Hahnodontidae, Hahn & Hahn, 2003

Hahn, Gerhard & Hahn, Renate, 2003, New multituberculate teeth from the Early Cretaceous of Morocco, Acta Palaeontologica Polonica 48 (3), pp. 349-356 : 351

Family Hahnodontidae Sigogneau−Russell, 1991a Hahnodontidae fam. nov.; Sigogneau−Russell 1991a: 123. Hahnodontidae ; Kielan−Jaworowska and Hurum: 2001: 413.

Sigogneau−Russell (1991a) described the genus Hahnodon , founded on an isolated m2, and established for it the family Hahnodontidae . Now, a second isolated m2has been found, representing a new genus, Denisodon , as well as two other teeth that cannot referred to a genus.

Genera included.— Hahnodon Sigogneau−Russell, 1991a and Denisodon gen. nov.

Distribution.—Early Cretaceous (?Berriasian) of Morocco.

Revised diagnosis.—A family of the Paulchoffatioidea , founded on m2only, with the following characteristics: m2 with an anterior buccal cusp (b1) and an anterior lingual cusp (l1); b1 closely attached to l1, b1 larger than l1. Posterior lingual cusp (l3) present, posterior buccal cusps completely reduced or rudimentary. Posterior half of the crown occupied by a central basin surrounded by an elevated border. Enamel ridges weakly expressed. Apex of b1 and l1 far behind in position, posterior wall of both cusps sloping down nearly vertically.

Discussion.—The basin−like structure of the posterior portion of the crown of m2confirms that the Hahnodontidae belong to the Paulchoffatioidea . The family differs from both other families, Paulchoffatiidae and Pinheirodontidae , in the following details:

– The anterior buccal cusp is preserved, whereas it is completely reduced in the other families.

– The anterior lingual cusp is a single, cohesive morphological unit, whereas it is composed of two confluent cusps, l1 and l2, in the Paulchoffatiidae (see Hahn and Hahn 1998: text−fig. 39 a–f).

– The enamel is nearly smooth. In both the other families the cusps have moderately to strongly developed enamel ridges, the border of the posterior basin is crenulated, and the bottom of this basin is covered with bars.

– The two anterior cusps are closely attached, whereas in both other families the lingual and buccal cusps are separated from one another by a broad longitudinal valley.

– The apex of b1 and l1 is located far posteriorly, whereas in the other family it is placed centrally. The presence of the anterobuccal cusp must be interpreted as a plesiomorphic character, as well as perhaps the smooth structure of the enamel. The evolutionary line leading to the Hahnodontidae must have separated from those leading to the two other families, before the anterior buccal cusp was reduced and the enamel had evolved ridges. So far the Hahnodontidae are the most primitive family of the Paulchoffatioidea . On the other hand, the closeness of the anterior cusps to each other can be interpreted as an autapomorphy of this family.

Chronologically, the Hahnodontidae are contemporaries of the Pinheirodontidae , whereas the Paulchoffatidae are distinctly older. In the structure of m2, the Hahnodontidae are more advanced than the Pinheirodontidae in the partial reduction of the lingual cusps (in the Pinheirodontidae all three lingual cusps are preserved, see Hahn and Hahn 1999b: fig. 1a), but more primitive in preservation of the anterobuccal cusp (which is reduced in the Pinheirodontidae ). It would be very interesting to know something about the structure of the lower premolars (chewing as in the Paulchoffatiidae , cutting as in the Pinheirodontidae ?), but, unfortunately, lower premolars have not yet been found.

The Hahnodontidae are isolated palaeogeographically. The two other families of the Paulchoffatioidea are known only from the Late Jurassic and the Early Cretaceous of Europe. The common ancestor of the Paulchoffatioidea and the geographical origin of this superfamily are unknown. The ancestral species has lived probably in the Middle Jurassic of Eurasia (as can be deduced from the distribution of the three attached families). Then, the Hahnodontidae must have been immigrated to Africa from Eurasia during the Late Jurassic. Also, descendants of the Hahnodontidae are not known. The only known younger Gondwana multituberculate from the Upper Cretaceous of Madagascar ( Krause and Grine 1996) is a poor fragment of one tooth which has never been described sufficiently. Its systematical position is unclear. It is more probable that this tooth represents a member of the Cimolodonta then a descendant of the Hahnodontidae .