Original paper

Tooth emergence and replacement in the European Hyrachyus minimus (Fischer, 1829) (Mammalia, Perissodactyla) from the Geiseltal Fossillagerstätte – a further example for 'Schultz's rule' in ungulates

Hellmund, Meinolf

Abstract

A reappraisal of dentitions of Hyrachyus minimus (Fischer, 1829) from the Eocene Geiseltal Fossillagerstätte reveals that ‘Schultz's rule' also applies for the perissodactyl Hyrachyus minimus and is connected with the replacement of the milk teeth and the emergence of the permanent dentition. The phenomenon, called ‘Schultz's rule' already known from primates is a characteristic for mammals that grow very quickly. The majority of Hyrachyus develops a P1/p1, the so called ‘persistent milk tooth', but within natural variability in the taxon, the tooth is sometimes missing. A descriptive, relative age determination can be ascertained for the juvenile, subadult (in transition to adult), adult and senile stages, in the sense of ‘younger/older than'. Suture closure also indicates the individuals are adult, especially combined with a fully developed permanent dentition. Hyrachyus is a conservative taxon with regard to its tooth morphologies; no molarisation trend is recognizable in its premolars. A marginal size increase in tooth parameters is verified along the entire middle Eocene time line (ranging from MP 11-MP 13 = Geiseltalian) at the Geiseltal location. Sexual dimorphism occurs in the morphology of the canines, but is not very striking in Hyrachyus. Putatively this character does not have much importance in their lives. In some well-preserved skulls of Hyrachyus minimus of the Geiseltal Collection, the tips of the nasals have nearly the same protruding outline as the premaxilla bones. This points to a short nose incision, so that it is very unlikely that Hyrachyus minimus had a trunk-like organ consisting of soft tissue. In two of three preserved adult skulls the sutures between the nasals, frontals and lacrimals are fully fused, such that no sutures are visible. The investigated specimens were collected from various-aged brown coal seams (MP 11-MP 13) of the former mining area of the Geiseltal Fossillagerstätte over several decades. Accordingly they do not represent members of a natural population or from a homogenous biocoenosis.

Keywords

replacement of dentitionsexual dimorphismschultz’s rulehyrachyus minimusobliterationshort nose incisionemergencegeiseltal fossillagerstätteeocene