Metaxytherium, de Christol, 1840
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publication ID |
https://doi.org/ 10.5281/zenodo.5376713 |
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persistent identifier |
https://treatment.plazi.org/id/DD158787-C31A-9604-48CC-D3F7FDC2F9C3 |
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treatment provided by |
Marcus |
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scientific name |
Metaxytherium |
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Metaxytherium cf. krahuletzi Depéret, 1895 ( Figs 2-5 View FIG View FIG View FIG View FIG ; Tables 1-4)
SYNONYMY. — See Domning & Pervesler 2001: 17.
REFERRED NEW SPECIMEN. — One additional specimen: MPNRL-MAN2000 skull, thoracic vertebrae and ribs.
LOCALITY. — La Rochette, Manosque (Provence, France).
AGE AND STRATIGRAPHIC OCCURRENCE. — Early Miocene, late Burdigalian, 16.5-17.5 Ma. “Molasse calcaire et sablo-marneuse” Formation.
DESCRIPTION
Preservation state
The cranium and the mandible are in anatomic connection, so the ventral side of the cranium and the dorsal side of the mandible are not visible. They are deformed, mediolaterally flattened with the right side still partially embedded in the molasse, and the posterior portion incomplete (lacking the right exoccipital and the basioccipital).
The rib cage is also embedded in the hard sandstone matrix; the left ribs are more or less in anatomic position, the right ones are posteriorly inclined, the vertebrae are crushed, lying on the right side, but more or less in anatomic connection.
The exoccipital-supraoccipital and the interpremaxillary sutures completely fused, and the M 3 is completely erupted, which indicates that the specimen was an adult.
The state of preservation and the incomplete preparation do not permit scoring of all the morphological characters used by Domning (1994) and Bajpai & Domning (1997). However, the features visible include those most critical for taxonomic determination in this case; the difficulty in making a specific identification does not result from incomplete preparation.
Premaxilla
The rostrum is incomplete and compressed, but long enough to be supposed enlarged relative to the cranium [c. 3(1?)]. Only the dorsal portion is preserved, but the dorsal keel is damaged so that the dorsal surface of the premaxillary symphysis appears to be concave. The posterior end of the rostrum seems to be upraised to form a slight boss in lateral view [c. 10(1?)]. The lateral edges are thin, not flaring. The tusks and the alveoli are not preserved, but, judging from the development of the rostrum, they were probably small, extending less than half the length of the symphysis [c. 140(0)].
The nasal process is long [c. 7(0)] and thin at its posterior end [c. 6(0)]. It contacts the frontal [c. 9(1)], the lacrimal, and abuts against the nasal, but the sutures with these bones are unclear. The premaxillarymaxillary suture is partially visible and its anterior end is estimated to be about 5 mm posterior to the dorsal tip of the symphysis. The contact with frontal and lacrimal appears to be about 86 mm long.
The mesorostral fossa is long and narrow and the external nares appear to be retracted and enlarged beyond the level of the anterior margin of the orbit [c. 8(1)], but these characters could be accentuated by compression and deformation. A nasal incisure at posterior end of mesorostral fossa is absent [c. 37(0)]. The rostral deflection is uncertain; probably c. 60°.
Nasal
The nasal region is abraded and incomplete, so the dorsal exposure of the nasal cannot be clearly delineated, but processes of the frontals seem to separate the nasals in the midline [c. 31(1?)].
Lacrimal
The lacrimal is about 5 cm long and has a prominent ridge, 3 cm long, on its lower lateral surface. It abuts posterodorsally against the supraorbital process of the frontal and is anteromedially in contact with
Condylobasal length 460e Height of jugal below orbit 56 Length of premaxillary symphysis 165e
Rear of occipital condyles to anterior end of interfrontal suture 284
Top of supraoccipital to ventral sides of occipital condyles 151 Length of frontals, level of tips of supraorbital processes to frontoparietal suture 187 Breadth across supraorbital processes 72 × 2e Breadth of cranium at frontoparietal suture 88 Length of mesorostral fossa 110e Maximum height of rostrum 58e Length of zygomatic process of squamosal 160 Anterior tip of zygomatic process to rear edge of squamosal below mastoid foramen 174 Length of parietals, frontoparietal suture to rear of external occipital protuberance 74
Height of posterior part of cranial portion of squamosal 118 Dorsoventral breadth of zygomatic process 59 Length of jugal 170e Length of frontals in midline 133e Height of supraoccipital 76e Width of supraoccipital 46 × 2e Deflection of masticating surface of rostrum from occlusal plane (degrees) 60° Parietal-supraoccipital angle (degrees) 125°
the jugal. The contact with the maxilla is unclear. A nasolacrimal canal is absent [c. 91(1)].
Frontal
The supraorbital process is well developed, about 2 cm thick dorsoventrally with a long, acute anterior point and a small posterolateral corner [c. 36(1)], flattened in the horizontal plane, with a dorsal surface inclined relatively gently ventrolaterad [c. 43(0)]; not divided by deep dorsoventral grooves indenting its lateral margin [c. 44(0)]. A large orbicular apophysis is present on each side. The lateral crests are not significantly overhanging. The medial wall of the temporal fossa is formed by a thin lamina orbitalis of the frontal [c. 38(0)] and the crista orbitotemporalis is present. The median portion of the frontal roof is convex [c. 42(0)] with a low longitudinal crest. The anterior portion is not well preserved and the contacts with nasal and premaxilla are not visible. The interfrontal sutural surface is fused.
Parietal
The parietal is 74 mm long in the midline and 119 mm long laterally. The interparietal suture is fused. The fronto-parietal suture is concave in shape. The frontal process of the parietal is about 65 mm long. The posterolateral corners of the roof are indented by the squamosals. The roof is nearly rectangular, posteriorly flat and anteriorly slightly concave. The temporal crests are lyriform, low, broad, confined to the lateral edges; minimum external distance between crests 74 mm, minimum internal distance 37 mm (Type B of Domning 1988). A median bump is present just in front of the external occipital protuberance.
Supraoccipital
The right ventral portion of the supraoccipital is lacking. The supraoccipital is hexagonal in outline with rounded dorsolateral corners; it forms an angle of 125° with the posterior part of the parietals. The parietal-supraoccipital suture is fused. The external occipital protuberance is very large and thick anteroposteriorly; it rises above the plane of the parietal roof, and the median ridge below it is present but short, the nuchal crest is distinct at its lateral ends near the squamosals, and the lateral border is thick, rounded and not overhanging. The areas of insertion for semispinalis capitis muscles are flattish, face posterodorsad, and extend about
Total length 302 Anterior tip to front of ascending ramus 207 Anterior tip to rear of mental foramen 83 Length of symphysis 106e Posterior extremity to front of ascending ramus 103 Distance between anterior and posterior ventral extremities 183 Height at condyle 226 Height at deflection point of horizontal ramus 133 Deflection point to rear of alveolar row 78e Minimum anteroposterior breadth of ascending ramus 95 Front of ascending ramus to rear of mental foramen 131 Top of ventral curvature of horizontal ramus to line connecting ventral extremities 48 Minimum dorsoventral breadth of horizontal ramus 82 Length of alveolar row 60e Deflection of symphyseal surface from occlusal plane (degrees) 64° M 3 Crown height 15 Crown length 19
halfway to the ventral end of the bone. The lower part of the supraoccipital is convex laterally below the semispinalis insertions. The sutural surfaces for the exoccipitals are not distinct. The ratio of width to height is c. 1.21.
Exoccipitals
Only the dorsal portion on the left side of exoccipital is preserved. The bone appears to be fused with the supraoccipital. The dorsolateral border is rounded, smooth and not flange-like [c. 70(0)]. The supracondylar fossa is deep and extends across the entire width of the occipital condyle [c. 67(2)].
Maxilla
Only a small dorsal portion of maxilla is visible laterally. The ventral side and the zygomatic-orbital bridge are concealed by matrix. The infraorbital foramen is not visible in anterior view because of the compression.
Squamosal
The squamosal is dorsally in contact with the squared posterior part of the parietal roof [c. 76(1)]. The sigmoid ridge is prominent [c. 74(0)], dorsally slightly sharp and ventrally more rounded. The surface of the cranial portion dorsal to the zygomatic root is nearly flat. The postglenoid process and the postarticular fossa are distinct and well developed. The processus retroversus is present, moderately inflected [c. 77(1)], and does not project below the line of the suture with the jugal; its posterior end is incomplete, but distinct dorsal and ventral protuberances are present. The temporal condyle is not visible. The zygomatic process is 160 mm long, narrow throughout its length [new character]. The posterodorsal edge of the process is almost straight (slightly concave) in outline and convex laterad. The external auditory meatus is partly obscured, but seems to be about as wide anteroposteriorly as high [c. 82(1?)].
Jugal
The preorbital process is relatively flat and thin [c. 88(0)], but with a protuberance on the posterior side. It presumably does not contact the premaxilla [c 87(0?)]. The ventralmost point of the jugal lies clearly under the orbit itself [c. 85(2)], as mentioned by Domning & Pervesler (2001: 43). The ventral tip is thick and rounded; the border behind is smooth. The ventral margin of the orbit is concave, smooth, large but not overhanging [c. 90(0)]. There is a raised postorbital process against which the squamosal abuts. The posterior process is incomplete, but seems to be as long as the diameter of the orbit [c. 89(0?)], tapering rapidly in outline with a longitudinal low sulcus on the lateral side.
Mandible
Left and right dentaries are preserved, but they are mediolaterally flattened. The left is exposed, but the right is enclosed in the matrix. The mandible is in anatomic position and the dorsal extremity of the ascending ramus is therefore covered by the zygomatic arch. The condyle, partially exposed, is elliptic and laterally overhanging. The mandibular notch and coronoid process are not exposed. The base of the anterior border of the coronoid process is exposed and extends slightly anterior to the base of the process [c. 126(1)]. The posterior border is thick and rounded. It has no distinct processus angularis superior, but has a broadly convex outline beginning well below the condyle [c. 125(2)]. Lateral to M 3 there is a shelf that slopes steeply. The horizontal ramus is deep and short [c. 128(1)], with a strongly concave ventral border [c. 122(3)] that is not tangent to the mandibular angle [c. 129(1)]. The ventral border is thickened and rounded anteriorly, tapering posteriorly. Only one large mental foramen is present [c. 123(1)], and it lies at the level of deflection of the dorsal edge of the ramus. The deflection of the masticating surface is abrupt, about 64° (60° according to Domning & Pervesler 2001: 44). The masticating surface is incomplete and mediolaterally compressed. The lateral edge is thin and overhanging. The posteroventral side of the symphysis is concave and the anteroventral side is convex transversely. The symphyseal suture is not well preserved, but appears to be fused. The labial side of M 3 is visible. The distance between M 3 and the coronoid process is about 25 mm.
Vertebrae
The cervical and the most anterior vertebrae are missing.Twelve thoracic vertebrae and a very small fragment representing a thirteenth thoracic vertebra are preserved in anatomic sequence (Ta-m). They are crushed, fragmented and lying on the right side. The centra seem to be collapsed and only the left sides of the neural spines and the left transverse processes are exposed. The neural spines are better preserved than the transverse processes. The first vertebrae are partially overlapping. The overlapping diminishes from the first to the seventh vertebra and the following vertebrae are in anatomic position.
The neural spines are laterally flattened with a slight backward slope. Their full height is around 100 mm. The posterior side of each spine has a rounded protuberance; posteriorly this protuberance gradually becomes confined to the lower part of each spine. The posterolateral edge of each spine is notched just above the postzygapophysis, the notch seems to become less developed from the eighth vertebra. The summit of each spine is expanded (mediolateral width about 20-25 mm; anteroposterior width around 60 mm), flattened, with anterior and posterior corners rounded. The transverse processes are massive and thick. Their mediolateral length is about 25 mm, anteroposterior length about 42 mm, and dorsoventral thickness about 10 mm. They are deformed, but seem to be horizontal and anterior to their neural spines. The zygapophyseal articular planes are nearly horizontal on anterior vertebrae and seem to slope inward beginning with the seventh vertebra.
Ribs
Ten ribs in anatomic sequence (Ra-j) and a small fragment (R?) are preserved on the left side; seven ribs posteriorly inclined are preserved on the right side (Re-j; Ri-l). Their contacts with the vertebrae are not visible. The ribs are massive and thick, smooth with a low ridge (iliocostalis tendon insertion) on the posterior edge; this ridge becomes longer on the posterior ribs. The shafts are elliptical in cross section and maintain their diameters for most of their length. The distal ends are flattened, swept back, and taper in the last 10-20 cm.
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