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A new alvarezsaurid dinosaur (Theropoda, Alvarezsauria) from the Upper Cretaceous Baruungoyot Formation of Mongolia provides insights for bird-like sleeping behavior in non-avian dinosaurs [1]

['Kohta Kubo', 'Department Of Natural History', 'Planetary Sciences', 'Hokkaido University', 'Kita-Ku', 'Sapporo', 'Hokkaido', 'Yoshitsugu Kobayashi', 'Hokkaido University Museum', 'Tsogtbaatar Chinzorig']

Date: 2023-12

Jaculinykus differs from all other alvarezsaurs in having a dorsoventrally high narial opening of the premaxilla, medially curved parasagittal crest on the parietal, slender and nearly straight dentaries, triangular-shaped deltopectoral crest being separated from the humeral head by a notch, strong medial tab of metacarpal I, weakly developed proximodorsal process of phalanx I-1, robust medial condyle of the tibia relative to the fibular condyle, and sharply indented base of the ascending process of astragalus. Jaculinykus is also distinguished from other alvarezsaurs by the unique combination of the following characters: slender ischial shaft relative to the pubic shaft, an open popliteal fossa of the femur, and prominent external projection of the ectocondylar tuber of the femur. It differs from the hypothesized sister taxon Shuvuuia deserti in possessing the following additional features: the deltopectoral crest being separated from the humeral head by a notch; the absence of the third manual digit. It also differs from stratigraphically same or older (the Baruungoyot Formation) alvarezsaurid, Ondogurvel alifanovi, from the same locality (the Nemegt locality) in possessing the following features in the manus: the metacarpal II being not incorporated into the fused metacarpal element, a sharply truncated metacarpal III, and a weakly-developed proximodorsal process of phalanx I-1. It differs from Parvicursor remotus of the Baruungoyot Formation in possessing the following features: a straight posterior margin of the medial distal condyle of femur, ectocondylar tuber on the lateral distal condyle not extended posteriorly beyond the level of the medial condyle, and robust and rounded medial condyle of the proximal end of tibia. Differs from Ceratonykus oculatus of the Baruungoyot Formation in possessing the following features: the ratio of the frontal length to the transverse width in Jaculinykus yaruui being smaller (1.7) than that in Ceratonykus oculatus (4.0); a well-developed cnemial crest of the tibia. It differs from Khulsanurus magnificus of the Baruungoyot Formation in possessing the following features: the presence of pleurocoels and epipophyses on the cervicals, dorsoventrally thin and subcircular in cross-section transverse process of the anterior caudals, and a notch between the humeral head and deltopectoral crest. It differs from stratigraphically younger (the Nemegt Formation) alvarezsaurid Mononykus olecranus in possessing the following features: the femoral head being more robust in Jaculinykus yaruui than that in Mononykus olecranus, a popliteal fossa on the distal end of the femur opening distally, and an anteriorly oriented cnemial crest of the tibia. It differs from Nemegtonykus citus of the Nemegt Formation in possessing the following features: the scapula unfused to the coracoid, a strongly curved femoral shaft, and the presence of an ectocondylar tuber on the femoral distal condyle.

Nemegt locality [ 35 , 36 , 42 ], Ömnögovi Province, Mongolia ( Fig 1 ). The specimen was collected from the upper section of the Baruungoyot (or Barun Goyot) Formation, suggested as the Campanian in age, ( Fig 1 ). The stratigraphic horizon of Jaculinykus belongs to a part of “Big Red” in the transitional stratigraphic interval (Zone 4), [ 36 , 42 ].

The type specimen (MPC-D 100/209) is a nearly complete skeleton with a skull, missing some cranial elements (vomers, nasals, postorbitals, and supraoccipitals), eighth or ninth cervical vertebra, posterior dorsal vertebrae, seven anterior caudal vertebrae, sternum, furcula, right manual phalanx (II-2), right manual ungual and left fibula ( Fig 2 ). It is housed in the Institute of Paleontology of Mongolian Academy of Sciences (IP-MAS), Ulaanbaatar, Mongolia.

Twenty-eight tiny teeth (Figs 3H and S1 ) are recovered. The teeth are not in situ but are preserved close to the premaxilla, maxilla, and dentary. Since all teeth are associated near the original position and are projecting the same orientation, these teeth likely retain the original relationships. The teeth are homodontous, ranging from 1 mm and 5 mm in size. Each tooth crown exhibits a subconical shape and lacks serrations as in Mononykus olecranus [ 2 , 33 ], Shuvuuia deserti [ 1 ], and some troodontids [ 48 ], but unlike early-branching alvarezsauroids (e.g., Haplocheirus sollers) which retain denticulate tooth crowns [ 12 ].

The angular is weakly bowed ventrally and covers the entire length of the external mandibular fenestra ventrally ( Fig 3B ). The angular meets the prearticular posteriorly and the surangular ventrolaterally. Although most parts of the retroarticular process of the articular is missing, a subcircular basin is present in it. The posterior half of both hyoids is straight relative to those of Shuvuuia deserti (MPC-D 100/977) and has a ridge for the attachment of the lingual muscle.

The surangular is anteroposteriorly long and dorsoventrally low ( Fig 3B ). The long anterior process composes more than half of the entire length of the surangular. It possesses a long groove on its lateral surface in the anterior half part and forms the dorsal border of the external mandibular fenestra ( Fig 3B ). The anterior process of the surangular does not extend anteriorly beyond the level of the anterior end of the angular as in Shuvuuia deserti. The surangular becomes low and transversely widens posteriorly towards a lateral ridge ( Fig 3B ). Posterior to the lateral ridge, the surangular overlaps the articular laterally and the angular dorsally.

The lateral curve of the dentary toward its posterior end is weak compared to Shuvuuia deserti (MPC-D 100/977 and 100/120), ( Fig 3B ). The ratio of the posterior width of the dentaries to its anteroposterior length is slightly smaller (9.5%) than that in Shuvuuia deserti (10.5%), (MPC-D 100/120, [ 22 ]). Anteriorly, the foramina are present on the lateral surface of the dentary and along the symphysis. The unfused symphysis is slightly inflected medially, forming a V-shaped symphysis in ventral view ( Fig 3B and 3H ). Posteriorly, there is a long alveolar groove along the dorsal border of the dentary, despite no observation of the teeth in the preserved alveolar groove because of a matrix. The splenial and dentary meet each other along a straight suture.

The palate elements (pterygoid and palatine) are partially preserved. Although both anterior parts of the pterygoids are broken partially or covered by matrix and other cranial elements, the pterygoid is anteroposteriorly long and straplike as in Shuvuuia deserti (MPC-D 100/1001). Posteriorly, the pterygoid has a lateral ramus for articulation with the quadrate ( Fig 3B ). The posterior portion of the palatine is long and slender. The posterior end of the palatine is slightly expanded transversely compared to its shaft, forming a flat and thin articular facet for the pterygoid ( Fig 3B ).

The paroccipital process is short and slightly expanded distally ( Fig 3G ). In posterior view, the paraoccipital process extends laterally as in Shuvuuia deserti (MPC-D 100/1304), but unlike the paroccipital process in an unnamed alvarezsaurid (HGM L08-59) from China which is oriented lateroventrally [ 16 ]. Two foramina for the vagus (X) nerve are present on each side of the exoccipital and at the same level as the paroccipital process ( Fig 3G ). Medial to the foramen of the vagus (X) nerve, the hypoglossal (XII) foramen is present near the base of the occipital condyle. In ventral view, the anterior surface of the exoccipital bears a large posterior tympanic recess ( Fig 3B ).

The exposed width of the foramen magnum relative to the occipital condyle ( Fig 3G ) suggests that its size is comparable to that of Shuvuuia deserti (MPC-D 100/1001, 100/1304), [ 1 , 3 ], which is known for its unusually large foramen magnum. The occipital condyle is transversely wider than dorsoventrally high, with an infracondylar recess in its ventral surface. Ventral to the occipital condyle, the basal tuber ( Fig 3B ) is well-developed and transversely wide. The subcondylar recess, which extends from the exoccipital, is more deeply excavated on the posterodorsal surface of the basal tubera than the exoccipital. The basal tubera are separated along the midline of the skull, continuing into the basisphenoid recess, as seen in Haplocheirus sollers and Shuvuuia deserti [ 12 ]. This gap between the basal tubera is proportionally narrower than that in Shuvuuia deserti.

The quadrate is tall and compressed anteroposteriorly. The quadrate is shallowly excavated posteriorly ( Fig 3B ) as seen in Shuvuuia deserti [ 1 , 3 ]. Although both tips of quadrate heads are missing, the lateral quadrate head extends along the lateral flange. Ventral to the base of the medial quadrate head, there is a distinct oval quadrate foramen at the mid-posterior surface of the quadrate as in Shuvuuia deserti (MPC-D 100/1001).

Although the maxillary process and anterior part of the jugal are missing, the preserved parts clearly show that the jugal is fused to the quadratojugal and together forms the rod-like jugal bar as in Shuvuuia deserti [ 1 , 3 ]. The jugal bar is circular in cross-section and weakly curved mediolaterally. Posteriorly, the jugal bar lacks the postorbital and squamosal processes ( Fig 3B ). The posterior end of the jugal bar has a shallowly concave facet for articulation with the quadrate.

Lateral to the prefrontal, the main body of the left lacrimal is preserved in articulation with the prefrontal ( Fig 3A ). Although the maxillary and jugal processes of this element are heavily damaged, it forms an inverted ‘L’-shaped outline. The lacrimal lacks a prominent posterior process. Unlike Shuvuuia deserti (MPC-D 100/977), Jaculinykus yaruui does not bear a distinct sutural contact between the maxillary process and the prefrontal.

The squamosal possesses postorbital, medial, and paraoccipital processes ( Fig 3F ). The features of these three processes in Jaculinykus yaruui are ambiguated because each process is either missing distally or covered by other cranial elements. The squamosal lacks the ventral process, forming a triradiate element as in Shuvuuia deserti [ 1 , 3 ] and Ceratonykus oculatus [ 30 ]. Ventrally, the paraoccipital process has a shallow furrow articulating with the quadrate heads. The dorsal surface of the squamosal has a more distinct depression around the junction between the medial and paraoccipital processes than that of Shuvuuia deserti (MPC-D 100/977).

The left and right parietals are completely fused in Jaculinykus yaruui as in Shuvuuia deserti [ 1 ], ( Fig 3A and 3E ). Medially, the fused parietals are horizontally flat without a sagittal crest and forms the posterior part of the skull roof. Lateral to the distinct parasagittal crests, the lateral surface and posterior process of the parietal slant ventrolaterally with a shallow angle, forming the medial margin of the supratemporal fossa ( Fig 3E ). Parasagittal crests are present on the dorsal surface of the parietals, and are strongly curved, unlike the straight parasagittal crests of Shuvuuia deserti (MPC-D 100/977). These crests extend more medially in Jaculinykus yaruui, suggesting that the supratemporal fossa is larger than that of Shuvuuia deserti.

The frontal forms the entire dorsal margin of the orbit ( Fig 3A ). It is slightly vaulted dorsally as in alvarezsaurids [ 1 , 30 ], but unlike a flattened skull roof of Haplocheirus sollers [ 12 ]. Dorsally, the frontal is anteroposteriorly long and becomes transversely narrow towards its anterior end ( Fig 3A ). Anteriorly, the frontal has a large recess constricting posteriorly for the articulation with the prefrontal. The contact between the frontals is straight without complex interdigitation, as in other alvarezsauroids [ 1 , 12 ]. A distinct orbital rim develops along the lateral rim of the frontal from the prefrontal facet to the postorbital process as in Shuvuuia deserti [ 1 , 3 ], troodontids [ 48 ], and some dromaeosaurids (e.g., Mahakala omnogovae [ 49 ]). The postorbital process of the frontal is anteroposteriorly short and laterally extensive, which is weaker than that in Haplocheirus sollers. There is a weak and small recess of the anterior margin of the supratemporal fossa on the postorbital process as in Shuvuuia deserti, but unlike early-branching alvarezsauroids in which the supratemporal fossa occupies a large portion of the frontal [ 8 , 11 ].

The prefrontal is a large, rhomboidal cranial element with a sharp posterior apex in dorsal view as in Shuvuuia deserti [ 3 ] and Ceratonykus oculatus [ 30 ], ( Fig 3A ). The prefrontal overlaps the frontal ventromedially, which is unique to alvarezsaurids [ 1 ]. In addition, the anterolateral surface of the prefrontal lacks a concave notch for articulation with the lacrimal angle unlike non-alvarezsaurid alvarezsauroids such as Haplocheirus sollers [ 12 ].

Although the right maxilla is severely broken into small pieces, it remains in the original position, allowing observation of some anatomical features. The maxilla has an elongated, roughly triangular outline in the lateral view ( Fig 3D ). The nasal ramus of the maxilla has a shallow sulcus along its dorsal rim as in Shuvuuia deserti (MPC-D 100/977, [ 1 , 3 ]). The ventral margin of the premaxillary process also slightly slants anterodorsally as in Shuvuuia deserti but unlike the non-alvarezsaurid alvarezsauroid Haplocheirus sollers, which has a straight ventral margin of the maxilla. Six maxillary teeth are aligned along the ventral margin of the premaxillary process.

The premaxilla is anteroposteriorly longer than high and has elongated nasal and short maxillary processes ( Fig 3A and 3C ). The nasal process forms the dorsal border of the elliptical external nares as in Shuvuuia deserti [ 1 , 3 ]. The anterodorsal margin of the nasal process is moderately curved unlike that of Shuvuuia deserti, which is straight. The horizontally oriented maxillary process forms a dorsoventrally wide narial opening in comparison to the narial opening of Shuvuuia deserti.

Skull in dorsal ( A ) and ventral ( B ) views. ( C) Right premaxilla in medial view (a broken line shows a ventral margin of the premaxilla). ( D ) Right maxilla in lateral view (reversed) (a broken line shows a ventral margin of the maxilla). ( E ) Parietals in dorsal view. ( F ) Left squamosal in dorsal view. ( G ) Braincase elements in posterior view (a broken line and shadow area represent an outline of the foramen magnum). ( H ) Teeth close to the right dentary. Gray-shaded areas indicate missing parts.

The three-dimensionally preserved skeleton of Jaculinykus yaruui is compressed dorsoventrally, but most elements of the skeleton are positioned nearly at the original position, despite some lateral displacement of some elements ( Fig 2A and 2B ). The anterior half of the skeleton is oriented ventral-side-up from the skull through the dorsal vertebrae, whereas the posterior half including pelvis and hind limbs oriented left-lateral-side-up. The hind limbs are folded on either side of the body as in Albinykus bataar [ 5 ]. The left forelimb is extended laterally and folded next to the body with the elbow, although the left manual elements are displaced. The neck curves posteriorly on the right side of the body, so that the skull lies on the right side of the body, above the right knee. Most of the tail lies on the left side of the body. The proximal end of the tail is directed to the left side of the body. The middle to distal parts of the tail are directed anteriorly and curve around the flexed hind limbs to the right, and then travel forward under the skull. This posture is nearly identical to the inferred sleeping posture of troodontids such as the type specimen of Sinornithoides youngi, IVPP V9612 [ 40 , 41 ] and Mei long [ 38 , 39 ].

Postcranial skeleton

Pelvic girdle. The ilium is dorsoventrally low, and its anteroposterior length is about four times as long as the iliac height above the center of the acetabulum (Fig 7A and 7B). As in other alvarezsaurids, the postacetabular process is anteroposteriorly longer and dorsoventrally lower than the preacetabular process. The iliac blades abut to the neural spines of the sacrum along all of the dorsal edges except the posterior part of the postacetabular process, where they diverge laterally (Fig 7D). The supracetabular crest overhangs laterally, especially along the anterior half part of the acetabulum, and terminates anterior to the ischial peduncle. Ventrally, the preacetabular process curves medially and lacks the cuppedicus fossa (Fig 7E). The brevis fossa on the postacetabular process is large and tapers towards the posterior end as in other alvarezsaurids [1,25]. Although the pubic and ischial peduncles are poorly preserved on both sides, they are deflected medially relative to the iliac blade as in Xixianykus zhangi [25]. The pubic peduncle is transversely compressed and more prominent than the ischial peduncle (Fig 7C). Lateral to the ischial peduncle, the antitrochanter is strongly expanded laterally and forms the posterior and anterior margins of the acetabulum and brevis fossa (Fig 7C and 7D). Similar to other alvarezsaurids, the pubis and ischium extend posteroventrally and abut each other along their entire lengths [1,25], (Fig 7A and 7B). The proximal end of the pubis is compressed transversely and is inclined lateroventrally. The preacetabular tubercle is elongated relative to those of Xixianykus zhangi [25] and Trierarchuncus prairiensis [18,62], (Fig 7C). The obturator notch is bordered ventrally by a weak ridge along the posterior margin of the pubic shaft. The gracile pubic shafts are mostly straight in lateral view and abuts its counterpart throughout the entire length. A pubic apron is absent as in other alvarezsaurids [1], but unlike the early-branching alvarezsauroid, Haplocheirus sollers, where it is developed on the three-quarters of the length of the pubis [50]. The ischium is more slender than the pubis and rod-like (Fig 7A–7C). The proximal end of the ischium extends posterodorsally. The ischial shaft has a sub-oval in cross-section and is less compressed transversely than the pubic shaft as in Ondogurvel alifanovi [31], but unlike those of Parvicursor remotus, Xixianykus zhangi and Shuvuuia deserti (MPC-D 100/99).

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