Coloniatherium cilinskii, Rougier & Forasiepi & Hill & Novacek, 2009

Rougier, Guillermo W., Forasiepi, Analía M., Hill, Robert V. & Novacek, Michael, 2009, New mammalian remains from the Late Cretaceous La Colonia Formation, Patagonia, Argentina, Acta Palaeontologica Polonica 54 (2), pp. 195-212 : 197-206

Coloniatherium cilinskii sp. nov.

Figs. 2–6 .

Etymology: After Mr. Juan Cilinski, a local rancher who provided great support to our field efforts while working at El Uruguayo.

Holotype: MPEF−PV 2087, fragmentary right lower jaw with complete p3, root fragments of p2, m1–m3, and alveoli for p1 and double rooted canine ( Fig. 2A ).

Type locality: El Uruguayo (43 ° 06’18.8’’ S; 67 ° 41’45.3” W), Estancia El Pino, belonging to the Avila family.

Type horizon: Upper part of La Colonia Formation, Campanian–Maastrichtian, Upper Cretaceous.

Material.—MPEF−PV 2085, fragmentary right lower jaw with very worn remnants of m1–m 3 in situ, and alveolus for p3 ( Fig. 2B ); MPEF−PV 2104, fragmentary lower jaw with worn down p1 and p2; MPEF−PV 2192, fragmentary jaw with partial canine; MPEF−PV 2070 left C; MPEF−PV 2223 P1; MPEF−PV 2066 P1; MPEF−PV 2088 right P2; MPEF−PV 2100 right P2; MPEF−PV 2103 left P2; MPEF−PV 2081 right P3; MPEF−PV 2079 left P3; MPEF−PV 2142 right P3; MPEF−PV 2260 right M1; MPEF−PV 2078 left M1 ( Fig. 3A); MPEF−PV 2301 right M1; MPEF−PV 2183 left M2; MPEF−PV 2163 right M3; MPEF−PV 2300 right M3; MPEF−PV 2059 right p2; MPEF−PV 2090 right p2; MPEF−PV 2148 right p2; MPEF−PV 2078 right p3; MPEF−PV 2073 left p3; MPEF−PV 2064 left m1; MPEF−PV 2011 left m1; MPEF−PV 2063 right m2; MPEF−PV 2299 right m2; MPEF−PV 2091 left m2 ( Fig. 3B); MPEF−PV 2092 left m3; MPEF−PV 2137 left m3; MPEF−PV 2203 left m3.

Diagnosis.—Large mesungulatid with a dental formula of I?C1P3M3. Length of the postcanine dental series 230 mm (average of three specimens); p1 small and implanted masseteric fossa mental foramina obliquely without diastema behind a double rooted canine; p2 with a broad posterior cingulid absent mesially; m1 possessing inflated crown with small mesial and distal cingulids and a variable number of small roots between the two principal ones; p3 trigonid proportionately smaller than in more posterior molars. Differs from Mesungulatum by its larger size, more bunoid appearance, proportionately broader molars, and a greater posterior reduction of the lower molars (assuming MACN−RN 6 represents m1–m2). The cingula of the upper and lower molars are broader, and more elevated into the crown; the cingular cusps are not as well−defined as in Mesungulatum . The lingual crest of the paracone that would contribute to the “Medianer Grat”, very distinct in Mesungulatum , is less conspicuous in Coloniatherium . The mesial cingulum in Coloniatherium extends to the base of the parastyle, while it lies mesial to the parastyle in Mesungulatum . The parastyle and metastyle are similar in size in Coloniatherium , while the parastyle is proportionately higher in Mesungulatum ; the stylocone is relatively higher in Coloniatherium . In some specimens of Coloniatherium the upper molars have a small labial rugosity or cingulum, which is absent in Mesungulatum ( Fig. 3A). The M3 is more mesiodistally compressed in the taxon from La Colonia than in that from Los Alamitos, with the stylocone more distally located directly connected to the metacrista.

Attributed specimens.—MPEF−PV 600, an incomplete right petrosal missing the back half of the promontorium exposing the broken cochlea and part of the vestibular apparatus ( Figs. 4 , 5 ); MPEF−PV 2130, a fragmentary right petrosal, missing most of the pars canalicularis and having the promontorium broken open; MPEF−PV 2273, an incomplete left petrosal missing the postpromontorial portion but well preserved otherwise ( Fig. 6 ); MPEF−PV 2280, a heavily weathered and fragmentary right petrosal; MPEF−PV 2281, an incomplete and moderately worn right petrosal, broken to expose cochlear and vestibular cavities. All the above mentioned specimens are referred to Coloniatherium cilinskii .

Locality and age of attributed specimens.—All petrosals were collected in La Colonia Formation. MPEF−PV 600 comes from stake 002 Estancia Baibián (in the vicinity of 43 ° 00’25.4” S; 67 ° 38’02.5” W), left margin of the Mirasol Chico Canyon; MPEF−PV 2130, MPEF−PV 2281, and MPEF−PV 2273 come from El Uruguayo Quarry, and MPEF−PV 2280 comes from the locality “Ulna” also on the left margin of the Mirasol Chico Canyon (38 ° 52’27.0” S; 69 ° 28’21.7” W).

Mandibular and dental description.—The jaws of Coloniatherium are very robust and proportionately short. The ventral border of the dentary forms a smooth convex curve. The buccal side of the alveolar border is much lower than the lingual side; this feature, related to the lingual shift of the posterior molar positions, results in a large retromolar space. In lateral view, the edges of the alveoli converge ventrally from both the front and back of the jaw towards the p2–m1 area. Two large mental foramina are located below the roots of the p2. The more anterior of these foramina is larger and determines a deep groove extending mesially to the level of the canine. The coronoid process is incomplete in all the jaws but their broken bases suggest that the coronoid ridge was close to vertical, forming almost a right angle with the alveolar process, a feature also seen in the close relatives Peligrotherium and Reigitherium . The masseteric fossa is delimited by a sharp coronoid crest and a more rounded ventral edge. A small masseteric foramen is present in some of the specimens, but absent in others. On the medial aspect of the dentary there is no clear evidence of either a coronoid bone or a Meckelian groove. The symphysis is well−preserved in MPEF−PV 2087; it is oval, relatively smooth, and extends posteriorly to the level of the anterior root of p2. A small genial tubercle is recognized immediately behind the symphysis.

The dental formula as presented in the diagnosis is obtained from several specimens. The total count of incisors is unknown but the likely number is 2 or 3 (based on MPEF−PV 2087), with the two more distal incisors somewhat transversely set. The lower canine is large, birradiculated and procumbent, and less bulging posteriorly than the upper ca−

ROUGIER ET AL.—LATE CRETACEOUS MAMMALS FROM ARGENTINA 199

1 mm mesial lingual mesial lingual

nine. The p1 is small, double rooted and obliquely inset in the jaw, but as preserved in MPEF−PV 2104 the crown characters are obliterated by wear. The p2 is the largest lower tooth, and has a molar−like appearance. This tooth has two stout roots, the mesial one anteriorly directed and the distal one vertical. The crown is dominated by a centrally−located protoconid. A large and distinct anterior accessory cusp (paraconid) and the rudiments of a cingulid give the mesial half of the tooth a triangular aspect. The lingual metaconid is low and small, connected to the protoconid by a blunt crest that is rapidly removed by wear; the trigonid angle is approximately 140 °. The distal half of the tooth is broad and dominated by a transverse posterior cingulum that culminates on a distinct labial cusp. The p3 is a large, bulbous tooth, with a poorly developed trigonid located centrally and two broad and expansive mesial and distal cingula. Typically the p3 has three or more roots; the smaller ones are located lingually, between the lager mesiolabial and distal roots. The larger roots are mesiodistally compressed. The molars ( Fig. 3B) uniformly diminish in size and all have a similar pattern to the p3, retaining the compressed mesial and distal roots, but without accessory roots. In the molars (m1–m3) the trigonid is formed by a tall protoconid located on the distolingual corner of the tooth, a distinct, anteriorly displaced paraconid, and a large metaconid aligned transversely with the protoconid and lingually with the paraconid. From the mesial and distolingual edges of the protoconid descend blunt, well−developed cristae: the paracristid and metacristid respectively, forming an angle of 45–50 ° to each other. In some specimens, a feeble crest extends labially from the labial slope of the metaconid towards the protoconid subdividing the basin of the trigonid. Mesial and distal cingula are broad, well developed, and relatively higher in the crown (elevated cingula), less distant from the occlusal surfaces than in Mesungulatum . Cingular cusps (cusps e, f, and d) are not present, unless the cuspules present in both mesial and distal cingulids are regarded as homologous of such structures. The cuspules in the cingula are not as distinct as those in Mesungulatum , and there is not a cristid obliqua as in Los Alamitos mesungulatids ( Bonaparte 1986b, 1987, 1990, 1994), differing from some Laurasian dryolestoids ( Prothero 1981). The m3 is a much reduced and simplified version of the mesial molars.

The upper incisor count is unknown. The upper canine is taller and more robust than the lower, with a prominent bulge of the base of the posterior root determining a sigmoid posterior outline, similar to the condition seen in Vincelestes and Peligrotherium . Three upper premolars are recognized based on morphology. The P1 is relatively small, double−rooted, with a mesially located paracone, and two distinct but small cingular cusps. The posterior cingular cusp is connected to the paracone by a crest that descends posteriorly from the tip of the paracone. Midway along this crest, and over the distal slope of the paracone, there is typically a conspicuous cusp or swelling but this is effaced by wear in many specimens. The posterior accessory cusp is also contacted by buccal and lingual cingula.

The P2 is a large, complex tooth with a small, cylindrical mesial root and a large, transverse, distal root. The crown is dominated by a centrally located, conical paracone. The paracone is loosely connected to the metastyle by a crest. A large, broad cingulum occupies the distal margin of the tooth, which culminates lingually in a thickening, or cuspule, similar to the condition seen in the molars. A small parastyle determines a high platform, which is supported by the small anterior root, hanging from the mesial slope of the massive paracone. The parastyle is connected to the lingual and buccal aspects of the paracone by distinct crests.

cochlear tensor cochlear canal tympani cochlear canal basal turn fossa cupula modiolus basal turn tractus foraminosus promontory goove anterior and lateral ampullae recesses hiatus Fallopii lateral flange secondary facial foramen facial sulcus prootic canal groove for the ramus superior of the stapedial artery

CN VIII vestibule aqueductus cochleae jugular notch aqueductus vestibuli crus commune sacculus posterior semicircular canal lateral semicircular canal stapedius fossa epitympanic recess paraoccipital caudal tympanic process process of petrosal

The last upper premolar (P3) is molarized; the trigons of the four last postcanines (P3–M1–3) resemble an isosceles triangle, wider labiolingually, and formed by the lingual paracone and the labial stylar cusps ( Fig. 3A). The overall shape of the crowns is subrectangular because of the more pronounced lingual development of the cingula and cingular cusps, which diminish in size posteriorly. As in the lower dentition, the P3 is massive and bunoid with a proportionately small trigon. The M1 is a nearly symmetrical molar dominated by a large paracone and stylocone that divide the tooth in mesial and distal halves ( Fig. 3A). The paracone is connected to the parastyle by a broad paracrista. The parastyle is a robust conical cusp that connects to the labial end of the broad mesial cingulum. A second large crest, the metacrista, rises labiodistally from the paracone and ends on the buccal margin of the tooth without contacting a procochlear branch of CN VIII prootic subarcuate fossa portionately large metastyle. The metastyle is conical and in close contact with the stylocone. No metacone swelling or cusp is present, a condition similar to that observed in certain mesungulatids (e.g., Mesungulatum, Bonaparte 1986b , 1987), but differing from the Laurasian dryolestoids ( Simpson 1927; Prothero 1981). Both mesial and distal cingula are broad and contribute a semi−quadrangular occlusal outline to the molar. Both cingula end lingually in a swelling, but do not form a cusp as seen in Mesungulatum . The stylocone is high; the lingual slope of this cusp bears a distinct mesocrista (Medianer Grat; Martin 1999) that does not reach the buccal aspect of the paracone. There is a variously developed lingual cingulum (e.g., MPEF−PV 2065, 2301, 2078, Fig. 3A) between the parastyle and the buccal surface of the metastyle, indicating what we believe is intraspecific variability. In the M2−M3, the metastylar portion of the molars is reduced (more so in the M3) to accommodate a sharp inturned distal portion of the alveolar line, a feature also seen in Mesungulatum (MACN−RN 05, Bonaparte 1986b), and therefore both M2 and M3 acquire the typical “last molar morphology”. The M3 lacks a metastyle, and the metacrista connects the stylocone, which in turn is relatively lower than the anterior molars (only slightly taller than the parastyle).

Justification of postcanine identification.—Among Recent mammals identification of premolar/molars is based on the observation of a developmental process, the replacement of a tooth by another, or the lack of such event ( Clemens and Lillegraven 1986). In fossils this information is seldom available ( Kielan−Jaworowska et al. 2004) and the specimens described here are no exception. Morphology is traditionally used as a proxy to determine the boundary between premolar and molars. In the case of Coloniatherium and all the other known Mesungulatidae and Mesungulatum −like taxa ( Reigitherium and Peligrotherium ), the identification of the boundary between premolars and molars is challenging. The first two postcanines in Coloniatherium are simple and semi−trenchant teeth; we feel quite comfortable calling them premolars. The third element, however, has all three of the main cusps forming a small trigon/trigonid high up in the crown, the tooth is the largest of the postcanine series, and is very bulbous and massive. This is unlike the three following teeth, which are not bulbous, except for the somewhat bunodont cusps, and subequal to each other in morphology. The P/p2 and the teeth here identified as P/p3 have supernumerary roots not known in the more posterior elements. Additionally, the roots of the premolars (P/p1–P/p3) are of cylindrical or oblong section while those of the last three postcanines are mesiodistally compressed and broad buccolingually. Despite the molarized nature of the third postcanine, we believe the differences outlined above suggest a tooth formula of P/p3–M/m3.

Further support for this interpretation is provided by the closely related Peligrotherium , which has a similar number of teeth and morphology, but because some of the specimens are very well preserved it is possible to note that the third postcanine is far less worn than the fourth. This, in turn, indicates that the third postcanine erupted after the fourth, a pattern consistent with replacement of the third position ( Rougier and Paez 2007; Paez 2008). Taking everything into consideration, we believe a postcanine formula of P/p3–M/m3 is the best supported.

Petrosal description.—All of the petrosals are heavily weathered and broken. The exposure to watery media has resulted in a polished surface patina on the fossils that can usually be distinguished from the relatively unpolished surfaces of recent breaks. In some instances, however, all the breakpromontorium tensor tympani fossa promontory groove hiatus Fallopii jugular notch fenestra cochleae deep pocket groove for stapedial artery groove for ramus inferior and post-trigeminal vein secondary facial foramen lateral flange prootic canal groove for the ramus superior groove for lateral head vein posterior semicircular canal stapedius fossa fenestra vestibuli age can be repolished by the elements and rendered indistinguishable from the normal bone surface. This introduces a degree of uncertainty on the interpretation of the petrosal morphology. Because MPEF−PV 600 is one of the better preserved petrosals, the description is mostly based on this specimen and completed with information provided by the other specimens.

The petrosals are large by comparison to those of other Mesozoic mammals (MPEF−PV 600: 12.6 mm long in ventral view; Figs. 4 , 5 ; perhaps a little longer when complete). The skull length is estimated at 87.5 mm based on the proportions of the prototribosphenidan Vincelestes ( Bonaparte and Rougier 1987b; Rougier et al. 1992); 161.5 mm based on Didelphis ; or 148.3 mm based on Leptictis . These differences in estimated skull length (based alternatively on comparisons with Vincelestes or therian mammals) reflect the progressive proportional reduction of petrosal length during mammaliaform evolution ( Rougier et al. 1996a; Rougier and Wible 2006). The smaller predicted skull length (based on Vincelestes proportions) agrees closely with the predicted skull length based on the jaws of Coloniatherium .

Mammalian petrosals comprise two major portions; the pars cochlearis, housing the auditory portion of the inner ear, and the pars canalicularis, housing the organs of equilibrium ( Klaauw 1931; MacIntyre 1972). In most Mesozoic mammals the pars cochlearis comprises (in addition to the cochlea and its bony enclosure) the promontorium and other posterior and lateral structures such as the rostral tympanic process, lateral trough, facial canal, and lateral flange ( Rougier and Wible 2006). The pars canalicularis is represented by the semicircular canals and the vestibule (in life occupied by the utriculus and sacculus), which would have been filled with endolymph and connected to the endocranial space (see below). The osteological features of the pars canalicularis are dominated by the subarcuate fossa which is circumscribed by the semicircular canals. The anterior semicircular canal forms the rim of the fossa, the lateral canal is located in the floor of the fossa. The posterior semicircular canal is located in the medial wall of the subarcuate fossa and the mastoid exposure of the petrosal ( De Beer 1937; MacPhee 1981; Meng and Fox 1995a, b; Wible et al. 2001). The vestibule occupies the central part of the osseous labyrinth, and communicates with the cochlear canal anteriorly and the semicircular canals posteriorly ( Meng and Fox 1995a, b; Wible et al. 2001).

The pars cochlearis is represented in MPEF−PV 600 by a low promontorium, which has been broken open, exposing the coiled cochlea and the vestibule ( Fig. 4 ). The promontorium is almost complete in MPEF−PV 2273 ( Fig. 6 ) except for the presence of one erosive hole developed at the most ventral projection of the promontorium where the bone is the thinnest because of the underlying cochlea. The surface of the promontorium is broad and slightly convex, and projects only moderately ventrally. The anterior pole of the promontorium is flat and has a fossa located anterolaterally that can be interpreted as an attachment site for the tensor tympani muscle. This fossa is shallow, although better pronounced than in therians, and similar to that found in Vincelestes ( Rougier et al. 1992; Rougier et al. 1996a). In the MEPV−PV 2273, lateral to the tensor tympani fossa, there is a broad, shallow sulcus that runs anteroposteriorly from the anterolateral corner of the promontorium to the fenestra vestibuli. This sulcus, the promontorial groove, is better developed anteriorly and fainter in the proximity of the fenestra vestibuli ( Fig. 6 ). The internal carotid (ICA) artery was the likely occupant of this groove, indicating a transpromontorial course for the vessel ( Wible 1986). Medial to the fossa for the tensor tympani, the petrosal is very thick with a deep medial margin, but despite the ossification of the anterior half of the petrosal, a rostral tympanic process is absent. In extant mammals the promontorium closely matches the size of the enclosed cochlea. In the petrosals from La Colonia and in most Mesozoic taxa, the promontorium encloses a proportionally small cochlea leaving a substantial part of the promontorium unoccupied by any inner ear structure. This anterior portion of the promontorium is remarkably stout and developed in the petrosals from La Colonia. The anterior border of the promontorium is eroded in all the specimens, exposing the trabecular structure of the bone. Based on the thickness of the preserved base, however, it is probable that this border protruded anteriorly forming a shelf, as in Vincelestes and some therians ( Rougier et al. 1992; Wible et al. 2001, in press; Wible 2003.

The back portion of the promontorium in MPEF−PV 600 is badly damaged, revealing the structures of the inner ear in ventral view ( Fig. 4 ). The cochlea is broken through the modiolar axis exposing the medial and lateral portions of the basal turn, and the impression of the cochlear cupula ventral to the modiolar axis. What is preserved of the petrosal suggests that the cochlea had at least one and a half turns arranged in a tight spiral. There are no remnants of the primary spiral lamina in the MPEF−PV 600, but in the more complete MPEF−PV 2273, a small portion of the primary spiral lamina can be seen through the fenestra cochleae. In MPEF−PV 600 there is only a faint ridge in the basalmost portion of the cochlea, which reflects the presence of a short secondary spiral lamina. Evidence of this structure in other petrosals (MPEF−PV 2281) is equally poor. In life, the primary and secondary spiral laminae separate the cochlear canal into the scala vestibuli and scala tympani. The space between the laminae contains the basilar membrane that supports the organ of Corti ( MacIntyre 1972; Meng and Fox 1975a, b). The modiolus is obliquely sectioned in MPEF−PV 600 and 2281.

In both specimens numerous minute openings can be clearly seen forming the tractus foraminosus, which would transmit branches of the cochlear nerve (CN VIII) into the primary spiral lamina. The edges forming the dorsal and ventral limits of the tractus foraminosus are smooth and seem to have finished edges. In specimen MPEF−PV 600 ( Fig. 4 ), behind the cochlea there is an elongated, deep depression that constitutes the vestibule. In the medial side of the roof of the vestibule are two openings, the larger of which corresponds to the vestibular opening of the crus commune (formed by the anterior and posterior semicircular canals). The second, tiny foramen, lateral to the crus commune, represents the internal opening of the vestibular aqueduct, which would have transmitted the endolymphatic duct. At the posterolateral corner of the vestibule, the anterior and lateral semicircular canals merge in a single rounded space that likely lodged their respective ampullae. Portions of the posterior and lateral semicircular canals are partially visible in ventral view of MPEF−PV 600 and 2281 due to breakage in the vestibular and postpromontorial area. In MPEF−PV 600 the lateral semicircular canal encircles the stapedial fossa ( Fig. 4 ), however most of its medial extension is damaged, exposing the broken canal. The posterior semicircular canal can be seen in the vicinity of the opening of the lateral semicircular canal as a small foramen directed laterodorsally towards the vestibule. In this area there is an expansion that likely lodged both part of the ampulla of the posterior semicircular canal and the saccular portion of the vestibule. The nearly vertical posterior semicircular canal is clearly visible in the medial wall of the subarcuate fossa of MPEF−PV 2273, opening immediately lateral to the connection of the lateral semicircular canal with the vestibule.

Between the vestibule and the cochlea in MPEF−PV 600 and 2281, there is a narrow sulcus limited posteriorly by a low, somewhat damaged, broad crest. This sulcus is the inner ear opening of the aqueductus cochleae that would have transmitted the perilymphatic duct. The aqueductus cochleae is long, so that the medial opening in the jugular notch is removed from the lateral aperture in the inner ear ( Figs. 4 , 5 ). In medial view, the external opening of the aqueductus cochleae is a small, dorsoventrally compressed fissure located dorsal to the jugular notch. The processus recessus, which floors the aqueductus cochleae ( De Beer 1937; MacPhee 1981), is relatively thin and horizontal. It is less extensively developed than in Recent mammals, because the medial aperture of the perilimphatic duct (in the inner ear) can be seen in posterior view through the fenestra cochleae.

The promontorium of petrosal MPEF−PV 2273 ( Fig. 6 ) is very well preserved and there are three major apertures around its posterior perimeter. From lateral to medial these are the secondary facial foramen, fenestra vestibuli, and fenestra cochleae. The fenestra cochleae is located in the posteromedial corner of the promontorium. It is narrow, elongated, and well−separated from the postpromontorial tympanic sinus by a low crest. It is also intimately connected with a groove that marks the contact between the processus recessus and the tympanic roof. The fenestra would have been, in life, closed by the secondary tympanic membrane, as in all living mammals. The fenestra cochleae sits in a relatively shallow fossula fenestra cochleae. Two weak ridges extend from the posteromedial margin of the fenestra cochleae to the edge of the jugular notch, forming a groove that continues a short distance inside the endocranial surface. The occupant of this groove is unknown.

The morphology seen in the petrosals, in particular that of MPEF−PV 2273, indicates that two structures are connecting the inner ear and the area in the vicinity of the jugular foramen. One is the occupant of the bony canal that we have called here aqueductus cochleae; the other is the occupant of the groove that runs along the processus recessus. To our knowledge, there is no model among Recent mammals for a morphology like this. It is known, however, that among some forms (e.g., Ornithorhynchus ; Zeller 1989, 1991) a small vein accompanies the perilymphatic duct. It is therefore conceivable that the smaller osseous canal carries the vein and the duct is left exposed to the middle ear cavity and the postpromontorial tympanic sinus. If this were to be the case, a true fenestra cochleae would be absent among mesungulatids. Until additional information in support of this possibility is gathered, we will continue to interpret the osseous canal in La Colonia mesungulatids as the aqueductus cochleae, and the fenestra in the rear of the promontorium as a true fenestra cochleae.

Immediately posteromedial to the fenestra cochleae and the processus recessus there is a deep, rounded excavation of the roof of the postpromontorial tympanic sinus (clearly seen in MPEF−PV 2273; Fig. 6 ). The area is partially circumscribed laterally by the crest indicating the posterior semicircular canal, and medially by the thickened edge of the petrosal. This depression is reminiscent of the “deep pocket medial to the paroccipital process” ( Rougier et al. 1996b). In the La Colonia petrosals, this depression seems to be simply an excavated portion of the postpromontorial sinus of the tympanic cavity.

The fenestra vestibuli, which in life accommodates the footplate of the stapes, is elongated with its major axis oriented anterolateral−posteromedial. It opens into a deep and elongated fossula fenestra vestibuli ( Fig. 6 ). The stapedial ratio ( Segall 1970) is approximately 1.5, that is, slightly oval, but with a smaller ratio than that of most eutherians ( Wible et al. 2001). A circular or slightly elliptical fenestra vestibuli, as seen in La Colonia petrosals, is the primitive morphology shared among basal mammals and retained in some therians ( Archibald 1979). The anteromedial margin of the fossula fenestra vestibuli is notched by a shallow sulcus that represents the pre−stapes portion of the stapedial artery. Slightly posterolateral to the fossula fenestra vestibuli, the subtle continuation of the stapedial artery can be observed, directed toward the groove interpreted here as for the ramus superior.

In most Mesozoic groups, a well−developed lateral flange partially delimits the lateral trough. Monotremes are the only Recent mammals that possess a lateral flange, which in turn is continuous dorsally with an intramembranous ossification, the anterior lamina ( Griffiths 1978; Kuhn and Zeller 1987; Hopson and Rougier 1993). In the petrosals MPEF 600 ( Fig. 4 ) and 2273 ( Fig. 6 ) there is a portion of the lateral flange preserved immediately anterior to the groove for the ramus superior of the stapedial artery (see below). This surface is concave and low, projecting ventrally in a less pronounced way than in Vincelestes . The dorsal broken surface of the lateral flange faces dorsolaterally, and therefore it is probable that at least a portion of the lateral flange, when complete, was exposed on lateral view of the skull. We are uncertain if there were dorsal and anterodorsal continuations of this surface resulting in an anterior lamina, but we regard this as likely. A slight roughness suggests a possible broken surface. Regardless of the extension of the connections of a putative anterior lamina, what remains of the lateral flange contributed to form part of the enclosure of the cavum epiptericum and cavum supracochleare.

The pars canalicularis is well preserved in MPEF−PV 2273 ( Fig. 6 ) and 600 ( Fig. 4 ). The facial sulcus is proportionally broad, with the hiatus Fallopii placed at the anterior edge of the petrosal. The hiatus Fallopii is a relatively small aperture and given its position on the anterior margin of the petrosal floor of the cavum epiptericum, it probably transmitted an intracranial palatine branch of the facial nerve. In MPEF−PV 2273 the large secondary facial foramen opens posterolateral to the promontorium. The primary facial foramen and the cavum supracochleare are visible through the somewhat damaged promontorium and secondary facial foramen in MPEF−PV 600 ( Fig. 4 ) and MPEF−PV 2281. The structures associated with the facial nerve are proportionally large, with a spacious cavum supracochleare and large foramina for the nerves branching off from the geniculate ganglion. The floor of the ganglion is extensive; this, in turn, determines that the hiatus Fallopii and the secondary facial foramen are far apart from each other.

Posterolateral to the secondary facial foramen there is a broad, shallow groove that probably housed the ramus superior of the stapedial artery ( Figs. 4 , 6 ). The area around the groove for the ramus superior is somewhat eroded in all the specimens, and despite the fact that the groove is open ventrally in MPEF PV 600 and 2273, it is possible that the artery was enclosed in a canal like that of Vincelestes ( Rougier et al. 1992) , which has been eroded. The groove for the ramus superior connects with another sulcus on the vertical posterior face of the petrosal (MPEF−PV 600 and 2281), which presumably was occupied by the arteria diploëtica magna. Yet another shallow but distinct groove runs anteroposteriorly on the tympanic roof, ventral to the floor of the cavum supracochleare and along the lateral trough. The likely occupants of this groove are the ramus inferior of the stapedial artery and postrigeminal vein (see also below). The position of the stapedial artery is indicated by a faint groove posterolateral to the fossula fenestra vestibuli. This groove and the more substantial one leading to it, anteromedial to the fossula, strongly suggests that the artery perforated the stapes as predicted for Vincelestes , some multituberculates, and some eutherians ( Kielan−Jaworowska et al. 1986; Rougier et al. 1996a, 2002; Wible et al. 2001). The exact position of the bifurcation of the stapedial artery into the rami superior and inferior is not clearly indicated in any of the specimens but likely occurred in the midst of the lateral trough. A similarly developed stapedial−internal carotid system is purported to be primitive for Mammalia ( Wible 1986; Rougier et al. 1992; Wible and Hopson 1995; Rougier and Wible 2006). A large horizontal canal, completely enclosed in the petrosal, runs dorsal to the shelf of bone that roofs the groove for the ramus superior. This canal continues in a deep sulcus on the lateral surface of the petrosal and in a broadly open sulcus running parallel to the facial nerve on the ventral surface. The close association of this canal with the facial nerve and the connection with a large sub−vertical sulcus on the lateral side of the petrosal indicates the presence of a prootic sinus and lateral head vein ( Wible 1990; Wible and Hopson 1995; Rougier and Wible 2006). The canal, therefore, is identified as the prootic canal. The aperture of the prootic canal is relatively large, at least as large as the secondary facial foramen. Anteriorly to the ventral opening of the prootic canal, there is a broad groove that forms the bulk of the lateral trough. This sizable groove enters the prootic canal, indicating the presence of a robust post−trigeminal vein. A post−trigeminal vein is a primitive amniote feature retained among adult extant mammals only in monotremes, but predicted in a variety of fossil mammaliaforms ( Wible and Hopson 1995; Wible et al. 1995; Rougier et al. 1996b; Rougier and Wible 2006).

Behind the groove for the ramus superior of the stapedial artery is the epitympanic recess ( Figs. 4–6 ), the portion of the tympanic cavity dorsal to the incudo−malleolar articulation ( Klaauw 1931). The recess is deep and approximately oval, with raised posterior and medial walls formed by the lateral extension of the damaged paroccipital process and the small, anteriorly directed crista parotica, respectively. A deep pocket on the posteromedial portion of the fossa likely represents the fossa incudis, for the articulation of the crus brevis of the incus ( Klaauw 1931). A low, anteroposteriorly directed crest marks the lateral limit of the fossa incudis.

The caudal tympanic process of the petrosal (CTPP) is somewhat damaged and extends medially from the paroccipital process as a ridge that becomes progressively low ( Fig. 4 ). The CTPP forms the posterolateral wall of the enormous stapedial fossa, which would have housed the stapedius muscle. The stapedial fossa is deep, subcircular, and positioned posteromedial to the fenestra vestibuli. The lateral semicircular canal curves around the stapedial fossa and is exposed in ventral view due to partial loss of the tympanic floor. Medial to the stapedial fossa, there is an extensive surface, slightly concave dorsally. This portion of the petrosal is homologous to the mastoid exposure of the eutherian petrosals described by MacIntyre (1972) and Wible (1990) from the Late Cretaceous Lance Formation, Wyoming. On the medial edge of the Patagonian petrosals, behind the promontorium and in front of the surface for the articulation with the exoccipital, the jugular notch is recognized ( Figs. 4–6 ). Judging from the size of the notch, the jugular foramen was small, a common condition among the petrosals of many Mesozoic cladotheres such as Vincelestes and the “symmetrodont” Khoobur (Wible et al. 1995; Rougier et al. 1996b).

In medial view, the La Colonia petrosals each have a rugose, triangular facet for articulation with the exoccipital. No evidence of an inferior petrosal sinus is found, though the appropriate areas of the petrosal are reasonably well preserved in most specimens. The anterior pole of the promontorium and its medial surface are highly trabeculated and were likely filled by a venous sinus, as in monotremes ( Zeller 1989), that was fed and drained by small foramina that riddle the petrosal. In lateral view, the most distinctive feature is the deep groove for the prootic sinus, which is almost vertical and positioned very posteriorly, behind the posterior edge of the subarcuate fossa. This feature can be clearly seen in MPEF PV 2273 ( Fig. 6 ), 2283, and 600 ( Fig. 5 ). At the level of the lateral flange, the groove for the prootic sinus curves sharply medially into the large, semi−horizontal prootic canal. The similar size of the prootic sinus groove and prootic canal, in addition to the absence of a groove continuing ventrally to the groove of the prootic sinus, suggest that a postglenoid vein was absent in the La Colonia petrosals, as in most Mesozoic groups ( Wible and Hopson 1995; Rougier and Wible 2006).

In endocranial, or dorsal view ( Fig. 5 ), the dominant features are the subarcuate fossa and the internal auditory meatus (IAM). The subarcuate fossa, which lodged the paraflocculus of the cerebellum, is deep, narrow, and more tubular than spherical, in contrast to the usual mammalian condition ( MacIntyre 1972; Wible 1990, 2003; Wible et al. 1995, 2001; Rougier et al. 1996b; Sánchez−Villagra 2002). On the ventromedial edge of the fossa, there is a narrow, elongated foramen interpreted as the aqueductus vestibuli, the internal aperture of the endolymphatic canal. The internal auditory meatus (IAM) is separated from the subarcuate fossa by a broad bony bridge. The morphology of the IAM varies among the specimens; in MPEF−PV 600 the IAM is subequal in size to the subarcuate fossa, it is deep and not very broad ( Fig. 5B ); in MPEF−PV 2273, 2130, and 2281 the IAM is considerably smaller and rounded in shape, even accounting for some erosion due to transport. The internal division inside the IAM of cranial nerve (CN) VII, vestibular branch of CN VIII, and cochlear branch of CN VIII is seen in MPEF PV 600. As preserved, all of these branches leave the IAM through relatively large foramina, but due to the rough preservation, this feature may be artificial. The foramen for the facial nerve leaves the IAM anterolaterally, piercing the thick commissura prefacialis. This structure is best preserved in MPEF−PV 600 ( Fig. 5B ).

The abundant cancellous bone on the medial and anterior pole of the promontorium coalesces in small sized canals and canaliculi in the substance of the petrosal; a particularly large one is exposed along a portion of the medial boundary of the petrosal MPEF−PV 2281 by breakage. This canal is, however, not an inferior petrosal sinus (contra Rougier and Wible 2006: 292) because it lacks an exit on the vicinity of the jugular foramen, but it might have been connected to it via foramina leading from the cancellous promontorium to the sinus.

The petrosals from La Colonia had a very small, subrectangular occipital exposure. Covering the lateral portion of the occipital exposure and forming the lateral wall of the canal for the substantial arteria diploëtica magna was the squamosal (not preserved in any of the available specimens but deduced from articular surfaces). The arteria diploëtica magna was directed anterolaterally as in Vincelestes , and, judging from the size of the grooves, it was of similar size to the ramus superior.

Stratigraphic and geographic range.—Upper Cretaceous La Colonia Formation in Chubut Province, Argentina.

Figures

Fig. 2. Right dentaries of cladotherid mammal Coloniatherium cilinskii gen. et sp. nov., from the Upper Cretaceous La Colonia Formation, Chubut, Argentina. A. Specimen MPEF−PV 2087. B. Specimen MPEF− PV 2085. C. Reconstruction of the dentary based on the two previous specimens. The fracture line posterior to the p3 indicates the junction of the two different elements.

Fig. 4. A. SEM stereophotograph of the fragmentary right petrosal of cladotherid mammal Coloniatherium cilinskii gen. et sp. nov., from the Upper Cretaceous La Colonia Formation, Chubut, Argentina, specimen MPEF−PV 600. B. Diagrammatic outline of the right petrosal of Coloniatherium cilinskii gen. et sp. nov., specimen MPEF−PV 600. Hatching indicates broken bone surfaces; the stipple pattern represents inner ear cavities or canals exposed by breakage of the petrosal.

Fig. 5. Stereophotograph (A) and interpretative diagram (B) of the endocranial view of the fragmentary right petrosal of cladotherid mammal Coloniatherium cilinskii gen. et sp. nov., from the Upper Cretaceous La Colonia Formation, Chubut, Argentina, specimen MPEF−PV 600.

Fig. 6 Stereophotograph (A) and interpretative diagram (B) of the fragmentary left petrosal of cladotherid mammal Coloniatherium cilinskii gen. et sp. nov., from the Upper Cretaceous La Colonia Formation, Chubut, Argentina; specimen MPEF−PV 2273 in tympanic view. Line pattern indicates broken bone surfaces.