Kulbeckia

Ekdale, Eric G., Archibald, J. David & Averianov, Alexander O., 2004, Petrosal bones of placental mammals from the Late Cretaceous of Uzbekistan, Acta Palaeontologica Polonica 49 (1), pp. 161-176 : 168-170

Kulbeckia ( type II)

Ear regions from the Late Cretaceous of Uzbekistan were assigned to the genus Kulbeckia based on similarity to the petrosal structure of other “zalambdalestid” taxa,including Zalambdalestes and Barunlestes ( Kielan−Jaworowska and Trofimov 1980; Kielan−Jaworowska 1984; Rougier et al. 1998) and other criteria discussed earlier. The overall structure of the Kulbeckia petrosal is described below.

Cerebellar surface.—Two conspicuous openings are found on the cerebellar surface of the petrosal of Kulbeckia : the internal auditory meatus anteriorly,and the subarcuate fossa posteriorly ( Fig. 3A ). The ovoid internal auditory meatus is subdivided by the crista transversa into the foramen acusticum inferius (medial) and the foramen acusticum superius (lateral). The foramen acusticum inferius is oval in shape and opens dorsolaterally, whereas the foramen acusticum superius is smaller,rounder, and opens dorsally and anteromedially.

The subarcuate fossa sits posterolateral to the internal auditory meatus. Unfortunately,none of the Kulbeckia specimens preserve a complete subarcuate fossa. The best preserved fossa can be seen on URBAC 00−16. Although the borders of the fossa on this specimen have been damaged,it can be determined that the fossa is indeed present in the Kulbeckia petrosal,and it forms a deep and circular pit with smooth and thin walls. In addition,because of the damaged borders,portions of the paths of the semicircular canals are exposed.

Traveling in an anterior−posterior direction along the medial edge of the petrosal bone is the sulcus for the inferior petrosal sinus. The sulcus skirts the medial rim of the cerebellar surface from the rostral apex (where the sulcus is quite broad,but narrows as it extends posteriorly) to a level at the posteriormost aspect of the internal auditory meatus ( Fig. 3A ).

Immediately posterior to the posterior terminus of the sulcus for the inferior petrosal sinus is the small and circular opening of the cochlear canalicularis. It opens medially and slightly posteriorly. A short sulcus can be seen traveling posteriorly from this small opening.

Posterior to the cochlear canalicularis is a jagged piece of bone that is marked by several sulci and pits. The area is best preserved in URBAC 00−16,but this region of the petrosal bone can be observed in the other Kulbeckia specimens to a lesser extent. This surface of the petrosal forms the lateral wall of the posterior lacerate foramen. Because the only basicranial elements known for Kulbeckia are isolated petrosal bones,the ultimate shape and size of the posterior lacerate foramen cannot be determined for this taxon at this time.

Squamosal surface.—In Kulbeckia ,the tegmen tympani is thin and uninflated. At the posterior aspect of the squamosal surface of the petrosal bone run two broad sulci,which are al − most parallel to each other,divided by a distinct crest ( Fig. 3B ). The anterior of the two sulci is the petrosal contribution to the ascending canal for the superior ramus of the stapedial artery (the other portion would be contributed by the squa−

EKDALE ET AL.—PLACENTAL PETROSALS FROM LATE CRETACEOUS 169

mosal in an articulated skull). This canal runs from a position just posterolateral to the anterior border of the subarcuate fossa to an anteroventral position on the tympanic surface of the bone. The posterior sulcus runs in a more anterior−posterior direction ventral to the first sulcus mentioned. The second sulcus,which is the petrosal contribution of the post − temporal canal,is shallower than the first,and it widens as it extends posteriorly.

Tympanic surface.—The tympanic surface of the Kulbeckia petrosal is marked by the semi−ovoid bulge of the promontorium ( Fig. 3C ). Although the internal structure of the Kulbeckia ear has yet to be determined,given the overall similarity of the external petrosal structure to that of other “zalambdalestids”,it is safe to assume that the cochlea com − pletes at least one full 360° coil,as is the case with all therian mammals ( Wible et al. 2001). The promontorium is tear−drop in shape,and the surface is fairly smooth,although it does possess a few dimples.

The fenestra cochleae is found along the posteromedial aspect of the promontorium,just ventral to the cochlear canalicularis. The fenestra is round,it opens posterodorsally, and it is subequal in size to the fenestra vestibuli sitting at the posteroventral aspect of the promontorium. The fenestra vestibuli opens dorsolaterally,and it has an average stapedial ratio of 1.9 (the range being 1.4–2.8 and the standard deviation is 0.4). The wide range of variation in the stapedial ratios does not appear to be a result of damage,and without associ − ated skeletal elements,it cannot be determined whether the variation is taxonomic (i.e.,these petrosals represent more than one taxon) or not. There is not a correlation between stapedial ratio and promontorium area,so it is unlikely that the variation is a result of body size.

Traversing the fenestra vestibuli in a medial to lateral direction is a distinct sulcus for the stapedial branch of the internal carotid artery. This groove can be seen clearly on all of the specimens extending from a position just lateral to the fenestra cochleae and across the fenestra vestibuli. Its position lies in line with the anterior sulcus found on the squamosal side of the petrosal as discussed above. On ZIN C. 85045,the sulcus notches the fenestra vestibuli on its medial aspect,but no other Kulbeckia petrosal specimens display this notch.

Anterior to the fenestra vestibuli on all Kulbeckia specimens is a slight depression,giving the promontorium a dim − pled appearance. The depression is in the same location as the sulcus for a promontorial branch of the internal carotid artery in other mammals,but upon closer inspection,it is doubtful that this depression carried a blood vessel in life,as it is merely a depression and not a sulcus.

Posterior to the promontorium are two depressions separated by a thick,curved crest. These two depressions are best preserved in URBAC 00−02 and 00−16. The medialmost depression,which sits posteriorly adjacent to the fenestra co − chleae,is the cochlear fossula. This rounded and shallow de − pression accommodated the cavum tympani in life. The fossa is bordered anteriorly by the posterior lip of the fenestra cochleae,medially by the posterior lacerate foramen,posteri − orly by a distinct wall of bone running the width of the petrosal known as the caudal tympanic process,and laterally by a posterior extension of the crista interfenestralis which, by connecting to the caudal tympanic process of the petrosal, divides the cochlear fossula from the second depression posterior to the promontorium. The crista interfenestralis is the bridge of bone separating the fenestrae vestibuli and cochleae.

On the portion of the caudal tympanic process bordering the cochlear fossula posteriorly is a “tympanic process”,as originally described for Asioryctes and Kennalestes by Kielan−Jaworowska (1981) and scored as present for Zalambdalestes by Rougier et al. (1998). The “tympanic process” is incomplete in all of the Kulbeckia petrosals studied here. A reason for this is that the process is composed of both petrosal and exoccipital in other taxa,and the same may be true for Kulbeckia ,for which no exoccipital is known. Nonetheless,a portion of the process can be seen in URBAC 00−16. The crista interfenestralis extends posteriorly to contact the “tympanic process”,and in so doing,divides the two depressions posterior to the promontorium.

The depression lateral to the posterior extension of the crista interfenestralis lies immediately posterior to the fenestra vestibuli. It is significantly larger and deeper than the cochlear fossula,and it is ovoid in shape. The medial portion of this depression is the fossa musculus minor,and the lateral aspect is the sulcus for the facial nerve. Posterolaterally,this depression is formed by a large and distinct paroccipital process of the petrosal bone,as is best preserved in URBAC 00−16,as well as the lateral portion of the caudal tympanic process of the petrosal. Unfortunately,because all of the Kulbeckia specimens are damaged posteriorly,none of them possess a complete paroccipital process.

At the anterior end of the facial sulcus and anterolateral to the fenestra vestibuli is the tympanic aperture of the facial canal. In Kulbeckia ,this opening is oriented posteriorly and slightly dorsally ( Fig. 3C ). The opening is rounded anteriorly.

Laterally adjacent to the fenestra vestibuli,and ventral to the tegmen tympani,is a small,flat,and uniformly concave epitympanic recess for the articulation of the auditory ossicles ( Fig. 3C ). Lateral to the epitympanic recess is the ventral terminus of the sulcus for the superior ramus of the stapedial artery. Traveling anteriorly from this sulcus on the ventral surface of the petrosal is a groove for the inferior ramus of the stapedial artery ( Fig. 3C ). The sulcus can be seen parallel to the promontorium along the ventral bony flooring of the facial canal. This sulcus is best observed in URBAC 00−16,but can be seen in the other specimens to a lesser extent.

Anterior to the tympanic aperture of the facial canal in URBAC 00−16 and 00−02 is the anteriorly directed hiatus Fallopii. In URBAC 00−16,the hiatus is positioned at the an − terior edge of the tegmen tympani,but in URBC 00−02,the hiatus appears to open on the tympanic surface of the bone followed by a short sulcus. The hiatus is not preserved in all Kulbeckia specimens,however. The area around the tym − panic aperture of the facial canal is too damaged to determine the presence/absence or orientation of the hiatus in URBAC 98−117,but it appears to be absent in ZIN C. 85513 and 85045. In these specimens that appear to lack a hiatus,a small sulcus can be seen leading anteriorly from the tympanic aperture of the facial canal,but this is most likely the sulcus for the inferior ramus of the stapedial artery and the ventral flooring of the aquaeductus Fallopii is broken.

Figures

Fig. 3. Stereophotographs of the petrosal of Kulbeckia kulbecke (URBAC 00−16) in cerebellar (A),squamosal (B),and tympanic views. Scale bar 1 mm. Anterior towards the top in all views.