Sinornithosaurus millenii

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Sinornithosaurus millenii


Life restoration of Sinornithosaurus. © @ Gabriel Ugueto.

Temporal range: Early Cretaceous; Aptian (~124.6-122 Ma)[1][2]

Scientific classification:

Life
Domain: Eukaryota
(unranked): Unikonta
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Clade: Olfactores
Clade: Craniata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Clade: Eugnathostomata
Clade: Teleostomi
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Reptilia or Sauropsida
Clade: Eureptilia
Clade: Romeriida
Clade: Diapsida
Clade: Neodiapsida
Clade: Archelosauria
Clade: Archosauromorpha
Clade: Archosauriformes
Clade: Crurotarsi
Clade: Archosauria
Clade: Avemetatarsalia
Clade: Ornithodira
Clade: Dinosauromorpha
Clade: Dinosauriformes
Clade: Dinosauria
Order: Saurischia
Clade: Eusaurischia
Clade: Theropoda
Clade: Neotheropoda
Clade: Averostra
Clade: Tetanurae
Clade: Orionides
Clade: Avetheropoda
Clade: Coelurosauria
Clade: Tyrannoraptora
Clade: Maniraptoriformes
Clade: Maniraptora
Clade: Pennaraptora
Clade: Paraves
Clade: Eumaniraptora
Family: †Dromaeosauridae
Clade: †Microraptoria
Genus: †Sinornithosaurus
Species: †S. millenii

Sinornithosaurus is a genus of microraptorian dromaeosaurid that lived in the Early Cretaceous of Liaoning, China ~124.6-122 million years ago.[1][2]

Taxonomy:
The type, and only confirmed, species of Sinornithosaurus is S. millenii. Although a second species, S. “haoiana”, was also described, a subsequent analysis found that the supposed synapomorphies of S. “haoiana” are either present in S. millenii or subject to variation in known Sinornithosaurus specimens. Thus, S. “haoiana” is considered a junior synonym of S. millenii.[3]

Description:
Gregory S. Paul estimated a total length of 1.2 meters, as well as a total body mass of 3 kg, for S. millenii. The head was large, long, and shallow.[4] Contrary to what Paul claimed[4], not all of the teeth were serrated. The premaxillary teeth were unserrated on both anterior and posterior carina with the exception of the first tooth, which has a few very faint denticles on the middle part of the anterior carina[3][5], while other teeth were indeed serrated.[4][5] Most of the teeth were mediolaterally compressed, moderately recurved, and inclined posteriorly[5] (thus, with their serrations, can be considered ziphodont). The premaxillary tooth crowns also had a distinct groove posterior to the anterior carina on the lingual surface.[3] The maxillary teeth have been considered fang-like with grooves on their lateral surfaces.[6] Some authors have considered the teeth slipped out of their sockets due to taphonomy[7], although there are apparently other specimens with fully articulated teeth of similar length and distribution.[8]

Sinornithosaurus did not have fused sternal plates.[4]

The feathers of Sinornithosaurus were generally 30-45 mm long and 1-3 mm wide. They were compound structures composed of multiple filaments with two types of branching structure only found in avian feathers: filaments joined in a basal tuft and filaments joined at their bases in a series along a central filament. However, they seemed to lack barbules, and so Sinornithosaurus’ feathers could not have formed a closed pennaceous vane.[9] The filaments contained either eumelanosomes or phaeomelanosomes (which imply black and russet coloration), indicating that its feathers had significantly different color tones.[10]


Image of specimen NGMC 91 ("Dave"). © @ Jonathan Chen.

Paleobiology:
Sinornithosaurus has been suggested to be the first venomous non-avian dinosaur, on the grounds that the teeth were fang-like, the presence of grooves on these teeth (serving as channels for venom), and an alleged pocket for venom glands.[6] This is highly unlikely, as grooved teeth are found in numerous other theropods, and there is no obvious sign of a venom gland[7] (as stated above, the maxillary teeth have also been argued to have slipped out of their sockets due to taphonomy[7], although other specimens supposedly suggest this to have been a real feature[8]). Grooved teeth are also found in other animals outside of the Dinosauria, and they are not necessarily associated with venom delivery. A few alternative hypotheses have been suggested for their function.[11]

The shape of the sclerotic ring of Sinornithosaurus suggests it was a cathemeral predator, being active at any light level.[12]

Sinornithosaurus had contour feathers on its hindlimbs, and appears to have been a glider that moved through the air in a biplane-like configuration.[13] As in other microraptorines, the low aspect ratio might have helped compensate for small adjustments in pitch while gliding.[14]

Gregory S. Paul considered S. millenii to have been capable of preying upon contemporaneous species of Sapeornis, Caudipteryx, Psittacosaurus, Jeholornis, and Tianyulong.[4] There is potential evidence that Sinornithosaurus itself was prey to larger predators, with a dromaeosaurid hindlimb (tentatively referred to Sinornithosaurus) found in the stomach contents of the holotype specimen of Sinocalliopteryx gigas (a large compsognathid).[15]


Art by Gregory S. Paul. Screen capture taken from [4].


Life restoration of Sinocalliopteryx feeding on Sinornithosaurus. © @ Cheung Chungtat. Note the possibly inaccurate coloration of the Sinornithosaurus.

References:

[1] Zhonghe, Z. (2006). Evolutionary radiation of the Jehol Biota: chronological and ecological perspectives. Geological Journal, 41(3‐4), 377-393.
[2] Chang, S. C., Zhang, H., Renne, P. R., & Fang, Y. (2009). High-precision 40Ar/39Ar age for the Jehol Biota. Palaeogeography, Palaeoclimatology, Palaeoecology, 280(1-2), 94-104.
[3] Turner, A. H., Makovicky, P. J., & Norell, M. A. (2012). A review of dromaeosaurid systematics and paravian phylogeny. Bulletin of the American museum of natural history, 2012(371), 1-206.
[4] Paul, G. S. (2016). The Princeton field guide to dinosaurs (Vol. 110). Princeton University Press. pp. 147, 161-162, 170, 267, & 273.
[5] Xu, X., & Wu, X. C. (2001). Cranial morphology of Sinornithosaurus millenii Xu et al. 1999 (Dinosauria: Theropoda: Dromaeosauridae) from the Yixian formation of Liaoning, China. Canadian Journal of Earth Sciences, 38(12), 1739-1752.
[6] Gong, E., Martin, L. D., Burnham, D. A., & Falk, A. R. (2010). The birdlike raptor Sinornithosaurus was venomous. Proceedings of the National Academy of Sciences, 107(2), 766-768.
[7] Gianechini, F. A., Agnolín, F. L., & Ezcurra, M. D. (2011). A reassessment of the purported venom delivery system of the bird-like raptor Sinornithosaurus. Paläontologische Zeitschrift, 85(1), 103-107.
[8] Gong, E., Martin, L. D., Burnham, D. A., & Falk, A. R. (2011). Evidence for a venomous Sinornithosaurus. Paläontologische Zeitschrift, 85(1), 109-111.
[9] Xu, X., Zhou, Zh. & Prum, R. Branched integumental structures in Sinornithosaurus and the origin of feathers. Nature 410, 200–204 (2001). doi.org/10.1038/35065589
[10] Zhang, Fucheng; Kearns, Stuart L.; Orr, Patrick J.; Benton, Michael J.; Zhou, Zhonghe; Johnson, Diane; Xu, Xing and Wang, Xiaolin (2010). Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature, 463(7284) p. 1075.
[11] Folinsbee, K. E., Müller, J., & Reisz, R. R. (2007). Canine grooves: morphology, function, and relevance to venom. Journal of Vertebrate Paleontology, 27(2), 547-551.
[12] Schmitz, L., & Motani, R. (2011). Nocturnality in dinosaurs inferred from scleral ring and orbit morphology. Science, 332(6030), 705-708.
[13] Chatterjee, S., & Templin, R. J. (2007). Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui. Proceedings of the National Academy of Sciences, 104(5), 1576-1580.
[14] Han, G., Chiappe, L. M., Ji, S. A., Habib, M., Turner, A. H., Chinsamy, A., ... & Han, L. (2014). A new raptorial dinosaur with exceptionally long feathering provides insights into dromaeosaurid flight performance. Nature Communications, 5(1), 1-9.
[15] Xing, L., Bell, P. R., Persons IV, W. S., Ji, S., Miyashita, T., Burns, M. E., ... & Currie, P. J. (2012). Abdominal contents from two large Early Cretaceous compsognathids (Dinosauria: Theropoda) demonstrate feeding on confuciusornithids and dromaeosaurids. PLoS One, 7(8), e44012.

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You might have noticed that I don't specifically mention any of the alternative hypotheses for grooved teeth in the OP. I didn't want to give my personal opinion there since then I felt like the OP would have been less of an accurate summary of what we know about this animal and more of me regurgitating my personal opinions.

Instead I'm going to offer my opinion on the potential reason behind Sinornithosaurus' grooved teeth here. There are three alternative hypotheses for the purpose of grooved teeth (when they're not for venom delivery); see [11] in the OP for details.

1.) Increasing the structural strength of the tooth (especially a slender tooth) by increasing the amount of surface area for enamel (which is harder than dentin).

2.) Storing bacteria for infectious bites.

3.) Permitting rapid penetration and withdrawal of the teeth during fights ("blood groove" hypothesis?).


An illustration of the skull of Sinornithosaurus for visual reference. © @ Sebastian Josephus Odenbacker.

Out of all of these alternative hypotheses, hypothesis 1 is the only one that's not obviously nonsense. In fact, it actually makes intuitive sense. There appear to be some conflicting opinions over just how elongated the maxillary and dentary teeth of Sinornithosaurus really are, but I'll take Gong et al.'s word for it that there are fully articulated teeth (from other specimens) that are similar in length to the specimen they originally examined (although, that doesn't mean absolutely no tooth slippage happened in the latter), since I don't see much reason to doubt them.

Assuming that is true, then, the tooth crowns of Sinornithosaurus were quite elongated and thus more susceptible to fracture. How to make them stronger? Have grooves that increase the amount of surface area for enamel (the hardest substance in teeth). As a result, the length and serrations of the teeth would allow them to inflict relatively large, debilitating wounds on prey, while the grooves on the teeth provide the structural strength needed to perform this action.

I think the fact that both maxillary and dentary teeth have these grooves, and even the premaxillary teeth have distinctive grooves, makes this hypothesis more plausible than the venomous bite one. If strengthening the teeth was the function of these grooves, we should expect to see them on both upper and lower teeth (and we do), as all of the teeth would be experiencing stress while biting and drawing through flesh. If Sinornithosaurus was venomous, shouldn't we expect to only see grooves on the maxillary teeth, since that's where the supposed "venom gland" is? Additionally, many other theropods possess grooves on their teeth as noted in the OP; as predatory animals, any adaptations to strengthen their teeth as they bite, cut, and/or hold onto prey would have been beneficial to them.

This isn't 100% proven, however. Biomechanical modeling would confirm whether or not grooves with added enamel surface area actually strengthens teeth. Also, it could be that the teeth really are slipped too far out of their sockets (although, the grooves still seem to continue quite close to the tooth tips).