Abstract
The skulls of two perinate paravians from Ukhaa Tolgod, Djadoktha Formation, Mongolia, are described here. The skulls are nearly unique in their combination of ontogenetic age and preservational quality and provide us with the first look at the morphology of such important anatomical regions as the rostrum, palate, and braincase at or near the onset of postnatal development in a nonavian paravian coelurosaur. Based on a number of derived characters, the skulls are allocated to a derived position within Troodontidae that is outside the clade consisting of Saurornithoides mongoliensis, Saurornithoides junior, Troodon formosus, and probably Sinornithoides youngi. A single synapomorphy, presence of a lateral maxillary groove, supports the Ukhaa perinates as Byronosaurus. The comparative morphology of the Ukhaa perinates with adult troodontids indicates a number of significant postnatal transformations (e.g., elongation and flattening of the rostrum, increase in the number of maxillary and dentary teeth, restructuring of the occipital plate and paroccipital process). These comparisons demonstrate that many characters historically considered important for phylogenetic and taxonomic assessments of adult maniraptorans are present at a relatively early stage of ontogeny. Differences in the developmental timing of various cranial characters have important implications for interpreting the fossil record as well as for understanding the role heterochrony has played in the evolution of derived coleurosaurs, including birds. The ontogenetic information provided by the Ukhaa perinates also allow us to comment on the enigmatic paravian Archaeornithoides deinosauriscus, which has been considered both the sister taxon to Avialae and a juvenile specimen of the troodontids Saurornithoides mongoliensis and Byronosaurus jaffei. We found no unique characters that support a priviledged relationship of Archaeornithoides deinosauriscus with avialans and only weak character support for this taxon as a basal troodontid—there is no known character evidence supporting it as a juvenile of either Saurornithoides or Byronosaurus.
Highlights
The remains of embryonic, neonate, and juvenile dinosaurs are emerging from the fossil record with increasing frequency (e.g., Norell et al, 1994; Mateus et al, 1997; Xu et al, 2001; Reisz et al, 2005; Goodwin et al, 2006; Schwarz et al, 2007; Balanoff and Rowe, 2007; Balanoff et al, 2008; Kundrat et al, 2008)
Our understanding of skeletal development and its phylogenetic patterns in this important clade, which includes the origin of birds, is limited
The purpose of this paper is to provide a detailed description of these two skulls, and in the process, critically assess both their taxo¬ nomic status and importance for our under¬ standing of ontogenetic and phylogenetic transformations in theropod cranial morphol¬ ogy
Summary
The remains of embryonic, neonate, and juvenile dinosaurs are emerging from the fossil record with increasing frequency (e.g., Norell et al, 1994; Mateus et al, 1997; Xu et al, 2001; Reisz et al, 2005; Goodwin et al, 2006; Schwarz et al, 2007; Balanoff and Rowe, 2007; Balanoff et al, 2008; Kundrat et al, 2008). Establishing the nature and pattern of skeletal development, and the phylogenetic distribution of the features defining that development, is neces¬ sary to fully understand the characters through which we interpret the evolutionary history of this, or any, group. The implications of this understanding—or lack thereof—range from the accuracy with which specimens of different ontogenetic ages are identified taxonomically, to the precision with which the characters we use to build phylogenetic hypotheses are defined, to our ability to recognize transformations in developmental patterns. The only way to establish ourselves on the right side of these implications is by finding new ontogenetically and phylogenetically informative specimens and describe them in detail
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
