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Intestinal preservation in a birdlike dinosaur supports conservatism in digestive canal evolution among theropods | Scientific Reports - Nature.com

Intestinal preservation in a birdlike dinosaur supports conservatism in digestive canal evolution among theropods | Scientific Reports - Nature.com

Intestinal preservation in a birdlike dinosaur supports conservatism in digestive canal evolution among theropods | Scientific Reports - Nature.com
Nov 19, 2022 9 mins, 48 secs

nov., a dromaeosaurid from the Lower Cretaceous Jehol Biota of Inner Mongolia, China.

Phylogenetically, Daurlong nests among a lineage of short-armed Jehol Biota species closer to eudromaeosaurs than microraptorines.

Dromaeosauridae is a clade of small- to mid-sized theropod dinosaurs known from the Cretaceous of both hemispheres1.

The Lower Cretaceous Jehol Biota from north-eastern China has provided a rich diversity of dromaeosaurids, the majority of which referred to Microraptorinae2,3,4,5,6,7.

The affinities of two other Jehol Biota dromaeosaurids, Tianyuraptor ostromi6 and Zhenyuanlong suni7, are more problematic.

The abundance of Jehol Biota dromaeosaurids, when not due to taxonomic oversplitting1, may be ecologically explained assuming niche segregation and avoidance of direct resource competition.

Here, we describe a new dromaeosaurid from the Lower Cretaceous of the Pigeon Hill locality, Inner Mongolia (China), which shows the first case of intestinal preservation in a theropod lineage very close to bird ancestry.

Dinosauria, Theropoda, Dromaeosauridae, Daurlong wangi gen.

(c), ventral part of the belly region.

(d), tail. Abbreviations: ca, caudotheca; cc, caudal centrum; co, coracoid; cv, cervical vertebrae; dr, dorsal rib; dv, dorsal vertebrae; f, feathers; fe, femur; ga, gastralia; I-/II-/III-, phalanges; il, ilium; is, ischium; mc, metacarpal; pu, pubis; sc, scapula; st, sternum, sr1, sternal rib1, sv, sacral vertebrae.

Locality and Horizon Pigeon Hill, Morin Dawa Daur Autonomous Banner, Inner Mongolia Autonomous Region (N 48°39′40.76″/E 123°52′ 41.15″); Longjiang Formation, Lower Cretaceous.

Diagnosis Mid-sized dromaeosaurid with (autapomorphies marked by asterisk): slender subnarial ramus of premaxilla extended caudally well beyond the external naris; large, trapezoid promaxillary recess placed at the rostroventral corner of antorbital fossa*; maxillary fossa large, shallow and caudodorsally located, so that the pila promaxillaris is wider than the pila interfenestralis*; stepped transition from the subcutaneous surface of maxillary ventral ramus to the antorbital fossa; fan-shaped distal end of first sternal rib*.

Daurlong further differs from Zhenyuanlong because it lacks a pitted ventral ramus of the antorbital fossa, lacks markedly concave distal margins in maxillary tooth crowns, bears a bowed scapula, a more robust radius, and a wider overlap of the semilunate carpal over metacarpal II (Fig. 3).

wangi: 1, slender subnarial ramus of premaxilla extended caudally well beyond the external naris; 2, large, trapezoid promaxillary recess placed at the rostroventral corner of antorbital fossa; 3, maxillary fossa large, shallow and caudodorsally located, so that the pila promaxillaris is wider than the pila interfenestralis; 4, stepped subcutaneous surface of the ventral ramus of maxilla; 5, absence of pitted ventral ramus of the antorbital fossa; 6, robust fang-like maxillary teeth with straight to slightly convex distal crown margins; 7, distal end of first sternal rib fan-shaped.

Abbreviations: af, antorbital fossa; de, dentary; ju, jugal; la, lacrimal; ma, maxilla; na, nasal; pm, premaxilla; su, surangular.

The specimen is 85% the size of Tianyuraptor ostromi holotype6, 93% the size of Zhenyuanlong suni holotype7, and between 115 and 350% the size of the Jehol Biota microraptorines5,8.

The skull is almost perfectly articulated, except for the missing nasal ramus of the left premaxilla and the partially displaced dorsal parts of the right nasal and lacrimal (Figs. 1, 3).

The rostral margin of the premaxilla forms a right angle with the occlusal margin, similar to Velociraptor, Zhenyuanlong and Saurornitholestes2,10,12, differing from the shallower premaxillae with acute rostroventral corner in Microraptor and Sinornithosaurus4,11 and the shallow platyrostral premaxilla of Halszkaraptor13.

The subnarial process of the premaxilla extends caudodorsally well beyond the caudal margin of the external naris, as in Velociraptor and Linheraptor12,14, differing from the relatively shorter processes in Microraptor and Zhenyuanlong2,5.

The antorbital fossa is large, longer than tall and covering more than two-thirds of the maxilla, similar to Zhenyuanlong and Wulong7,15, differing from the relatively shorter excavation seen in some eudromaeosaurs (e.g., Acheroraptor17).

As in Zhenyuanlong, the rostral margin of the antorbital fossa is at the level of the second maxillary tooth, more rostral than in other dromaeosaurids4,7,12.

The promaxillary fenestra is relatively large, as in other Jehol Biota dromaeosaurids6 and is placed adjacent to the rostroventral corner of the antorbital fossa, differing from the more caudally placed fenestra in Sinornithosaurus and Zhenyuanlong7,11.

The promaxillary fenestra is trapezoid in lateral view, with straight rostral, caudal and ventral margins, and a caudodorsally slanted dorsal margin.

The maxillary recess is a large shallow fossa delimited by curved ridges paralleling the rostral margin of the antorbital fenestra.

In microraptorines4,11 the area around the pila promaxillaris and the region underneath the antorbital fenestra are excavated by a series of small pits and ridges, absent in Daurlong.

The caudoventral ramus of the antorbital fossa lacks the pits and crests present in Zhenyuanlong7.

The dorsoventrally shallow subcutaneous surface of the ventral ramus is separated from the antorbital fossa by a stepped margin, differing from Zhenyuanlong which shows a distinct antorbital rim7.

The frontal widely contributes to the dorsal margin of the orbit.

The rostral ramus of the postorbital is upturned, forming an obtuse angle with the caudal ramus, which is instead perpendicular to the ventral ramus.

The dorsal margin is slanted rostroventrally at its rostralmost end, then, caudal to the second alveolus, it is parallel to most of the ventral margin, as in most non-dromaeosaurine dromaeosaurids4,10.

The ventral margin of the dentary gradually tapers rostrodorsally from the rostral sixth of the bone, and then is inclined caudodorsally along the caudal third.

The cranial neural spines of the dorsal series are rectangular and strongly inclined caudodorsally.

The caudal neural spines of the dorsal series are square-shaped, with their long axis oriented dorsally, almost vertical to the corresponding centra (Fig. 2).

Daurlong is similar to other Jehol Biota dromaeosaurids4,6,7 in having the caudotheca extended through the rostralmost vertebrae, differing from eudromaeosaurs where the bony rods bundle begins distal to caudal 620,21.

The pubic foot is broadly rounded and dorsoventrally expanded, as in other Jehol Biota dromaeosaurids2.

The ischium is about half the pubis in length, lacks dorsal processes along the straight dorsal margin and ends distally in a poorly curved tip.

The obturator process is triangular, as long as deep, and placed at mid-lenght of the ventral margin, differing from the prominent and more distally-placed process in microraptorines5.

The plumage is preserved along the dorsal margin of the postorbital part of the skull, adjacent to the presacral neural spines (Fig. 1d) and along the edges of the tail.

No feathers are preserved close to the limbs or along the ventral margin of the body: it is unclear if long pennaceous remiges and rectrices were present as in Zhenyuanlong7.

The plumage along the dorsal margin of the pre-caudal part of the skeleton is preserved as a series of compound structures containing several filaments joined at their proximal ends, similar to the condition in Sinornithosaurus33.

Short pennaceous feathers are preserved along most of the margin of the caudotheca.

The rostral margin of the layer is placed ventral to the 9th dorsal centrum, where it extends dorsoventrally along the dorsal half of the abdominal cavity.

The black-bluish layer reaches its maximum depth between the 10th and the 11th dorsal vertebrae, where it reaches the ventralmost part of the gastral basket.

Caudal to the 11th dorsal vertebra, the layer is limited to the dorsal end of the abdominal cavity.

In particular, Daurlong shares with these dromaeosaurids the antorbital region which is very extensive rostrally (resulting in a very short subcutaneous part of the preantorbital maxilla), a reduced deltopectoral crest of the humerus, and the elongation of the preacetabular process of the ilium6,7.

These three dromaeosaurids are significantly larger in body size than the other Jehol Biota paravians (e.g.,3,4).

Daurlong differs from Tianyuraptor and Zhenyuanlong in the absence of accessory antorbital pits, in the robustness and curvature of the maxillary teeth, and in the relative robustness of the forearm elements.

It is noteworthy that the "Tianyuraptor-like" taxa retain features ancestral to both the above mentioned clades, shared with earlier-diverging dromaeosaurids (i.e., halszkaraptorines and unenlagiines) or with non-dromaeosaurid paravians (e.g., troodontids), such as the absence of a fossa bounding the maxillary recess4,18, the relatively short forelimbs (i.e., shared with troodontids, halszkaraptorines and some unenlagiines9,19), and the elongation of the preacetabular process of the ilium (i.e., shared with the halszkaraptorines, some unenlagiines, and avialans9,19).

As in Scipionyx intestine, the bluish layer does not extend cranial to the 9th dorsal vertebra and shows its maximum dorsoventral extent at the level of the 10th-12th dorsal vertebrae, where it reaches the gastral basket.

In both cases, an “empty” region separates the intestinal mass from the cranial margin of the pubis shaft41.

Ovoid structures in the abdominal cavity of a specimen of Sinosauropteryx, and argued to be eggs42, closely match in shape and position the duodenal portion of Scipionyx intestine41 and the ventral part of the bluish layer in Daurlong, and are here re-interpreted as intestinal remnants.

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