Alcidedorbignya inopinata Muizon & Marshall, 1987

Muizon, Christian de, Billet, Guillaume, Argot, Christine, Ladevèze, Sandrine & Goussard, Florent, 2015, Alcidedorbignya inopinata, a basal pantodont (Placentalia, Mammalia) from the early Palaeocene of Bolivia: anatomy, phylogeny and palaeobiology, Geodiversitas 37 (4), pp. 397-634 : 410-412

Alcidedorbignya inopinata Muizon & Marshall, 1987

Alcidedorbignya inopinata Muizon & Marshall, 1987c: 205 .

HOLOTYPE. — A partial maxilla with P4-M3 (YPFB Pal 6121).

HYPODIGM. — The hypodigm is that given in Muizon & Marshall (1992), to which we add the following specimens: MHNC 8372, an almost complete skeleton, which is missing only the left upper incisors, the two i1s, a few phalanges and some carpal and tarsal bones and a few caudal vertebrae; MHNC 8373, a partial skull with complete braincase and mandibles of a very young individual, possibly a near-term foetus or a new-born individual (some postcranial elements are preserved); MHNC 8399, a partial skull with the braincase and basicranium perfectly preserved; the rostrum has been partly weathered and part of the left maxilla and the dorsal bones of the rostrum are missing; the right maxilla and premaxilla and the dentaries are almost complete; the specimen also includes half of the atlas; MHNC 8416, a partial skull (with crushed basicranium but the two petrosals well-preserved) and mandibles of a juvenile individual, with a partial right limb, and some ribs; MHNC 8423 a partial skull and mandibles of a juvenile missing part of the occipital region and bearing right dC, erupting P1s, DP2s-DP4s, erupting M1s; di2s, right di3, dcs, p1s in crypt, dp2s-dp4s, and erupting m1s; MHNC 8400, a left maxilla (with C-M3) with jugal and lacrimal; MHNC 8401 a right maxilla with roots of C-P2 and P3-M3; MHNC 8402, associated left maxilla and dentary, with P1-M3 and p3-m3; MHNC 8403, associated right maxilla and dentary, with C-M3 and right i2-trigonid of m3 and left i1-C; MHNC 8404, left maxilla with P3-M3; MHNC 8405, left maxilla with root of canine and P1-M3; MHNC 8406 left maxilla with root of P2 and P3-M3; MHNC 8407, left maxilla with roots of P4, partial M1 and M2-M3; MHNC 8408, maxilla with P4-M1 and partial M2; MHNC 8409, right dentary with c-m3, MHNC 8410, fragment of left dentary with trigonid of m2 and m3; MHNC 8411, fragment of right dentary with m2-m3; MHNC 8412, fragment of right dentary with m2-m3; MHNC 8413, fragment of left dentary with talonid of m2 and m3; MHNC 8414, left dentary with roots of p3-m1 and weathered m2-m3; MHNC 8415, m3; MHNC 8359; right periotic; MHNC 8360, left periotic; MHNC 8361, right periotic; MHNC 8362, left periotic; MHNC 8419, right periotic; MHNC 8420, right periotic; MHNC 8421, left periotic; MHNC 8422, right periotic (pars cochlearis); MHNC 8423, a sub-complete skull and mandibles of a juvenile, missing most of the occipital and the premaxillae.

GEOLOGICAL SETTING AND AGE. — The Tiupampa mammals occur in the middle section of the Santa Lucia Formation ( Marshall et al. 1997). This section has been referred to the early Palaeocene by Gayet et al. (1992), Bonaparte et al. (1993), and Muizon (1998). However, an early late Palaeocene age has been suggested by Marshall et al. (1997) and Pascual & Ortiz-Jaureguizar (2007). Recently, Gelfo et al. (2009) have reassessed the age of the Tiupampa mammal fauna and concluded that it is of early Palaeocene age. The Tiupampa mammal-bearing beds are regarded by these authors as a probable equivalent of the late Puercan (Pu3) of North America and would therefore be as old as 64 to 64.5 Ma. This hypothesis is based on the fact that the Tiupampa mammal-bearing beds are included in a single reversed stratigraphic series, which is likely to correspond to Chron 28r (see Gelfo et al. 2009 for discussion). However, when Gelfo et al. (2009) wrote their paper, the reference for calibration of Chron 28r was Lofgren et al. (2004), and the correlation was with the late Puercan (Pu3). New dates and calibration of the NALMAs have slightly modified the age and position of Chron 28r. According to Wilson (2013, 2014) and Sprain et al. (2014) Chron 28r is totally included in the base of the early Torrejonian (To1) and its absolute age is bracketed between c. 65 Ma and 64.866. This represents the very base of the early Torrejonian (To1), which spans from 65.118 Ma to c. 63.5 Ma. In other words, the absolute age for the Tiupampa fauna is c. 65 Ma.

EMENDED DIAGNOSIS

This following extensive diagnosis presents a list of the major morphological features of the skeleton and teeth of Alcidedorbignya inopinata and represents, in fact, a summarized description. Below a differential diagnosis is also provided.

General morphology

Small size by pantodonts standards; tail, long, 70% of the head and body length.

Teeth

Dental formula I3/3, C1/1, P4/4, M3/3; incisors spatulate; I1 distinctly smaller than I2-3; contralateral I1s widely separated from each other; i1 <i2> i3; C and c large, protruding, splayed labially; P1-2 and p1-2 single rooted and proodont (i.e. single elevated cusp); P3-4 and p3-4 semimolariform; P3-4 with strongly V-shaped paracone and protocone (double V-shaped); M1-3 with distinct pre- and postcingula; paraconule well developed; metaconule weak to indistinct; protocone anteroposteriorly broad; paracone and metacone separated at base (i.e. not connate); M1 with paracone and metacone subequal in size; M2 with paracone slightly higher than metacone; M3 with paracone distinctly higher and much more voluminous than metacone; M1-2 with moderately dilambdodont ectoloph (i.e. slightly V-shaped centrocrista); well-developed wing-like preparacrista (including parastyle) and postmetacrista (including metastyle); well-developed stylar shelf approximately 30% of tooth width on M2; no mesostyle; M3 with wing-like preparacrista (including parastyle) and virtually no postmetacrista; lower molars increase in size posteriorly with m1 distinctly smaller than m2; trigonids wider than talonids; protoconid and metaconid subequal in size, paraconid smaller; antero- and posterocingulid weakly developed; talonid basined; m3 talonid elongate.

Skull

Dome-shaped skull in lateral view, with the nasals sloping anteriorly; greatest width of the skull at the level of the posterior root of the zygomatic arch, slightly anterior to the level of the anterior edge of the dentary-squamosal articulation; rostrum approximately twice as wide at base (level of the anterior edge of the orbits) as at apex; in dorsal view nasals distinctly wider posteriorly; frontal-parietal suture on sagittal plane located at mid-length of the skull; numerous foramina for temporal rami on the parietal and at the parietal-squamosal suture; small sagittal crest; bilobate nuchal crest strongly protruding posteriorly; palate narrow (less than two M2 widths); long parallel-sided basipharyngeal canal (approximately 20% of the total length of the skull); large triangular ectopterygoid process of the alisphenoid; small posterior opening of the alisphenoid canal opening just anterior to the foramen ovale; anterior end of the canal opening internally just posterior to the foramen rotundum; foramen rotundum and sphenorbital fissure separated but adjacent one to the other and opening anteriorly in the same common fossa; large ascending process of the palatine in the orbit; large descending process of the frontal posterior to the palatine process; maxilla does not contribute to the lateral wall of the skull; two large diploic frontal foramina above the orbit; double lacrimal foramen; large wing of the lacrimal on face; jugal-maxilla suture bifurcated; jugal-lacrimal contact; zygomatic arch relatively slender; postglenoid process narrow and elevated (more elevated than wide), approximately half the transverse width of the glenoid cavity; postglenoid foramen on the medial edge of postglenoid process; presence of a tubercle (postglenoid eminence) on the posteromedial edge of the foramen; relatively flat promontorium with a distinct sigmoid groove for the internal carotid artery; promontorium totally separated from the alisphenoid, basioccipital and basisphenoid; deep notch for the internal carotid artery in the posterolateral angle of the basisphenoid; apex of the promontorium truncated; epitympanic recess opened ventrally and formed by the squamosal laterally and the petrosal medially; large foramen for the ramus superior of the stapedial artery opening in the petrosal; triangular post-promontorial tympanic sinus posterior to the external aperture of the cochlear fossula; spur-like medial caudal tympanic process on posterior angle of postpromontorial tympanic sinus; conical subarcuate fossa; vestigial anterior lamina of the petrosal; presence of an intracranial cavity (the petrosquamosal fossa) for the passage of the ramus superior of the stapedial artery and its tributary vessels and, capsuloparietal emissary vein and tributaries; long and deep groove on cerebral face of the braincase for the orbitotemporal artery and vein; large mastoid exposure on occiput; large post-temporal foramen in the squamosal petrosal suture; ectotympanic horseshoe-shaped, with distinct styliform process ventrally; anterior crus of the ectotympanic with a double articulation with squamosal laterally and petrosal medially.

Dentary

Stout body with a roughly constant height; symphysis fused in adults; ramus triangular with a large masseteric fossa, coronoid process recurved posteriorly and almost as wide at apex as at base; strong and salient coronoid crest at anterior border of masseteric fossa; condyloid process well above occlusal plane; angular process extends posterior to the level of the condyle; angular process hook-like but robust and almost as long as high; anteroposteriorly oriented crest on medial face of the angular process for the insertion of the pterygoideus medialis.

Postcranial skeleton

Axis with anteroposteriorly elongated neural process; 13 thoracic vertebrae, 9 lumbar vertebrae, 4 sacral vertebrae, and approximately 18 caudal vertebrae; long neural processes of the thoracic vertebrae; scapula with a large triangular supraspinatus fossa and a narrow and deep infraspinatus fossa; spine elevated; acromion anteriorly and distally projected; humerus with greater tubercle slightly higher than the head; deltopectoral crest limited to proximal half of the bone; very robust, long, and medially projecting medial epicondyle; well-developed epicondylar ridge; medial crest of the humeral trochlea projecting more distally than the lateral; ulna bent anteriorly and medially; deep fossa for the flexor muscles on medial face of olecranon and diaphysis; trochlear notch wide open; angle between humeral and radial facets (in anterodistal view) obtuse; radius with ovalshaped proximal epiphysis; centrale present; five anterior digits with short metacarpals; no size reduction of any digit; ungual phalanges with robust palmar tubercle; innominate with a distinctly everted ilium and a ventrolaterally deflected anteroventral spine; acetabulum relatively shallow and open; femur with a spherical head; greater trochanter as high as the head; lesser trochanter triangular, blade-like, and posteromedially oriented; small third trochanter; distal epiphysis as wide as deep; tibia with a salient tibial crest and tubercle; fibula with a well-developed interosseous crest on its proximal half; tibial trochlea on the astragalus relatively flat and angle between the medial and lateral tibial facets more than 120°; angle between lateral tibial facet and fibular facet close to 90°; astragalus with a distinct cuboid facet on the head; calcaneofibular facet present on the calcaneus lateral to ectal facet; well-developed peroneal process of the calcaneus; calcaneus with a strongly oblique (in dorsal view) cuboid facet, which faces distolaterally; presence of a supplementary articular facet for the astragalus on the medial edge of the cuboid facet; five pedal digits slightly longer than the manual ones, ungual phalanges cleft, with distal half expanded transversely (scutiform) in dorsal view and deflected palamarly or plantarly in lateral view, with robust plantar tubercle.

DIFFERENTIAL DIAGNOSIS

Alcidedorbignya inopinata is similar in size to Harpyodus euros and H. decorus but it is much smaller than all the other Pantodonta .

Alcidedorbignya inopinata differs from all the other Pantodonta in having single rooted P2 and p2, and a hook-like angular process of the dentary.

Alcidedorbignya inopinata differs from all the other Pantodonta except Bemalambda in having a distinct (although short) neck of the astragalus.

It further differs from Pantolambda bathmodon in the following characters: size approximately 60% (not 40% as stated by Muizon & Marshall 1992) smaller, lack of a mesostyle on the upper molars, broader stylar shelf, posteriorly protruding bilobate nuchal crest, a horse-shoe shaped ectotympanic (not expanded medially as in Pantolambda ), more gracile limb bones, ulna distinctly bent anteriorly and medially, obtuse angle between the anterior edge of the radial and trochlear notches of the ulna (acute angle in Pantolambda ), much less developed and less everted anteroventral iliac spine, medial condyle of the femur similar in size to the lateral one (i.e. not projecting posteriorly), presence of a well-developed interosseous crest on the medial edge of the fibula, presence of a distinct (although short) neck of the astragalus, cleft pedal ungual phalanges (In pantolambda the anterior pedal phalanges are cleft; the posterior are not), distal half of ungual phalanges deflected palmarly or plantarly.

Alcidedorbignya inopinata further differs from Harpyodus decorus and H. euros in the well-separated paracone and metacone, the V-shaped centrocrista and the much thinner postcingulum, whereas in Harpyodus the cusps are connate at base, the postcingumlum is much thicker and the centrocrista is straight.

Alcidedorbignya inopinata further differs from Bemalambda in the following characters: size approximately 75% smaller, well-separated paracone and metacone at base (not connate), lack of the anteroposterior compression of the upper molars, lack of the great enlargement of the metaconid observed in Bemalambda , much lower sagittal crest, bilobate nuchal crest, lack of a well-developed postorbital process, much lower sagittal crest, dorsoventrally lower dentary in its anterior half, condyloid process located well above the level of the occlusal plane of the molars, much longer neural process of the axis, presence of 13 thoracic, nine lumbar, and four sacral vertebrae (respectively 17T, 8L, and 3S in Bemalambda ), more gracile limb bones, ulna distinctly bent anteriorly and medially, presence of a much less developed and everted anteroventral iliac spine, medial condyle of the femur similar in size to the lateral (i.e. not projecting posteriorly as in Bemalambda ), and presence of a well-developed interosseous crest on the medial edge of the fibula.

NB: some of the above-mentioned morphological differences between Alcidedorbignya and larger pantodonts may be related to allometric phenomenons, which yet remain to be investigated (e.g., robustness of limb bones).