Nemegtomaia

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Nemegtomaia
Temporal range:
Ma
↓
Specimen tentatively assigned to Nemegtomaia,
Central Museum of Mongolian Dinosaurs
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: Theropoda
Family: †Oviraptoridae
Subfamily: †
Heyuanninae
Genus: †Nemegtomaia
Lü et al., 2005
Type species
†Nemegtomaia barsboldi
Lü et al., 2005
Synonyms
  • Nemegtia barsboldi
    Lü et al., 2004

Nemegtomaia is a

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

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 5 to 6 cm (2 to 2.3 in) wide and 14 to 16 cm (5 to 6 in) long when intact. The specimen was found in a

arid
environments that coexisted in the same area.

History of discovery

Maps showing the Nemegt locality (a, b) and where Nemegtomaia specimens have been found (c)

In 1996 the Japanese palaeontologist Yoshitsugu Kobayashi (as part of the "Mongolian Highland International Dinosaur Project" team) found an incomplete skeleton of an

oviraptorosaurs and birds.[1][2][3]

Nest diagram (A), bones of the nesting specimen that covered eggs (B–D), and a lower layer of eggs (E–G)

In 2004 Lü and colleagues determined that the skeleton belonged to a new, distinct taxon, and made it the

roundworm (Nematoda).[9]

Assigned specimens

Skeletal diagrams showing the preserved elements of the three known specimens in grey, composite on right

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-D 107/15, was found by Fanti (who nicknamed it "Mary") in the

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.[2][5]

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-D 107/16, 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-D 107/15. 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.[2]

Description

Size compared to a human

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.

sacral vertebrae (eight), is unique to this taxon.[11]

Skull

Preserved parts and reconstruction of specimen MPC-D 107/15's skull

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

nares (external nostrils) were relatively small and placed high on the skull.[1][2]

The jaws of Nemegtomaia were toothless, and like other oviraptorid dinosaurs, it had a short snout with a deep, robust, and somewhat

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.[1][2][12][3] As in most other oviraptorids, the front of the lower jaw was down-turned.[13]

Postcranial skeleton

Life restoration

The neural spines of the neck (

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).[1][2]

Classification

Henan Geological Museum

In their 2004

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 an ingeniine.[2][14] 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".[12][10]

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).[2] The subfamily name Ingeniinae has since been replaced by the name Heyuanninae (since Ingenia was preoccupied).[15] The cladogram below shows the placement of Nemegtomaia within Oviraptoridae, according to Fanti et al., 2012:[2]

oviraptorid
heads shown to scale; K (bottom middle) is Nemegtomaia
Oviraptoridae

Oviraptor

unnamed

Rinchenia

Citipati

unnamed

Khaan

unnamed

Conchoraptor

unnamed

Machairasaurus

unnamed

"Ingenia" (=

Ajancingenia
)

unnamed

Nemegtomaia

Heyuannia

Evolution

oviraptorosaurs
compared; P (far right) is the hand of Nemegtomaia, grouped within Ingeniinae (=Heyuanninae)

The

pennaceous feathers (feathers with a stalk).[16]

Oviraptorosaurs are known from Asia (where they may have originated) and North America, and are mainly known from deposits that date from the

Upper Cretaceous of Asia, with most genera having been discovered in the Gobi Desert of Mongolia and China.[14] Including Nemegtomaia, at least nine oviraptorid genera have been discovered in a relatively small geographical area in the Gobi Desert.[11][17]

Palaeobiology

Reproduction

Diagram showing the remains and body outline of the nesting specimen

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-D 107/15 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.[2]

Details of eggs preserved with the nesting specimen

Oviraptorid eggs appear to have been 17 cm (6 in) long on average, and the most complete eggs found with MPC-D 107/15 are thought to have been 5 to 6 cm (2 to 2.3 in) wide and 14 to 16 cm (5 to 6 in) long when intact. The eggs are nearly identical to some that have previously been found in Mongolia, and have therefore been assigned to the

micro-structure of the eggshells could not be properly studied, as the calcite has been heavily altered and re-crystallized.[2]

The nesting specimen was found in a

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.[2]

Restoration of a nesting individual

In 2018, the Taiwanese palaeontologist Tzu-Ruei Yang and colleagues identified

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.[18]

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.

courtship rituals.[20]

Diet and feeding

Lower jaw of the holotype specimen, with beak on the right

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

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.[14]

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

frugivorous diet that incorporated nuts and seeds.[11]

Diagrams showing the hands of specimen MPC-D 107/15 and 107/16

In 1977 Barsbold suggested that oviraptorids

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.[14]

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

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.[14]

In 2004 Lü and colleagues proposed that the articulation between the

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.[22]

Palaeoenvironment

Baruungoyot and Nemegt Formations

Nemegtomaia is known from the Nemegt and Baruungoyot Formations, which date to the upper Campanian–lower Maastrichtian ages of the Late Cretaceous period, about 70 million years ago. Though this taxon is known only from the Nemegt locality, unidentified oviraptorid remains from other localities may belong to it. The Nemegt

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 David A. 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.[2][11][23][24]

Photos of the interfingering contacts between the formations

The environment of the Nemegt Formation has been compared to the

ornithomimosaurians such as Deinocheirus, Anserimimus, and Gallimimus.[26][27][28]

Other oviraptorosaur genera known from the Nemegt Formation include the basal

niche partitioning in diet. The environment of the Nemegt Formation may have acted as an oasis and thereby attracted oviraptorids.[11]

Taphonomy

skin beetles
on the nesting specimen

The nesting specimen MPC-D 107/15 has provided much information about the

invertebrates.[2]

Borings in the bones, burrows, and reworked sediments (perhaps caused by the construction 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-D 107/15 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.[2]

See also

References

  1. ^ a b c d e f g Lü, J.; Tomida, Y.; Azuma, Y.; Dong, Z.; Lee, Y.-N. (2004). "New oviraptorid dinosaur (Dinosauria: Oviraptorosauria) from the Nemegt Formation of southwestern Mongolia" (PDF). Bulletin of the National Science Museum, Tokyo, Series C. 30: 95–130.
  2. ^
    PMID 22347465
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  3. ^ a b c Lü, J., Dong, Z., Azuma, Y., Barsbold, R. & Tomida, Y. (2002). "Oviraptorosaurs compared to birds." In Zhou, Z. & Zhang, F. (eds). Proceedings of the 5th Symposium of the Society of Avian Paleontology and Evolution. Beijing: Science Press. pp. 175–189.
  4. ^ Lü, J.; Tomida, Y.; Azuma, Y.; Dong, Z.; Lee, Y.-N.; et al. (2005). "Nemegtomaia gen. nov., a replacement name for the oviraptorosaurian dinosaur Nemegtia Lü et al. 2004, a preoccupied name". Bulletin of the National Science Museum, Tokyo, Series C. 31: 51.
  5. ^ a b Arbour, V.M. (2012). "Gobi Desert Diaries: Nemegtomaia Edition". pseudoplocephalus.blogspot.com. Retrieved 10 January 2017.
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  16. ^ a b Hendrickx, C.; Hartman, S.A.; Mateus, O. (2015). "An overview on non-avian theropod discoveries and classification". PalArch's Journal of Vertebrate Palaeontology. 12 (1): 1–73. Archived from the original on 2018-06-22. Retrieved 2016-12-27.
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  26. ^ Lucas, S.G.; Estep, J.W. (1998). "Vertebrate biostratigraphy and biochronology of the Cretaceous of China". New Mexico Museum of Natural History and Science Bulletin. 14: 1–20.
  27. ^ Watabe, M.; Tsogtbaatar, K.; Suzuki, S.; Saneyoshi, M. (2010). "Geology of dinosaur-fossil-bearing localities (Jurassic and Cretaceous: Mesozoic) in the Gobi Desert: Results of the HMNS-MPC Joint Paleontological Expedition". Hayashibara Museum of Natural Sciences Research Bulletin. 3: 11–18.
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