Petrosal anatomy of Necrolestes
The pars cochlearis and the pars canalicularis are separated by a large breakage, which does not allow us to see the exact contours of the two openings lying in the outer border of the promontorium. Nonetheless, this part of the petrosal is better preserved in the isolated broken left petrosal. The rounded fenestra cochleae (Zeller, 1985) is evident posteriorly. Posterolaterally visible is the oval-shaped fenestra vestibuli, which was in life closed by the footplate of the stapes. The stapedial ratio is 1.5.
The promontorium of Necrolestes is nearly smooth, and except for its anterior pole does not exhibit any sulci for either a stapedial artery or a transpromontorial internal carotid artery together with the carotid plexus. These two features indicate an arterial pathway pattern that is typical for metatherian mammals (see Discussion). In metatherians, the internal carotid generally follows an extrabullar pathway, lying medially to the auditory bulla or near the basioccipital-petrosal suture (Wible, 1986). In Necrolestes, a deep groove at the anterior pole of the promontorium is evident in front of the cochlear housing, which may be for the internal carotid artery. However, this is not the equivalent of a transpromontorial course, which is within the middle ear, but a variant on the extrabullar pathway (Rougier & Wible, 2006).
Projecting anteromedially from the promontorium is a flat shelf of bone, similar to the ‘epitympanic wing’ (sensu MacPhee, 1981, p. 22). This feature is present in many therians but is wholly lacking in extinct non-therian mammals (Rougier et al. 1996, 1998).
The promontorium is laterally bordered by a large bony projection – the posterior part of which is missing – which resembles the eutherian tegmen tympani (MacPhee, 1981, p. 64). This bony flange holds a ventrally directed process that resembles the tuberculum tympani described by Wible (2003) for the opossum Monodelphis brevicaudata and which may be homologous to the eutherian tegmen tympani (Kuhn & Zeller, 1987; see also Maier, 1989; Schmelzle, 2003). The tegmen tympani of Necrolestes ventrally covers a deep sulcus, perhaps the sulcus facialis running posterolaterally to the promontorium. The sulcus facialis received the main or hyomandibular branch of the facial nerve that was transmitted by the secondary facial foramen into the middle-ear space. The tympanic opening of the facial canal (what in this context some authors would call the secondary facial foramen in spite of the lack of the osseous floor referred to below) is situated between the tegmen tympani and the promontorium and consists of a large and almost rounded aperture. As in all extant mammals, the facial nerve leaves the cranial cavity via the internal acoustic meatus, runs beneath the prefacial commissure (Wible, 1990), and enters into the cavum supracochleare by the primary facial foramen (Wible, 1990; Wible & Hopson, 1993). The cavum supracochleare, which encloses the geniculate ganglion of the facial nerve (Gaupp, 1908), is not ventrally closed by an osseous floor in Necrolestes. As a matter of fact, the aperture for the hyomandibular branch of the facial nerve is not differentiated from the aperture for the second branch of the facial nerve, the greater petrosal nerve, which is normally transmitted by the hiatus Fallopii in extant mammals. Therefore, and considering the probable breakage of the floor of the cavum, Necrolestes must have exhibited the following pattern of the facial nerve course: the two branches of the facial nerve leave the posterior aspect of the geniculate ganglion and the main branch or hyomandibular ramus of the facial nerve enters the middle-ear space via the tympanic aperture or secondary facial foramen. There it runs posteroventrally in the sulcus facialis and exits the skull by the stylomastoid notch or foramen (see discussion below). The greater petrosal nerve or palatine ramus of the facial nerve exits via the hiatus Fallopii, which was opened on the tympanic side of petrosal and anterior to the secondary facial foramen. The greater petrosal nerve there runs anteroventrally to the posterior opening of the pterygoid canal.
Posteromedial to the secondary facial foramen and anterolateral to the fenestra vestibuli is a pronounced fossa (fossa muscularis major) that was probably for the origin of the tensor tympani muscle.
On the ventral aspect of the tegmen tympani, a deep fossa is bordered laterally and medially by two distinct crests. This fossa, which is not complete as it lacks the posterior part, most likely housed the mallear-incudal articulation and is thus identified as a part of the epitympanic recess. This fossa is lateral to and at a level with the tympanic aperture for the facial canal. Only its anteriormost part is preserved on the tegmen tympani. This fossa is separated from the facial sulcus by a prominent ridge of bone.
Posteromedial to the epitympanic recess is a great excavation that creates a medial accessory cavity of the epitympanic recess (sensu Archer, 1976; MacPhee, 1981), perhaps a mastoid recess formed by pneumatization. This striking feature of the pars canalicularis is well visible in lateral view (Fig. 1C). This accessory cavity is posterolateral to the articulation between malleus and incus, normally housed within the epitympanic recess, but was nevertheless called ‘medial’ by Archer (1976) and MacPhee (1981) for peramelid marsupials and lorises.
Figure 1. Right petrosal of Necrolestes patagonensis (YPM-PU 15384) in ventral (A), dorsal (B), lateral (C), and medial (D) views. cc, crus commune; cp, crista parotica; ctpp, caudal tympanic process of petrosal; eP, element of Paauw; er, epitympanic recess; fai, foramen acousticum inferius; fas, foramen acousticum superius; fc, fenestra cochleae; fs, facial sulcus; fsa, fossa subarcuata; fv, fenestra vestibuli; iam, internal auditory meatus; lapc, lateral aperture of the prootic canal; mp, mastoid process; pfc, prefacial commissure; pr, promontorium; rtpp, rostral tympanic process of petrosal; sf, stapedius fossa; sff, secondary facial foramen; sica, sulcus for the internal carotid artery; sips, sulcus for the inferior petrosal sinus; smn, stylomastoid notch; sss, sulcus for the sigmoid sinus; th, tympanohyal; tt, tegmen tympani; ttf, tensor tympani fossa; vs, vascular sulcus; 1, probable accessory auditory cavity formed by the inflation of the area containing the mastoid and caudal processes and the large excavation and aperture medial to it; 2, accessory cavity of the epitympanic recess (sensu Archer, 1976; MacPhee, 1981).
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There is no evidence of a fossa for the crus breve of the incus (fossa incudis). It might have been in the missing part of the tegmen tympani, as it is usually situated on the pars canalicularis posterolateral to the promontorium and secondary facial foramen and posterior to the epitympanic recess.
A salient crista parotica posterolaterally borders the deep sulcus facialis and must have formed the medial wall of the fossa incudis. This crest supports a thick and prominent protuberance, the tympanohyal (the ossified proximal segment of Reichert's cartilage), posterior to which is the stylomastoid notch by which the facial nerve probably left the middle ear.
The stylomastoid notch is bounded medially by a ridge running posteromedially, the caudal tympanic process of petrosal (MacPhee, 1981). The latter extends medially from the stylomastoid notch to the jugular foramen (posterior lacerate foramen of Archer, 1976) showing a slight decrease in height. The medial end of the caudal tympanic process of the petrosal probably contacted the paracondylar process of the exoccipital bone.
Dorsal to the caudal tympanic process there is a deep depression, hidden in ventral view. The narrower medial part of this depression is the postpromontorial tympanic sinus, partially covered by the expansion of the caudal tympanic process; its broader and deeper lateral part is the fossa for the stapedius muscle. According to the size and depth of this fossa, the stapedius muscle must have been greatly developed. A prominent process lies anteromedially to the sulcus facialis and projects anteroventrally from the stapedius fossa. This could be interpreted as the ossified element of Paauw (the os quatrum of De Beer, 1937), this element being an ossification within the tendon of the stapedius muscle (Maier, 1987), although preservation of such a structure would be exceptional. The element of Paauw plays a functional role, concentrating and transmitting forces between the stapedius muscle and the stapes (Sánchez-Villagra et al. 2002). On the medial part of the pars canalicularis (Fig. 1D), just medial to the stapedius fossa and postpromontorial sulcus, a striking structure remains difficult to interpret as it has never been observed by the authors in any therian petrosal. This structure consists in a great inflation of the medial side of the caudal tympanic process that contains a large aperture on the medial side of the petrosal. It may represent a surface area for attachment of the stapedius muscle, part of an accessory auditory chamber (MacPhee, 1981), or facial nerve enclosure. If the last interpretation is right, the facial nerve of Necrolestes would have not exited the middle ear via the stylomastoid notch (as described above) but instead via a stylomastoid foramen (as it is the case of some metatherians and most eutherians). Nonetheless, we should also consider the hypothesis that this part of the petrosal is broken, as this specimen lacks the adjacent exoccipital and paroccipital process. The whole area containing the mastoid and caudal processes and the large excavation and aperture medial to it is clearly inflated and may have played the role of an accessory auditory cavity.
Just anterolateral to the ossified element of Paauw is a small foramen that we interpret as the tympanic opening of the prootic canal (contra Asher et al. 2007). This opening corresponds with a lateral one, contained in the accessory cavity of the epitympanic process, and shows the prootic canal was small and horizontal. The opening is quite small; nevertheless it is clear in our 3D reconstruction based on CT (Fig. 2). A tiny sulcus for the lateral head vein runs from the tympanic aperture of the prootic canal to the stylomastoid notch.
Figure 2. 3D reconstruction and 2D slices of the left petrosal of Necrolestes patagonensis (YPM-PU 15384). The petrosal is oriented so that the tympanic aperture of the prootic canal is visible. The cross-sections in the slices show the tympanic opening of the prootic canal. A, anterior; B, bottom; L, left; P, posterior; R, right; T, top; fv, fenestra vestibuli; mp, mastoid process; pr, promontorium; rtpp, rostral tympanic process of petrosal; sf, stapedius fossa; tt, tegmen tympani. Not to scale.
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The surface of the pars canalicularis posterolateral to the caudal tympanic process of the petrosal is the mastoid exposure, the petrosal surface exposed on the occiput. On its anteriormost part the mastoid exhibits a large transverse process, which is likely to be the mastoid process of petrosal, to which the sternocleidomastoideus muscle was attached. On the posteriormost aspect of the mastoid, two tiny sulci are observable but do not correspond to the trajectories of the diploetic vessels.
The dorsal or endocranial view of the petrosal of Necrolestes is poorly preserved; nonetheless, two dominant features of this view are evident (Fig. 1B). Anteromedially, the internal acoustic meatus for the facial and vestibulocochlear nerves lies on the roof of the pars cochlearis. The extent of the prefacial commissure in Necrolestes was underestimated by Asher et al. (2007, Fig. 5B) based on their emphasis of a left petrosal fragment of YPM-PU 15384 that exposed the region around the internal acoustic meatus. In fact, based on our CT reconstruction from the more complete right side (Fig. 1B), the internal acoustic meatus exhibits a broad transverse septum and is bordered laterally by a quite broad prefacial commissure, broader than that of Monodelphis brevicaudata (Wible, 2003; Fig. 7). The floor of the internal acoustic meatus has a rough depression, the medial and lateral extremities of which are larger than the centre.
Posterodorsally, the three semicircular canals (see the description of the inner ear below) constitute the boundary of the non-preserved fossa subarcuata, which accommodated the paraflocculus of the cerebellum. According to the respective positions of the three semicircular canals and to the depth of the space inside the bone, the fossa subarcuata is likely to have been conical and deep. The posterior semicircular canal joins the anterior semicircular canal at the crus commune that forms the posteromedial rim of the fossa subarcuata. The anterior semicircular canal, which can be evidenced by its preserved dorsolateral part and the anterior ampulla, forms the rim for the orifice into the fossa subarcuata.
On the dorsoposterolateral corner of the pars canalicularis, behind the crus commune and posterior semicircular canal, is the narrow groove for the sigmoid sinus. This sinus is the posterodorsal branch of the transverse sinus (the anteroventral branch being the prootic sinus and sphenoparietal emissary vein) (Dom et al. 1970). There is no clear sulcus to link with the prootic sinus pathway; however, a tiny sulcus on the posterolateral corner of the pars canalicularis is likely to be for the transverse sinus. At this point the transverse sinus bifurcates. Its anteroventral branch is the prootic sinus which must have passed through the foramen just anteroventral to the point of bifurcation of the transverse sinus. Anterior to the sigmoid sinus sulcus there is a sulcus running along the lateral border of the fossa subarcuata. This short, shallow sulcus for the superior petrosal sinus appears to enter a tiny foramen at the anterior rim of the fossa subarcuata.
A broad sulcus running along the dorsomedial edge of the promontorium received the inferior petrosal sinus, which originated at the cavernous sinus around the hypophysis and left the skull via its own foramen or via the jugular foramen (see Wible, 2003).
We were not able to reconstruct the aqueductus vestibuli for passage of the endolymphatic duct. The aqueductus cochleae for passage of the perilymphatic duct is posteromedial to the internal acoustic meatus, but hidden by a bony bar behind the foramen acusticum inferius in dorsal view. The aqueductus cochleae opens into the jugular foramen, which transmitted cranial nerves (presumably the glossopharyngeal, vagus, and accessory nerves as in didelphids; Wible, 2003), and occasionally also a very small branch of the sigmoid sinus to the internal jugular vein. In certain metatherians (e.g. Monodelphis, Didelphis, Dasyurus; Wible, 2003; Pucadelphys; Marshall & de Muizon, 1995) the jugular foramen has a separate foramen for the inferior petrosal sinus anterior to it.
On the ventroposterior part of the pars canalicularis there is a tiny sulcus that runs mediolaterally (Fig. 1D). This sulcus is difficult to identify but it is not for the diploetic vessels, as it has no connection with the pathways of the prootic sinus or sphenoparietal emissary vein.