云南兽(三列齿类爬行动物)的耳区结构

本文介绍了以短吻云南兽为代表的一种耳区结构.它表明在三列齿类爬行动物里已经出现有发育的耳蜗壳以及在其内侧通过的颈内动脉等进步性质,听腔亦趋封闭.云南兽的中耳腔外侧出现了一条曲折的骨质外耳道,侧枕骨突外侧明显的沟可能表明方骨后耳膜之存在.

OTIC REGION IN TRITYLODONT YUNNANODON

Yunnanodon brevirostre (formerly named as Yunnania, Cui, 1976) is a tritylodont of small size, with short and broad snout, and low and posteriorly situated parietal crest. It is also characterized by possessing less number of postcanine teeth and a cusp formula of 2-3-2. The type skull came from the Lower Lufeng Series in the Lufeng Basin, Yunnan Province. The basic structure of the pterygoid and basisphenoid region of Yunnanodon brevirostre is exactly Bienotherium-like, i. e. narrow and ridged, which is distinguished from the flat and wide type as in the Bienotheroides. Recently, a further preparation reveals the auditory region of the type skull (V5017). Unexpectedly, it is found to be of a totally different structural pattern. Firstly, the skul possesses a pair of buldges at the rear end of the basisphenoid wing (plate I) in front of the fenestra ovalis and fenestra rotunda. It gives no choice but to interpret the buldges as the cochlear housing, 'promontorium' in Morganucodon and Sinoconodon. Up to the present, there is no such elevations appeared in any cynodonts and other tritylodonts. Figure 1 illustrates the internal morphology of the cochlea when the cover of the right side has been taken off. The buldge itself is filled with matrix, most of the fillings are quartz-like mineral grains. Inside the cochlea is a hollow space, its anterior end extends right into the prootic bone. A distinct round opening at the outer ridge indicates the exit of auditory nerve, the opening being considerably large. Separated from the nerve opening by a narrow bony bridge is a deep fossa which locates at the inner side of the fenestra ovalis. The fossa stretches dorsally and terminated at an opening, which penetrates the braincase. This opening in the braincase is anterior to the jugular foramen. This fossa should be the osseous vestibule and the dorsal open- ing to the braincase, a passage to the superior semicircular canal. Three small openings were also appeared at the lateral broken surface, one of them shows its connection with the vestibule fossa. These holes and passages may represent the remains of semicircular canals. On the other side of the skull, a tiny foramen has been exposed at the antero-medial corner of the cochlear buldge, and followed by a distinct groove. It is reasonable to assume this groove as the path of the internal carotid artery, and the foramen, its entrance, more precisely, the medial internal carotid artery by mammalian term. It is worth notice that this animal pos- sesses both the mammalian characters of the cochlear housing and the position of the mentioned artery. The structure of the middle ear is also unusual. Resulted from the development of the 'promontorium', the petrosal is enlarged and in contact with the basisphenoid end and the lateral flange of the prootic bone; consequently, a bony wall is formed, separating the cavum epipterycure from the tympanic cavity. In comparison with the other tritylodonts, the tympanic cavity is more closed. A small foramen at the anterior border of the paroccipital process represents the pterygo-paroccipital foramen. Opposite to the fenestra ovalis, there is a small piece of bone resting on the corner, it extends across the cavity and points to the fenestra ovalis, but failed to reach it because of breakage. The weakness and fragility of the small bone prevent the possibility of further preparing. This should be explained as the stapes. In front of it is another opening on the wall of prootic, which is identified as the internal opening of the vena capitis lateralis. On the paroccipital process, the attachment of the stapedial muscle leaves a very distinct scar, it is ellipsoid instead of being rounded as that in Morganucodon and Sinoconodon. The posterior paroccipital process is very long and slender, its isolated rear end stretches ventrally, while the anterior process is quite normal in position. The peculiar ball-like structure outside the paroccipital process is really interesting. The 'ball' is swollen dorsally and almost reaches the level of the skull roof. Ventrally, between the 'ball' and the paroccipital process lies a very distinct sulcus. The anterior half of the sulcus is relatively deep but tends shallow and flat posteriorly. The sulcus curves somehow outwards and runs between the end of stapes and the posterior paroccipital process. The quadrate is assumed resting at the place where the stapes ended. The authors here prefer to accept the 'postquadrate tympanum' statement based on the sulcus at this position. We consider the only explanation of the sulcus is the hanging with the ear drum. The lateral edge of it could be compared to the squamosal lip in Diademodon (Watson, 1911) and other cynodonts, which supports the tympanic membrane, as some authors held. If the explanation of the tympanic membrane is acceptable, it is most likely that the 'ball' should be represent a part of the squamosal bone, which contains the external audi[ory meatus. The meatus has been shown in various mammal-like reptiles. It exists in tritylodonts as well, except turns laterally rather then posteriorly (Kuhne, 1956). In Yunnanodon, the three openings on the 'ball' connect with each other, the external auditory meatus must be long and tortuous. The lateral opening sitting high up the top probably indicates the end of the meatus, and the basal plate inside the window may indicate certain device of the external ear. All these unusual morphological structures of the ear region speaks a certain particular mode of life of Yunnanodon brevirostre.