Theropods of the Judith River Formation of dinosaur Provincial Park, Alberta, Canada Author Currie, P. J. text 1987 1987-12-31 Tyrrell Museum of Paleontology Drumheller, Alberta Editor Currie P. J. Editor Koster E. H. Fourth Symposium on Mesozoic Terrestrial Ecosystems 51 60 book chapter http://doi.org/10.5281/zenodo.3676391 246dae96-99ed-4bb9-9ae7-b0223b873cf6 3676391 Ornithomimid frontals are very rare. Cranial bones in these animals are much lighter than those of other theropods both in terms of relative thickness , and the density of the spongy bone layer. Consequently, they may have been preserved less frequently because of their fragility. As in troodontids, the frontal of Dromiceiomimus is long and triangular ( Fig. 1b ). There is a distinct but shallow sutural surface for the prefrontal rostrodorsally. The lacrimal contact is unusual in that it is a long tapering suture on the rostrolateral surface of the frontal. Unlike most other theropods , there is a simple process of the nasal overlapping the frontal. The dorsal surface of the frontal is almost flat in the interorbital region, but caudally curves posteroventrally in an almost bulbous fashion to meet the parietal. The anterior margin of the supratemporal fenestra is not marked by a well-defined rim . The frontals of Struthiomimus altus ( AMNH 5355 ) and Ornithomimus edmontonicus ( ROM 851 , Horseshoe Canyon Formation ) are essentially identical to that of Dromiceiomimus samueli . Sues (1978) briefly described an indeterminate frontal in the collections of the National Museum of Canada ( NMC 12355 , Fig. 1C ) and referred it to the Theropoda . However , comparison with Mongolian specimens suggests that it may represent Erlicosaurus ( Currie , in preparation), a genus that is currently regarded as a Late Cretaceous prosaurupod (Paul 1984). The frontal suture of an isolated caenagnathid parietal ( TMP 81. 1 9. 25 2 , Currie, in preparation) shows that NMC 12355 is not a caenagnsthid , but it still may turn out to be an elmisaurid or other theropod . This frontal is an elongate triangle, like those of troodontids and ornithomimids . A shallow, rostrodorsal sutural surface suggests that the prefrontal was present and in the same position as in ornithomimids . The caudolateral margin of the ventral suture of the lacrimal is a transverse wall of bone similar to that of Troodon . A longitudinal trough runs along the dorsal surface of the frontal between the midline and the orbital rim . A poorly defined ridge marks the rostral boundary of the supratemporal fenestra. Dromaeosaurid frontals ( Figs . 1d, 1e , 3 ) are readily distinguished from those of other theropods. They are relatively shorter and broader than troodontid and ornithomimid frontals. In ventral view , the orbital rim is split rostrolaterally, and the lacrimal passes through this slot onto the dorsal surface of the bone. This is very different from the broad lacrimal suture on ventral surface of the frontals of Troodon and NMC 12355 , but is comparable with this part of the frontolacrimal suture of tyrannosaurids. Rostrodorsally, the dromaeosaurid frontal has a squamose sutural surface for the nasal and prefrontal bones. In dorsal view, the postorbital process of the frontal diverges strongly from the rest of the orbital rim . The postorbital suture extends onto the caudodorsal surface of the postorbital process, and is not well defined. The frontal is only shallowly dished out in the interorbital region.