The paraphyletic Archeoceti is the stem group of the Neoceti (modern cetaceans: toothed and baleen whales), which is regarded as monophyletic by
In the following description, the taxa Protocetidae, Basilosauridae and Dorudontinae are referred to, although they have been regarded as paraphyletic (
A phylogenetic analysis was carried out using a matrix of 31 characters. Characters were selected from
Cetacea
Pelagiceti
Basilosauridae
The skull of MNHN.F.PRU 10 (
The infraorbital foramen is located above the posterior edge of P2. The anterior edge of the nasals is missing but it is possible to evaluate that it reached the level of the anterior edge of P2. Lacrymal and jugal are unfused and form respectively the anterior and the ventral edges of the orbit. As in
The palate is very narrow anteriorly (mostly formed by the ventral surface of the premaxilla) and greatly widens posteriorly from P1 to M2 (mostly formed by the ventral surface of the maxilla). From the anterior edge of the canine, the premaxillae are wedged posteriorly between the maxillae until the level of P2. The palate bears deep embrasure pits, which receive the corresponding lower tooth. On the premaxilla, they are lateral to the tooth row and progressively move medially in the maxilla. Posterior to P4 the palate narrows and has a triangular morphology, with a pointed extremity, which extends well posterior to M2, at about the anterior third of the temporal fossa. This triangular region is formed by the horizontal part of the palatines, which have a bell-shaped suture with the maxillae. A small palatine foramen is located roughly in the middle of each palatine. Laterally, the vertical portion of the palatine participates to the medial wall of the orbital region. The palatine articulates posteriorly with the pterygoid.
The Fr-Pa suture is anterolaterally oriented from the midline to the lateral border of the intertemporal constriction. At the level of the postorbital process it turns posteroventrally on 3 cm and then turns anteroventrally to form a long anteriorly convex suture on half of the height of the lateral wall of the skull. At this point, the anterior edge of the parietal forms a posteroventrally-oriented suture with the alisphenoid, which reaches the anterior edge of the squamosal, anterodorsal to the subtemporal crest. Anteriorly, the alisphenoid forms the ventrolateral wall of the braincase, while parietal forms its dorsolateral wall. From the anterior border of the subtemporal crest, the Pa-Sq suture runs posterodorsally and then turns posteroventrally at about the level of the mid-height of the supraoccipital. Here, it reaches the nuchal crest roughly at the level of the supraoccipital-exoccipital suture. The intertemporal constriction is much longer than in neocetes and the sagittal crest reaches the vertex of the skull posteriorly. The parietal exhibits a large foramen and the parasagittal crest is hardly present as a slight inflation of the lateral edge of the intertemporal constriction. From the vertex, on each side of the skull, the parietals and the supraoccipital form the nuchal crest, which is posterodorsally oriented as in
The pterygoid articulates anteriorly with the palatine (
The basisphenoid-basioccipital suture is fused but distinctly thickened. The massive falcate process of the basioccipital extends ventrolaterally. The exoccipital participates to the falcate process (
The periotic is tightly articulated to the skull. The short anterior process and the lateral face of the tegmen tympani articulate laterally with the medial side of the falciform process of the squamosal, while the elongate posterior process separates the external auditory meatus from the paraoccipital process of the exoccipital (
Dentaries of MNHN.F.PRU 10 are virtually complete (
All premolars are double-rooted and compressed transversely. Except for p1, all premolars bear accessory denticles on mesial and/or distal margins (
Only the right forelimb is preserved in MNHN.F.PRU 10. It is near complete and includes the scapula, humerus, radius, ulna, five carpals, four partial metacarpals and two partial phalanges (
As all other basilosaurids, the humerus bears a prominent deltopectoral crest which is anteriorly oriented and occupies most of the shaft. The humerus contacts the radius and the ulna by a single articular trochlea. The morphology of the trochlea only allows flexion and extension of the arm inhibiting pronation and supination movements. Both radius and ulna are transversally flattened. Proportionally, the forelimb of
The left and most of the right hindlimb are missing. Only a non-diagnostic fragment of the innominate and incomplete femur, tibia, fibula and three first phalanges are preserved. The hindlimb seems to be quite similar in general morphology to that of
One most parsimonious tree of 83 steps (RI = 0.825) was obtained using TNT 1.1, Branch and Bound research (search by implicit enumeration option) (
The result differs from
Neoceti and Pelagiceti appear as monophyletic. At the base within Basilosauridae is a clade formed by
As a matter of fact, the basilosaurid skull results to be very constant in all genera of the family and the differences observed on the basicranium appear to be the result of the ontogenetic stage of the specimens compared. The main differences between basilosaurids are observed on the postcranial skeleton. Nevertheless, this preliminary study must be complemented with the construction of a larger matrix including more cranial and postcranial characters. The construction of another matrix based on single specimens instead of taxa will increase the reliability of this study and will reduce the probability of misinterpretations due to polymorphism (in preparation).
Special acknowledgements to D. Bohaska, P. Gingerich, G. Gunnell, J. Geisler, G. Phillips and A. Sanders for allowing us the access to the archaeocetes and early neocetes collections under their care. The specimen studied (MNHN.F.PRU 10) was collected with funds of the Institut Français d’études Andines (Lima, Peru) and prepared at the Muséum national d’Histoire naturelle (Paris, France). Photos in
Orbital region of
Région orbitaire de
Basicranium of
Basicranium de
Anterior view of the cervical vertebra of A)
Vue antérieure des vertèbres cervicales de A)
Consensus tree showing the phylogenetic relantionships of
Arbre consensus montrant les relations phylogénétiques de