Oksoko

Oksoko avarsan was a small toothless oviraptorid from Late Cretaceous Mongolia. Unlike genera based on one isolated bone, it is represented by a series of well-preserved skeletons at different growth stages. Nearly complete juveniles have skulls, while a partial adult and additional specimens preserve rare regions such as the hand and pygostyle.

Its most striking feature is a short forelimb with two working fingers. The third did not vanish entirely: a slender metacarpal and one tiny phalanx remained inside the hand. Equally important is a block containing young animals that died side by side in resting postures, direct evidence that juveniles sometimes stayed together.

Quick facts

A rescued fossil complex

The genus name refers to a three-headed eagle in Altaic mythology, chosen because the confiscated complex contained three skulls. The species name avarsan means rescued in Mongolian. Gregory Funston, Tsogtbaatar Chinzorig, Khishigjav Tsogtbaatar, Yoshitsugu Kobayashi, Corwin Sullivan and Philip Currie named the genus and its only known species in 2020.

Mongolian authorities confiscated MPC-D 102/110 and MPC-D 102/11 from illegal fossil dealers in December 2006 before the specimens could be exported. The fossils entered the Institute of Paleontology in Ulaanbaatar and became available for research. Illegal excavation nevertheless destroyed part of their scientific context: exact coordinates, position in the rock succession and a detailed record of the surrounding sediment were not preserved.

Rock chemistry is consistent with the Nemegt Formation. The researchers considered Bugiin Tsav and Guriliin Tsav plausible source areas in the north-western Nemegt Basin. An ankylosaur skeleton abandoned by looters and rediscovered at Guriliin Tsav in 2018 carried unusually similar plaster, blue bandage fibres and yellow plastic. This packaging links the fossils to a group of looters, but does not prove that the animals came from the same quarry.

Two other specimens have better documented provenance. MPC-D 100/33 was found at Bugiin Tsav in 1974, while adult MPC-D 102/12 was collected at Guriliin Tsav in 1998. All four catalogue groups remain in Mongolia.

Six individuals and an incomplete adult

The four catalogue numbers represent at least six animals. MPC-D 102/110 contains three juveniles. The holotype MPC-D 102/110.a lacks mainly the far half of the tail, while MPC-D 102/110.b is also nearly complete. A third animal is represented by parts of the pelvis and hind limb, including elements that were recognised inside the block with computed tomography.

MPC-D 102/11 is a partial juvenile with a skull. Three additional skull bones beside it belong to another animal. Because this material probably came from the same illegally excavated accumulation as the main block, isolated bones cannot simply be added to the minimum count without risking counting one individual twice.

MPC-D 100/33 is a partial postcranial skeleton of a growing animal. MPC-D 102/12 is a partial adult without a skull. Collectively the specimens preserve nearly every region of the skeleton, but the heads belong to young animals and the most mature individual cannot provide either adult skull shape or a directly measured complete body length.

Age of the fossils

The original description assigned Oksoko to the lower Maastrichtian part of the Nemegt Formation. The Maastrichtian is the final stage of the Cretaceous, but the formation’s position within it has not been fixed to one narrow numerical interval. Loss of the exact quarry level for the confiscated blocks further prevents precise correlation.

The defensible statement is therefore that Oksoko formed part of the Late Cretaceous Nemegt fauna, relatively close to the end of the period. Giving one exact age to all six individuals would imply precision that their geological records do not preserve.

Skull, beak and crest

The skull is short and high, and both jaws are toothless. A keratinous beak covered their front in life, but its soft outline is not preserved. The lower jaw has a ventral projection and a high coronoid region for jaw-closing muscles.

A rounded crest rises from fused nasal and frontal bones, with the parietals contributing more strongly behind than in many other oviraptorids. Air spaces linked to nasal passages entered the crest. The robust jugal and postorbital framed the rear of the orbit.

The crest was developed in juveniles. It could still have functioned in display, but it was not exclusively an ornament appearing only at sexual maturity. Its pneumatic passages make sound modification possible, yet neither the voice nor soft-tissue acoustics are known.

Two fingers and a remnant of the third

The hand retained three metacarpals, but only digits I and II formed full fingers. The first was strongest and ended in a large curved claw. The second was more slender with a straighter, smaller claw.

Metacarpal III was greatly reduced and carried one short blunt phalanx without a normal continuation joint. It did not reach the end of metacarpal II and probably remained buried in soft tissue. The hand looked two-fingered externally while preserving a rudimentary third ray internally.

Manipulation tests found less flexion and weaker grasping than in long-handed oviraptorosaurs. The arm was not useless: muscle attachments show it could pull strongly backward. Digging substrate, tending a nest, grooming and display have all been suggested, but no scratch marks or biomechanical study identifies one function.

Two tiny wrist ossicles were interpreted in 2020 as possible sesamoids previously unknown in theropod wrists. A 2024 redescription kept open the possibility that they are reduced intermedium and ulnare bones. Their unusual position and poor comparison sample prevent a final identification.

Body, legs and tail

Oksoko combined long hind limbs with short forelimbs and a long, low pelvis. An additional ridge crosses the hollow for tail musculature on the ilium, while the pubic bones curved forward. The foot retained ordinary oviraptorid proportions rather than the proximally pinched third metatarsal seen in the arctometatarsalian feet of some running theropods.

The tail was comparatively short. Adult MPC-D 102/12 preserves three terminal vertebrae fused into a pygostyle. This structure may have supported a fan of long feathers, but no tail feathers occur with the fossils. Their number, length and arrangement therefore come from comparison with other oviraptorosaurs.

The first chevrons beneath the tails of two similarly sized juveniles differ conspicuously: in MPC-D 102/110.b the first chevron is less than half the height seen in its neighbours. The contrast has been compared with possible sexual dimorphism in other oviraptorids, but a sample of two cannot distinguish sex from injury, individual variation or another cause.

Size without false precision

Comparative formulas based on limb dimensions produced masses of about 44–45 kilograms for three juveniles in MPC-D 102/110 and about 31 kilograms for MPC-D 102/11. These values are model outputs, not direct weighing or a complete volumetric reconstruction.

Adult MPC-D 102/12 was larger, but is incomplete and has no skull. The published evidence does not provide a secure total adult length or mass. The skeletons support a small or medium-sized oviraptorid, while more exact figures require restoration of missing regions.

Growth recorded inside the bones

Thin sections of femora and fibulae distinguish growth stages. Juvenile bones contain strongly vascularised fibrolamellar tissue associated with rapid growth and at least one cyclical mark. The animals were more than a year old but still depositing bone quickly.

In the adult, blood-vessel spaces become sparser near the outer surface, slowly deposited parallel-fibred tissue increases and growth lines crowd together. At least five cycles were counted in the femur. Earlier marks may have been erased as the marrow cavity expanded, so the animal was no younger than five and probably older.

Limb proportions changed relatively little, but individual bones became more robust. Fusion did not proceed in one simple head-to-tail sequence: juveniles retained separate parts of the sacrum and vertebral arches, whereas the adult had a fused pygostyle, joined distal ankle bones and many closed sutures. The cranial crest was already prominent in animals at roughly half the adult mass.

What the grouped burial proves

Three juveniles in the main block lie close together. Two nearly complete animals face different directions but both tucked their legs beneath the body, folded their forelimbs and brought the head close to the right hand. A juvenile in another confiscated block preserves almost the same resting posture. This differs from the familiar post-mortem pose in which a dinosaur’s neck and tail arch backward.

Articulated eye ossicles, connected joints and clean bone surfaces imply little decay, limited transport and rapid burial. Similar poses in animals oriented differently are difficult to explain as water arranging loose carcasses. The most economical interpretation is that the juveniles had been resting together before death.

This is strong evidence for temporary grouping among young oviraptorids. It does not establish lifelong herds, cooperative feeding, parental care or the composition of adult groups. No adult, nest or clutch occurs in the block. Because looting removed the sedimentary context, the shared cause of death cannot be identified as a flood, collapse or other event.

What did Oksoko eat?

No teeth, stomach contents, coprolites or associated food remains identify its diet. The toothless beak could have processed vegetation, small animals, eggs or a mixture. Jaw shape by itself cannot select one of those alternatives.

A 2020 evolutionary analysis linked shortening of the hand among heyuannines with reduced dependence on grasping prey and considered a more herbivorous diet plausible. The skull and lower jaw remain broadly similar to related oviraptorids with longer arms, however, and direct dietary evidence is absent.

The strong first finger and developed arm musculature could have served tasks that did not require precise grasping. Digging through substrate for food, arranging a nest, grooming plumage and display have all been proposed. None is yet identified by claw marks, stomach contents or a biomechanical model specific to Oksoko.

Position among oviraptorosaurs

Oksoko belongs to Oviraptoridae and is usually placed within Heyuanninae, a branch showing a trend toward shorter forelimbs and reduction of the outer finger. The 2020 analysis grouped it near the Chinese genera Banji and Jiangxisaurus and connected hand reduction with a reconstructed spread from southern China toward the Gobi.

A detailed 2024 revision showed that this exact placement is less stable than it first appeared. Banji shares mainly two palate characters with Oksoko, while Jiangxisaurus is linked by one metacarpal proportion. The matrix contains no single feature shared by all three and excluded from every other oviraptorid. Altering character treatment can therefore change the closest relatives.

The inferred geographic movement is a result derived from an evolutionary tree, not a migration directly observed in the fossil record. It suggests ecological change but does not reveal why a two-fingered hand was advantageous or prove that diet caused its evolution.

The Nemegt environment

Nemegt sediments accumulated in river channels, on floodplains and around temporary bodies of water. Channel sands, fine overbank deposits and soil horizons record a relatively moist alluvial landscape, although conditions varied through time and across the basin.

Deinocheirus, Gallimimus, tyrannosaurs, hadrosaurs, ankylosaurs, sauropods, turtles, crocodylomorphs and numerous other oviraptorosaurs occur in the wider formation. Different sites and levels mean that every listed animal was not necessarily a direct neighbour. By 2020, Oksoko was the ninth named oviraptorosaur genus and the sixth oviraptorid reported from the Nemegt Formation, illustrating the exceptional local diversity of the group.

What is known about its appearance?

The bones directly establish a high toothless skull with a rounded crest, short forelimbs with two working fingers, long hind limbs and a short tail. Articulated skeletons in resting poses provide unusually firm evidence for how the limbs could fold beneath and beside the body.

Feathers are highly probable from oviraptorosaur relationships, but none is impressed around Oksoko. The pygostyle permits a tail fan without revealing its dimensions. Colour, pattern, the keratinous outline of the beak, soft tissues over the crest, calls and sex differences remain reconstruction.

Evidence, inference and reconstruction