What is Petrolacosaurus?
{{Italic title}} {{Taxobox | name = ''Petrolacosaurus'' | fossil_range = {{Fossil range|Upper Carboniferous}} | image = Petrolacosaurus_BW.jpg | image_width = 250px | image_caption = ''Petrolacosaurus kansensis'' | regnum = [[Animal]]ia | phylum = [[Chordate|Chordata]] | classis = [[Sauropsida]] | subclassis = [[Diapsida]] | familia = '''Petrolacosauridae''' | familia_authority = Peabody, 1952 | genus = '''''Petrolacosaurus''''' | genus_authority = Lane, 1945}} '''''Petrolacosaurus''''' was a small, {{convert|40|cm|in|adj=on}} long animal, the earliest [[diapsid]] known. It lived during the later [[Carboniferous]] period. The strata where it was found in [[Kansas]] are of [[Pennsylvanian]] age, approximately 302 million years old.<ref>Falcon-Lang H.J; Benton M.J. & Stimson M. 2007. Ecology of early reptiles inferred from Lower Pennsylvanian trackways. ''Journal of the Geological Society'', London, '''164''', 6, 1113-1118. [http://findarticles.com/p/articles/mi_qa3721/is_200711/ai_n21137484/?tag=content;col1 article]{{Dead link|date=October 2022 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Its diet was probably small [[insect]]s. ''Petrolacosaurus'' had distinctive canine-like secondary-sized teeth, a [[Trait (biology)|trait]] found mainly in [[therapsids]], and later in [[mammals]]. Its fossils were found in [[Kansas]], USA.<ref name=EoDP>{{cite book |editor=Palmer D.|year=1999 |title= The Marshall illustrated encyclopedia of dinosaurs and prehistoric animals|url=https://archive.org/details/illustratedencyc0000unse_y8e7|publisher= Marshall Editions|location=London|page= [https://archive.org/details/illustratedencyc0000unse_y8e7/page/n319 82]|isbn= 1-84028-152-9}}</ref> ''Petrolacosaurus'' was too [[derived]] to be the ancestor of all modern forms. In fact, it was already a [[diapsid]], with two openings on each side of its skull to add attachment points for [[jaw]] muscles. Hence, it cannot have been the ancestor of any [[synapsids]], which have only one such opening. Synapsids diverged from the comm