Pawpawsaurus
Pawpawsaurus, meaning "Pawpaw Lizard", is a nodosaurid ankylosaur from the Cretaceous (late Albian) of Tarrant County, Texas, discovered in May 1992. The only species yet assigned to this taxon, "Pawpawsaurus campbelli," is based on a complete skull (lacking mandibles) from the marine Paw Paw Formation (Wachita Group). Discovery. "Pawpawsaurus" was found in the Paw Paw Formation in Tarrant County, Texas, in May 1992, by Cameron Campbell. A complete skull is the only specimen, from which the binomial was named "Pawpawsaurus campbelli". The Paw Paw Formation has produced another nodosaurid, "Texasetes pleurohalio" (Coombs, 1995), which may prove to be a  senior synonym of "Pawpawsaurus". This is the only nodosaurid known to have possessed the bony eyelids commonly found in ankylosaurids. The skull of "Pawpawsaurus" bears some notable similarities to that of "Silvisaurus", such as the presence of teeth in the premaxilla and the restriction of the osseous secondary palate to the rostral portion of the palatal region. Description. "Pawpawsaurus" is known from a relatively well preserved and complete skull. The skull is complete enough for a 2016 study to be able to identify and reconstruct the brain, nasal cavities and inner ear of the specimen, and CT scan it to  find new anatomical features. The brain of "Pawpawsaurus" is very similar to other ankylosaurians, but along with more derived taxa it is most different from "Kunbarrasaurus". The brain is 30% of the skull length, very similar to "Panoplosaurus", higher than "Euoplocephalus", and lower than "Kunbarrasaurus". The brain of "Pawpawsaurus" is long and wide. The inner ear is also similar to ankylosaurians to the exclusion of "Kunbarrasaurus", being tall and wide at the level of the semicircular canals. The nasal cavities of "Pawpawsaurus" resemble that of "Panoplosaurus" and "Euoplocephalus", although the thin dividing medians are not preserved as they were likely  cartilaginous. Classification. Based on the dimensions of the skull, "Pawpawsaurus" is believed to have been approximately 4.5 meters in length. Vickaryous et al. (2004) have stated that ""Sauropelta edwardsorum", "Silvisaurus condrayi", and "Pawpawsaurus campbelli" form a basal polytomy nested deep to "Cedarpelta"." Alternatively, "Pawpawsaurus" has been identified as a relative of "Struthiosaurus", with cladogram below showing the results of the 2018 phylogenetic analysis of Rivera-Sylva and colleagues, resolving "Pawpawsaurus" within the clade Struthiosaurini as labelled by Madzia "et al.", with relationships outside Struthiosaurini excluded for simplicity. Palaeobiology. "Pawpawsaurus" was primitive in regards to its sensory abilities when compared to  later ankylosaurs. CT scans of its skull and brain by Paulina-Carabajal and Louis Jacobs et al. revealed that "Pawpawsaurus"' hearing was on par with that of modern crocodilians, indicating that its hearing was relatively poor. Its sense of smell meanwhile, while not as advanced as that of later ankylosaurs, was still more powerful and acute than many of the theropods existent at the time thanks to its large nasal cavities. The enlarged nasal cavities of "Pawpawsaurus" would also likely have been useful for cooling blood that was entering the brain, finding food and potential mates, or alerting it to  danger.