Evidence for a venomous Sinornithosaurus

Recent work (Fry et al. 2008) showing the widespread presence of venom glands in lepidosaurs (including the common iguana) demonstrates that they are not “uncommon”. Both maxillary and mandibular glands were originally present and many lizards have retained their maxillary glands even though the mandibular glands are the primary source of toxin. Apparently, venom was present at the base of the lepidosaur radiation, and might be expected in a sister group (archosaurs). In modern reptilian taxa, teeth with pronounced labial grooves have proved to be venomous, and this type of tooth has often been suggested as indicating venom delivery in extinct taxa including conodonts (Szaniawski 2009) and synapsids (Hotton 1991). Gianechini et al. (2010) make a fundamental error by assuming that archosaurs with grooved teeth had no venomous taxa in their ancestry and were not venomous themselves. They support speculation about an extinct genus and additional speculation about other extinct genera. We have no way to conclusively prove that any of the animals with labial tooth grooves were not venomous and, considering recent studies of lepidosaurs, might reasonably expect them to be.

It is interesting that almost all of their examples with labial tooth grooves are drawn from a clade including Sinornithosaurus with other feathered taxa. We must point out that the grooves in the holotype do extend to the tip of the tooth leading us to wonder if the authors have ever examined the actual material. Our view that venom may be basal for archosaurs will allow occasional examples outside that clade. The only reasonable way to evaluate evidence for physiology and behavior in extinct taxa is by analogy with living forms in which the presence or absence of venom glands can be observed directly. The absence of teeth in modern birds severely limits our expectations of finding venom in modern archosaurs.

Our restoration of the skull of Sinornithosaurus is based on that of Xu and Wu (2001) as modified by our microscopic restudy of their specimen and two additional skulls (Fig. 1). The region with the subfenestral fossa is most poorly preserved in the type material of S. milleni and does not appear in the skull restoration of the describers. These changes were supported by our published photographs. In so doing we considered the exact questions raised by Gianechini et al. (2010). Our restoration shows the teeth with a little more of the crown exposed than illustrated by Xu and Wu (2001) but the extreme length of these teeth is not in question. Examination of the available alveoli and the potential socket depth based on the maxillary, indicates that any potential slippage was inadequate to affect their interpretation, as was confirmed by examination using radiographic techniques on related taxa. The relative length and distribution of teeth is supported by the Dalian specimen and one from Tianyu, in which the teeth remain in the sockets, approximately in their original position. The presence of the longest teeth posterior in the jaw but not entirely to its back is characteristic of modern venomous rear-fanged snakes (Fry et al. 2008) and provides striking evidence for the venomous interpretation.

Fig. 1
figure1

Sinornithosaurus showing elongated rear fangs, subfenestral fossa, and supradental groove in an articulated skull (Zheng 2009)

Their definition of a venom groove is arbitrary as was admitted by its author (Nydam 2000), and venom grooves in a variety of modern venomous animals do not fit his criteria. Modern researchers recognize a variety of morphologies (Fry et al. 2008) and our teeth are well within known variability.

Nowhere do we claim that the antorbital fenestra housed the venom gland, and the subfenestral fossa is clearly separated from the antorbital fossa by a bony ridge. Gianechini et al. (2010) offer no explanation why this is so. We did not associate the pits in the subfenestral fossa with the venom system and do not claim to understand their origin or function. Pit-like ornamentation was reported by Xu et al. (1999) as a diagnostic feature of Sinornithosaurus.

The reservoir pits would not be expected to be coincident with the tooth sockets, as is shown by Fry et al. (2008). The unusual nature of the pits and supradentary groove was recognized by Xu et al. (1999) in the original description of Sinornithosaurus. They should not be confused with foramina which are, by definition, penetrating structures.

Our speculation about the long fangs in Sinornithosaurus having evolved to penetrate a covering of feathers and the possibility that it was largely a predator on feathered taxa is unaffected by their comments. Feathers make such a thick keratinous covering that shorter teeth would have been unsuccessful. In a situation in which a deep tearing wound was difficult to achieve, addition of poison may have been favored. However their rejection of that hypothesis seems to result from inadequate knowledge of the original fossil material and the anatomy of extant venomous taxa. It is our hope that our paper will stimulate further research into the distribution of venom in both extinct and modern taxa.

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Correspondence to David A. Burnham.

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Gong, E., Martin, L.D., Burnham, D.A. et al. Evidence for a venomous Sinornithosaurus . Paläontol Z 85, 109–111 (2011). https://doi.org/10.1007/s12542-010-0076-7

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Keywords

  • Mandibular Gland
  • Venom Gland
  • Extinct Taxon
  • Venomous Animal
  • Extinct Genus