Microhyus Teilhard de Chardin, 1927

Tabuce, Rodolphe, Antunes, Miguel Telles, Smith, Richard & Smith, Thierry, 2006, Dental and tarsal morphology of the European Paleocene / Eocene “ condylarth ” mammal Microhyus, Acta Palaeontologica Polonica 51 (1), pp. 37-52 : 39-43

publication ID	https://doi.org/ 10.5281/zenodo.13272043
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scientific name	Microhyus Teilhard de Chardin, 1927
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Microhyus Teilhard de Chardin, 1927 View in CoL

Type species: Microhyus musculus Teilhard de Chardin, 1927 , earliest Ypresian, Dormaal, Belgium.

Microhyus musculus Teilhard de Chardin, 1927 Holotype: IRSNB M115 View Materials , left M1.

Localities: Dormaal (type locality), Belgium; Suffolk Pebble Beds, England;?Tremp, Spain.

Age :?MP6b and MP7 (late Thanetian to early Ypresian, Paleocene– Eocene).

Referred material from Dormaal.—IRSNB M1340 (= DI−2077RS) left p2; DIIC2037RS, right p2; IRSNB M1336, left p4; DIIA1905RS, left p4; IRSNB M1337 (= DIII794RS), left m1; D62, right m2; IRSNB M1338, right m2; IRSNB M1339, left m3; IRSNB M1341, right DP4; D61, left M1 (or M2); DIIA1280RS left?M2; DIIC1934RS, right M3; IRSNB M1342 (= DIIC623RS), right M3.

Measurements.—IRSNB M115, L = 2.4, W = 2.8; DIIC 2037RS, L = 2.1, W = 1.3; IRSNB M1340, L = 1.85, W = 1.2; IRSNB M1336, L = 3.0, W = 2.0; DIIA1905RS, L = 2.75, W = 1.85; IRSNB M1337, L = 2.6, W = 2.2; D62, L = 2.9, W = 2.35; IRSNB M1338, L = 3, W = 2.3; IRSNB M1339, L = 2.7, W = 2.1; IRSNB M1341, L> 2.6, W = 2.55; D61, L = 2.6, W> 2.7; DIIA1280RS, L = 2.55, W = 3.0; DIIC1934RS, L = 2.6, W = 2.3; IRSNB M1342, L = 2.55, W = 2.45.

Revised diagnosis.— M. musculus differs from M. reisi by its more exodaenodont p4; both the protoconid and hypoconid of p4 are sharper, forming a blade−like structure. On p4 to m3, M. musculus shows an ectostylid. On m1–m2, M. musculus differs from M. reisi by more pronounced exodaenodonty, a shorter cristid obliqua, the cusps are more bulbous and closely appressed to one another in the center of the crown. On upper teeth, M. musculus differs from M. reisi in having a more prominent mesial cingulum bearing an ectocone, and lingual cusps that are less peripheral and more concentrated at the center of the crown.

Description of the new specimens from Dormaal ( Fig. 2 View Fig ).— The p2 (IRSNB M1340, Fig. 2E View Fig ) is biradiculate, exodaenodont, and relatively broad. The protoconid is rather central on the crown and pointed; it bears a well developed mesial crest. The talonid is represented by a small hypoconid. On DIIC2037RS, a possible p3, this cusp is larger and relatively well defined.

The p4 ( Fig. 2A View Fig ) is rectangular in outline, very robust, longer than m1–m2, high−crowned, and shows a considerable exodaenodonty. The only two differentiated cusps are connected by a longitudinal shearing crest which bears clear apical serration. The protoconid constitutes about two−thirds the tooth length. An incipient paraconid can occur (DIIA1905RS) on the long paracristid. There is no cingulid or trigonid basin.

The lower molars are characterized by swollen crowns and bulbous cusps. The lingual cusps are clearly higher than the labial ones. The four principal cusps are positioned relatively close to the long axis of the teeth. The trigonid and talonid are equal in length, and the trigonid is slightly higher. A mesial cingulid is present; the labial one is absent, but an ectostylid is visible. On the m1 IRSNB M1337 ( Fig. 2B View Fig ), the labial exodaenodonty is well developed, the paraconid is crestiform and mesially positioned. The protoconid and the metaconid are subequal in size and transversely aligned; these cusps are connected by a short, notched protocristid. The metaconid bears a long premetacristid and a reduced postmetacristid. On the talonid, the cristid obliqua is very short; the entoconid and hypoconid are transversely aligned; the entoconid is more developed than the hypoconid and displays an entostylid. There is a notch between the metaconid and the entoconid, and the hypoconulid is lacking. On the m2, the four main cusps are more bunodont, the exodaenodonty is less pronounced, the trigonid is mesiodistally shorter, the paraconid is absent, the paracristid surrounds the mesial border of the tooth and forms a paralophid, the ectostylid is large, and the hypoconulid is distinct between the hypoconid and entoconid at the distal margin of the crown. The m3 is reduced, the paraconid and hypoconulid are lacking, the two talonid cusps are crestifom and smaller than those of the trigonid, the talonid basin is extended.

Of the upper premolars, only a DP4 is documented ( Fig. 2F View Fig ). This tooth was identified as an M1 by Sudre and Russell (1982), because it is essentially molariform and the four cusps are relatively close to the center of the crown as in molars. The large parastylar region is broken, but the crown appears clearly trapezoidal in shape being longer than wide, with the labial margin longer than the lingual one. The paracone is larger than the metacone; there is no preparacrista; the ectoflexus is deep. The metastyle is reduced but the metastylar region is expanded labially. The metacone is compressed labio−lingually. The mesial cingulum is robust and bears an ectocone. This cusp appears well−developed in lingual view. The protocone is larger than the hypocone. The posthypocrista descends to form the distal cingulum, continuing labially to the metastyle. There are no conules.

The holotype IRSNB M115 View Materials ( Fig. 2G View Fig ), a probable M1, was described by Sudre and Russell (1982). The three M1s (or M2s) present both ectocone and incipient hypostyle. The hypocone is taller than the protocone. Except for the holotype, the paraconule and metaconule are small but perceptible and twinned with the protocone and hypocone respectively. DIIA 1280 RS could be an M2 due to the minor labial expansion of the paracone; the ectocone is strong on this specimen .

The M3 is the smallest of the molars ( Fig. 2H View Fig ). The distal part of the tooth is particularly simplified by the lack of hypocone and the crestiform metacone, which forms a distal lobe. The protocone is bulbous and bears a preprotocrista that reaches the mesial cingulum.

Microhyus musculus? Teilhard de Chardin, 1927 Locality : Pourcy, France.

Age View in CoL : MP7 (early Ypresian, Eocene).

Material and measurements.— MNHN PYL 57 left M1, L = 2.0, W = 2.0.

Discussion ( Fig. 1B).—The enamel is chemically eroded on most of the crown. The observed morphology is, however, characteristic of Microhyus and reminiscent of that of M. musculus in the following diagnostic characters of that species: the mesial cingulum is large and bears an ectocone, both the hypocone and protocone are labially positioned, and the hypocone is higher than the protocone. We assign the tooth from Pourcy to M. musculus tentatively because of the paucity of the material and because comparisons with lower molars and premolars of M. musculus and M. reisi are not possible.

Microhyus reisi Antunes, Estravis, and Russell, 1987 Holotype: UNLSV3−3 (partial left dentary with p3–m1 and alveoli of p1–p2) ( Fig. 3D View Fig ).

Type and only known locality: Silveirinha, Portugal.

Age : MP 7 (early Ypresian, Eocene).

Referred material.—446 complete teeth, documenting all molars and premolars (deciduous teeth included), except P1. Astragali (UNLSNC−671, 672, 673, and 674). Calcanei (UNLSNC−675, 676, 677, 678, 679, 680, 681, 682, 683).

Measurements of teeth.—(See Table 1).

Description of new specimens ( Figs. 3–6 View Fig View Fig View Fig View Fig ).—The teeth of Microhyus reisi were adequately described by Antunes et al. (1987). We describe here the mandible, the dp4 and fragments of maxilla before discussing the dental variability of the species.

The holotype UNLSV3−3 ( Fig. 3D View Fig ) and the specimens UNLSV3−418 (partial left dentary with p3–m2 and p2 alveoli), UNLSNC−11 (partial right dentary with m1–m3) ( Fig. 3A View Fig ), UNLSNC−87 (partial right dentary with p4 and p2–p3 alveoli), and UNLSNC−153 (partial left dentary with p2 plus

c−p1 and p3 alveoli) ( Fig. 3E View Fig ) provide evidence of the mandibular morphology of M. reisi . A composite construction of the mandible is given in Fig. 4 View Fig . In lateral view, the horizontal

m3–m1 p4–p1 2 mm

ramus appears relatively robust and increases in depth distally. The c−m3 form a continuous series without diastema. The elongate and strongly ankylosed symphysis extends distally to p2. Two mental foramina are present, one beneath the mesial root of p1 and another beneath the mesial root of m1. The masseteric crest and fossa are well developed.

The specimen UNLSV 1−65 of the original collection is interpreted here as a dp3; the dp4 being represented in the new material by the specimen UNLSNC−155 (partial right lower jaw with dp4–m1 ( Fig. 3F View Fig ). This tooth is exodaenodont, longer than the m1, and 8−shaped in outline. The crown is narrow under the metaconid and relatively broad under both the paraconid and talonid. The trigonid represents more than half the length of the crown. The three cusps of the trigonid are labio−lingually compressed, the paraconid is particularly crestiform. The paracristid is long and rectilinear. The metaconid is lower than the protoconid and distally offset, and both cusps are separated by a deep notch. The talonid is similar to that of m1 of M. musculus .

The specimens UNLSNC−14 (a right maxilla fragment with P4–M2) ( Fig. 5A View Fig ), UNLSNC−205 (a right maxilla fragment with P4–M3) ( Fig. 5C View Fig ), and UNLSNC−283 (a left maxilla fragment with P4–M2) uphold the identifications of isolated upper cheek teeth made by Antunes et al. (1987). As P4 is molariform (i.e., the hypocone is present, the conules are bulbous and larger than those of the molars), this tooth might have been identified as a molar of another taxon. The zygomatic process of the maxilla arises between M1 and M2.