
birds (Aves) has often been noted. Fossils of oviraptorosaurs in the
family Caenagnathidae have historically been confused with those of
birds, and some researchers have gone so far as to consider
oviraptorosaurs as a whole more closely related to birds than to other
non-avian dinosaurs. In 2002 Lü and colleagues used the then unnamed
'Nemegtomaia' to show similarities between birds and oviraptorosaurs,
and found the latter group to be closer to birds than to bird-like
dinosaurs such as dromaeosaurs. They therefore concluded that
oviraptorosaurs were flightless birds rather than non-avian dinosaurs,
and noted that the boundary between birds and dinosaurs was becoming
more and more difficult to delineate. Other researchers have instead
found dromaeosaurs and troodontids to be most closely related to
birds, together forming the group Paraves; oviraptorosaurs,
therizinosaurs, and alvarezsaurs are just outside this group. The
wider group that includes oviraptorosaurs and Paraves is called
Pennaraptora, and this group is defined by the presence of pennaceous
feathers (feathers with a stalk).
Oviraptorosaurs are known from Asia (where they may have originated)
and North America, and are mainly known from deposits that date from
the Campanian-Maastrichtian ages of the Cretaceous period. The group
includes small to large members, and they are characterised by their
short skulls and beaks, elongated fingers, and short tails. Basal (or
"primitive") members had teeth, which disappeared in derived members
of the group (those within the superfamily Caenagnathoidea, which
includes Oviraptoridae). They were at least partially herbivorous, and
brooded their nests in a bird-like posture. Though they are all
thought to have to been feathered, they appear to have been
flightless. Cranial crests appear to have evolved convergently in
different lineages within the group. The family Oviraptoridae (to
which 'Nemegtomaia' belongs) consisted of generally small members, and
is exclusively known from the Upper Cretaceous of Asia, with most
genera having been discovered in the Gobi Desert of Mongolia and
China. Including 'Nemegtomaia', at least nine oviraptorid genera have
been discovered in a relatively small geographical area in the Gobi
Desert.
Reproduction
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The 'Nemegtomaia' specimen MPC-D 107/15 was found associated with a
nest with eggs; its feet were placed in the centre of what was
probably a ring of eggs, with the arms folded across the tops of the
eggs on each side of the body, a posture similar to what is seen in
other fossils of brooding oviraptorids. The collected part of the nest
is about 90 cm (35 in) wide and 100 cm (30 in) long; the skeleton
occupies the upper 25 cm (10 in) of the block, whereas the remaining
20 cm (8 in) is occupied by broken eggs and shells. There is no
evidence of plant material in the nest, but there are fragments of
undetermined bones. The nest does not preserve any complete eggs or
embryos, which prevents determination of the size, shape, number, and
arrangement of the eggs in the nest. It is probable that there were
originally two layers of eggs below the body, and there do not appear
to have been eggs in the centre of the nest. Most eggs (seven distinct
eggs have been identified) and egg fragments were recovered either in
the lower layer of the nest or under the skull, neck, and limbs of the
specimen, and the bones either rested directly on the eggs or were
within 5 mm (0.2 in) of their surfaces. That the skeleton was directly
positioned on top of it shows that the nest was not completely covered
by sand. Though the placement of the eggs does not suggest a specific
arrangement in the nest, most other oviraptorid nests show that the
eggs were arranged in pairs in up to three levels of concentric
circles. The eggs of MPC-D107/15 were therefore most likely displaced
during burial, or by external factors, such as strong winds,
sandstorms, or predators. This also supports the idea that the upper
layer of eggs was not buried, as fully buried eggs would have been
less likely to be transported by external factors.
Oviraptorid eggs appear to have been 17 cm (6 in) long on average, and
the most complete eggs found with MPC-D107/15 are thought to have been
5to 6 cm (2to 2.3in) wide and 14 to 16 cm (5to 6in) long when intact.
The eggs are nearly identical to some that have previously been found
in Mongolia, and have therefore been assigned to the oofamily
(egg-taxon family) Elongatoolithidae. The eggshells are relatively
thin, between 1and 1.2 mm (0.03and 0.04in), and their outer surface is
covered by ridges and nodes that rise about 0.3 mm (0.01 in) above the
shell. The micro-structure of the eggshells could not be properly
studied, as the calcite has been heavily altered and re-crystallized.
The nesting specimen was found in a stratigraphic area indicating that
oviraptorids preferred nesting near streams that provided soft, sandy
substrate and food in environments that were otherwise xeric
(receiving a small amount of moisture). Many oviraptorids have been
found in brooding positions, indicating they may have brooded for
relatively long periods, similar to modern birds such as the ostrich,
emu, and black-breasted buzzard, which brood for more than 40 days
with a limited supply of sustenance. Nesting in desert environments
can be harmful to adults that stay in the nest for large parts of the
day, and for eggs and nestlings, due to heat stress. The choice of
nesting area may therefore have been a mechanism for successful
incubation in extreme heat. It has also been suggested that the
evolution of tail-feathers in oviraptorosaurs was an adaptation for
shading and protecting eggs in their nests. That the second finger of
heyuannine oviraptorids was reduced in size compared to the robust
first finger may be explained by a change in function; it may be
related to the presence of long wing feathers that were attached to
the second finger. These wing feathers were probably used to protect
the eggs during nesting. When the second finger began functioning as a
feather support, its ability to grasp was reduced, and this function
was taken over by the first finger, which therefore became more
robust. The third finger was reduced in size, too, probably because it
was positioned behind the wing feathers in a way where it would not be
effective for grasping.
In 2018, the Taiwanese palaeontologist Tzu-Ruei Yang and colleagues
identified cuticle layers on egg-shells of maniraptoran dinosaurs,
including oviraptorids. In modern birds, such layers (which consist
mainly of lipids and hydroxyapatite) serve to protect the eggs from
dehydration and invasion of microbes. The researchers suggested that
the cuticle-coated eggs would have been a trait adapted for enhancing
their reproductive success in the variable environments where
'Nemegtomaia' and other oviraptorids nested.
Various studies have suggested that several individuals would gather
eggs in a single nest, and arrange them so they could be protected by
one individual, possibly a male. In 2010 the American palaeontologist
David J. Varricchio and colleagues found that the relatively large
clutch-size of oviraptorids and troodontids is most similar to those
of modern archosaurs (birds and crocodilians, the closest living
relatives of dinosaurs) that practice polygamous mating and extensive
male parental care (as seen in paleognaths such as ostriches and
emus). This reproductive system pre-dates the origin of birds and
would therefore be the ancestral condition for modern birds, with
biparental care (where both parents participate) being a later
development. Many oviraptorosaurs are known to have had pygostyles on
the end of their tails, which suggests the presence of feather-fans;
the American palaeontologist W. Scott Persons and colleagues suggested
in 2013 that these could have been used for intraspecific
communication such as courtship rituals.
Diet and feeding
==================
The diet of oviraptorids has been interpreted in various ways since
the time 'Oviraptor' was wrongly thought to have been a predator of
eggs. It has been suggested that oviraptorosaurs as a whole were
herbivores, which is supported by the gastroliths (stomach stones)
found in 'Caudipteryx', and the wear facets in the teeth of
'Incisivosaurus'. In 2010 Longrich and colleagues found that
oviraptorid jaws had features similar to those seen in herbivorous
tetrapods (four-limbed animals), especially those of dicynodonts, an
extinct group of synapsid stem-mammals. Oviraptorids and dicynodonts
share features such as short, deep, and toothless mandibles; elongated
dentary symphyses; elongated mandibular fenestrae; and a
downwards-projecting bar in the palate. Modern animals with jaws that
resemble those of oviraptorids include parrots and tortoises; the
latter group also has tooth-like projections on their premaxillae.
Longrich and colleagues concluded that due to the similarities between
oviraptorids and herbivorous animals, the bulk of their diet would
most likely have been formed by plant matter. Oviraptorids are found
at high frequencies in the formations they are known from, similar to
the pattern seen in dinosaurs that are known to be herbivorous; these
animals were more abundant than carnivorous dinosaurs, as more energy
was available at their lower trophic level in the food chain. The jaws
of oviraptorids may have been specialised for processing food, such as
xerophytic vegetation (adapted for environments with little water),
that would have grown in their environment, but this is not possible
to demonstrate, as little is known about the flora of the area at the
time. A 2013 study by Lü and colleagues found that oviraptorids appear
to have retained their hind limb proportions throughout ontogeny
(growth), which is also a pattern mainly seen in herbivorous animals.
In 2017, the Canadian palaeontologist Gregory F. Funston and
colleagues suggested that the parrot-like jaws of oviraptorids may
indicate a frugivorous diet that incorporated nuts and seeds.
In 1977 Barsbold suggested that oviraptorids fed on molluscs, but
Longrich and colleagues rejected the idea that they practised
shell-crushing altogether, since such animals tend to have teeth with
broad crushing surfaces. Instead, the shape of the dentary bones in
the lower jaws of oviraptorids suggests they had a sharp-edged beak
used for shearing tough food, not for cracking hard food items such as
bivalves or eggs. The symphyseal shelf at the front of the dentary may
have given some ability for crushing, but as this was a relatively
small area, it was probably not the main function of the jaws. The
fact that most oviraptorids have been found in sediments that are
interpreted as having been xeric and arid or semi-arid environments
also argues against them having been specialised eaters of shellfish
and eggs, as it is unlikely there would have been enough of these
items under such conditions to support them.
Longrich and colleagues pointed out that the robust forelimbs and
enlargement of a single finger in heyuannine oviraptorids is similar
to that seen in modern animals that eat ants and termites, such as
anteaters and pangolins, but the morpholology of heyuannine jaws does
not support them being insectivorous. The researchers found that the
function of heyuannine forelimbs was unclear, but suggested that they
could have been used for scratching, tearing, or digging, though not
prey capture.
In 2004 Lü and colleagues proposed that the articulation between the
quadrate and quadratojugal bones in the skull of 'Nemegtomaia'
suggested that these bones were movable in relation to each other,
which would have affected how the jaws functioned. In 2015 the Belgian
palaeontologist Christophe Hendrickx and colleagues found it unlikely
that 'Nemegtomaia' and other oviraptords had bird-like kinesis in
their skulls, due to the quadrate bone being immobile.
Palaeoecology
======================================================================
'Nemegtomaia' is known from the Nemegt and Baruungoyot Formations,
which date to the upper Campanian-lower Maastrichtian ages of the Late
Cretaceous period, about 70million years ago. Though this taxon is
known only from the Nemegt locality, unidentified oviraptorid remains
from other localities may belong to it. The Nemegt massif has numerous
canyons or gorges, up to 45 m deep, which have some of the best
exposures of these formations. The rock facies of the Nemegt Formation
are thought to represent a humid, fluvial (associated with rivers and
streams) environment, whereas those of the Baruungoyot Formation are
thought to represent an arid or semi-arid environment, with aeolian
(affected by wind) beds. These two formations with their diverse
fossils were historically thought to represent sequential time periods
with different environments, but in 2009 the Canadian palaeontologist
DavidA. Eberth and colleagues found that there was partial overlap
across the transition between them. The two formations "interfinger"
across a stratigraphic interval that is about 25 m (82 ft) thick,
which suggests that the fluvial and aeolian environments coexisted
when the area was sedimented.
The environment of the Nemegt Formation has been compared to the
Okavango Delta of present-day Botswana. The habitats in and around the
Nemegt rivers provided a home for a wide array of organisms. Aquatic
animals include molluscs, fish, turtles, and the crocodylomorph
'Shamosuchus'. Fossils of mammals such as multituberculates have been
found, and birds such as 'Gurilynia', 'Judinornis', and 'Teviornis'
are known. Herbivorous dinosaurs of the Nemegt Formation include
ankylosaurids such as 'Tarchia', the pachycephalosaurian
'Prenocephale', hadrosaurids such as 'Saurolophus' and 'Barsboldia',
and sauropods such as 'Nemegtosaurus', and 'Opisthocoelicaudia'. Other
theropods include tyrannosauroids such as 'Tarbosaurus', 'Alioramus',
and 'Bagaraatan', troodontids such as 'Borogovia', 'Tochisaurus', and
'Saurornithoides', therizinosaurs such as 'Therizinosaurus', and
ornithomimosaurians such as 'Deinocheirus', 'Anserimimus', and
'Gallimimus'.
Other oviraptorosaur genera known from the Nemegt Formation include
the basal 'Avimimus', the oviraptorids 'Rinchenia', 'Nomingia',
'Conchoraptor' and 'Ajancingenia', and the caenagnathid 'Elmisaurus'.
In spite of the high number of oviraptorid taxa in these formations
(the Nemegt has the highest known diversity of them anywhere), none of
them were closely related. The Nemegt Formation is unique in having
both oviraptorid and caenagnathid oviraptorosaurs, and in 1993 the
Canadian palaeontologist Phillip J. Currie and colleagues suggested
this diversity was due to the two groups preferring different
environments in the area. In 2016 the Japanese palaeontologist
Takanobu Tsuihiji and colleagues suggested that oviraptorids may have
preferred drier environments, while caenagnathids preferred fluvial
environments, based on the type of formations they have been found in.
Funston and colleagues suggested that oviraptorids were found in both
xeric and mesic environments (but were more abundant in the former),
whereas the other oviraptorosaur groups avoided the xeric
environments, and that the coexistence of the families can be
explained by niche partitioning in diet. The environment of the Nemegt
Formation may have acted as an oasis and thereby attracted
oviraptorids.
Taphonomy
===========
The nesting specimen MPC-D 107/15 has provided much information about
the taphonomic processes (changes during decay and fossilisation) in
the Baruungoyot Formation. The specimen is preserved in facies that
are thought to have been deposited through a sandstorm or dune-shift.
It does not seem to have been transported after death, but the body
appears to have shifted slightly to the right, indicating the sediment
in which it was deposited came towards it from its left side. The neck
is curved to the left, the left hand is folded backwards, and the legs
are folded into a crouching position. The vertebral column, neck, and
hips deteriorated during burial, and much of the damage to the
skeleton is thought to have been caused by the activity of
invertebrates.
Borings in the bones, burrows, and reworked sediments (perhaps caused
by the construction of pupal chambers) in the specimen indicate it was
scavenged upon by colonies of skin beetles (Dermestidae) and perhaps
other scavenging insects. There are many feeding traces in the joints
of the skeleton, and almost all the surfaces where the bones
articulated have been obliterated. There are also tunnels in the nest
under the neck and skull, and no egg remains have been found in parts
with such traces. Modern skin beetles mainly feed on muscle tissue but
not on moist materials, and their activity is prevented by rapid
burial. It is therefore thought that specimen MPC-D107/15 was only
partially buried at first, with the upper part being exposed enough
for a colony of skin beetles to develop. Some damage to the skeleton
(especially in the vertebral column) may also have been caused by the
scavenging of small mammals.
See also
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