Archaeornithoides

Bever, Gabe S. & Norell, Mark A., 2009, The Perinate Skull of Byronosaurus (Troodontidae) with Observations on the Cranial Ontogeny of Paravian Theropods, American Museum Novitates 3657, pp. 1-52 : 44-45

ARCHAEORNITHOIDES

Archaeornithoides deinosauriscus Elzanowski and Wellnhofer, 1992 , was named based on a partial rostrum and mandible from the Djadokhta Formation at Bayn Dzak, Mongolia ( Elzanowski and Wellnhofer, 1993). As recently reviewed by Averianov and Sues (2007), Archaeornithoides deinosauriscus originally was hypothesized as having a privileged phylogenetic relationship with birds among nonavian theropods. Support for this relationship was drawn from the absence of interdental plates and lack of serrations on the teeth, and the presence of a paradental groove on the dentary and wide palatal shelves on the maxilla. The presence of each of these ‘‘avian’’ characteristics is now known to occur within troodontids, with Byronosaurus (both the holotype of Byronosaurus jaffei and the Ukhaa perinates) exhibiting all four characters. The avialan status of Archaeornithoides deinosauriscus was challenged by Clark et al. (2002) who indicated the holotype probably had passed through the digestive tract of a larger animal. These authors considered the holotype to be a poorly preserved juvenile specimen of a nonavialan coelurosaurian taxon ( Clark et al., 2002: 39). Currie (2000) suggested that the holotype of Archaeornithoides deinosauriscus might be a juvenile Saurornithoides mongoliensis , which also is known from Bayn Dzak. This possibility was considered by Elzanowski and Wellnhofer (1993) but rejected based on the presence of expanded palatal shelves of the maxillae in Archaeornithoides deinosauriscus —shelves that are present in basal avialans but also present in Byronosaurus (and probably both species of Saurornithoides ). Averianov and Sues (2007) put forth the idea that Archaeornithoides deinosauriscus could be a juvenile Byronosaurus jaffei ; however, they did not synonymize Byronosaurus under Archaeornithoides stating that it was preferable to retain both as distinct taxa until more information on troodontid ontogeny was available. The discovery and description of the Ukhaa perinates, which are highly comparable in size to the holotype of Archaeornithoides deinosauriscus , provide us with enough new information on troodontid skeletal ontogeny to reconsider this enigmatic taxon.

The presence of a distinct groove on the buccal surface of the dentary housing neurovascular foramina, a relatively large number of small teeth that are packed closely together (most notably at the rostral end of the lower jaw), dentary teeth that lie within a medially open groove, and a flat internarial bar are synapomorphies that are present in all known troodontids and that at least could be preserved in the holotype of Archaeornithoides deinosauriscus . The presence of each of these characters in the Ukhaa perinates establishes that in at least one troodontid lineage, these characters are present early in postnatal ontogeny and therefore likely present in Archaeornithoides deinosauriscus if that taxon is indeed a troodontid.

The morphology of the internarial bar of Archaeornithoides deinosauriscus cannot be discerned. The dentary teeth are larger than those of the Ukhaa perinates and are evenly spaced, with no obvious concentration at the rostral end of the lower jaw. The dentary teeth are described as sitting within distinct alveoli; however, the lingual margin of the tooth row is lower than the labial margin and is separated from the tooth row by a distinct paradental groove. The presence of an open groove for the dentary teeth as described by Currie (1987), therefore, may be present. The buccal surface of the lower jaw contains nutrient foramina lying within a distinct groove that is delimited dorsally by a ridge, as in the Ukhaa perinates. The groove is not as deep as that of adult troodontids but is similar in depth to the same structure in the Ukhaa perinates. The size and distribution of the dentary teeth suggests that Archaeornithoides deinosauriscus falls outside the troodontid clade defined by the most recent common ancestor of Sinovenator changii and Troodon formosus . The presence of a distinct groove on the buccal surface of the dentary is present in all troodontids but also is known in a small number of non-troodontid paravians (e.g., Buitreraptor gonzalezorum ; Makovicky et al., 2005). If the dentary teeth actually do lie within an open groove, this may represent an unambiguous troodontid synapomorphy (see above). These two characters tentatively support the conclusions of Currie (2000) and Averianov and Sues (2007) that Archaeornithoides deinosauriscus is a troodontid—but one that is outside the clade comprised of the other currently recognized troodontid taxa. Further complicating the issue of Archaeornithoides deinosauriscus is its possession of unconstricted teeth. A continuous transition between root and crown is a derived feature within Paraves that is shared by dromaeosaurs. If Archaeornithoides deinosauriscus is a basal troodontid then unconstricted teeth may be plesiomorphic for Paraves with the apomorphically reversed condition of constricted teeth occurring in troodontids, avialians, and Microraptor zhaoianus . The perinate holotype of Archaeornithoides deinosauriscus indicates that the transformation from constricted to unrestricted teeth occurs relatively early in ontogeny, and if the presence of constricted teeth in adult troodontids and avialans does reflect a paedomorphic phylogenetic transformation then the transformation likely affected the embryological rather than postnatal development.

Archaeornithoides deinosauriscus , despite being based on a highly fragmentary single specimen, provides further complexity to the issue of ontogeny and phylogeny of paranasal pneumaticity in nonavian theropods. The antorbital fossa, maxillary antrum, and premaxillary recess all are present and well defined in the holotype. The palatine of Archaeornithoides deinosauriscus , however, exhibits no evidence of pneumatization—in stark contrast to the Ukhaa perinates. The palatine also differs from that of the Ukhaa perinates in its possession of a more plesiomorphic tetraradiate shape that includes a well-defined pterygoid process. The establishment of pneumatic-related rostral morphologies early in the postnatal ontogeny of crocodilians, birds, allosaurs, and the Ukhaa perinates, indicates that the apparent absence of pneumatic fossae in the palatine of Archaeornithoides deinosauriscus is a derived condition (either the palatine in this taxon lacks pneumatization or pneumatization is apomorphically delayed into latter stages of postnatal ontogeny).