about its brain. Probainognathus’ cerebral hemispheres were elongated, it had established olfactory bulbs, and a significant cerebellum and flooculi. These were among other, less pronounced, structures such as anterior colliculi. Because of these features, it has been suggested that Probainognathus’ brain had well-developed visual, auditory, motor, and body sensory systems. And, as a whole, the brain of Probainognathus indicates that it was in the process of developing endothermy. Skull The snout of Probainognathus extended beyond the premaxillary processes, and was very thin. The apparent fragility of this structure has led to it being broken off several specimens, and has led to an increased difficulty in studying it. There are several features that have been identified, however. The suborbital arch and the zygomatic arch were both shaped relatively sharply, and there was an extension of the squamosal posteriorly on the zygomatic arch. Probainognathus had a long secondary palate as it stretched all the way to the posterior end of the tooth row. The secondary palate was formed largely by the palatine, and it was somewhat narrow. This narrowing of the palate allowed for the conservation of space to be able to fit the back, lower postcanine teeth. Moreover, the vomer stretched posteriorly to the secondary palate, and significant pterygoid flanges were present, as well. The skull of Probainognathus was on average around in length, and lacked the pineal foramen. The skull was broad in proportion to its length, the face was short and narrow, but its zygomatic region was relatively large, seemingly for jaw muscle attachment. The skull also possessed a double occipital condyle, which is a typical mammalian feature. In comparison to close relatives, such as Chiniquodon, the braincase of Probainognathus was broader in the parietal region, and the oticoccipital region of the skull was deeper. Likewise, the parietals had more depth, the occipital crest was higher, and the orbits were proportionately larger relative to the rest of its head. The sagittal crest was also lengthened posteriorly, and split into the occipital crests. This led to a straighter posterior margin, and a more concave occiput in Probainognathus. Jaw The jaw of Probainognathus is of particular phylogenetic importance. Morphologically, the dentary made up most of the lower jaw, and it curved and extended down posteriorly to the area of the articular and jaw articulation. Correspondingly, in the upper jaw, the squamosal bone became situated next to the quadrate. Like some other