Discussion AND CONCLUSIONS

Dentaries and teeth demonstrate the presence of at least two sauropods in Malawi, Malawisaurus and Karongasaurus. Based on differences in the morphology of the caudal vertebrae that have been generally recognized as significant, titanosaurian caudal vertebrae from Malawi also represent at least two taxa including Malawisaurus and Titanosauridae indet. Malawisaurus has platycoelous medial caudal vertebrae, whereas a procoelous middle or posterior caudal vertebra is distinct from Malawisaurus and referred to Titanosauridae indet. A third taxon may be represented by vertebra referred to Titanosauria indet.

Phylogenetic analyses that include Aeolosaurus, Alamosaurus, Andesaurus, Antarctosaurus, Argentinosaurus, Epachthosaurus, Malawisaurus, Nemegtosaurus, Neuquensaurus, Opisthocoelicaudia, Quaesitosaurus, Rapetosaurus, Saltasaurus, and Titanosaurus (Upchurch 1995, 1998, 1999; Salgado et al. 1997; Curry Rogers and Forster 2001; see also Wilson 2002) indicate that Andesaurus and Malawisaurus are basal titanosaurians. In fact, Malawisaurus is the most complete Early Cretaceous titanosaurian known. It is represented by cranial elements, 18 cervical, 10 dorsal, six sacral, and 51 caudal vertebrae, 24 chevrons, pectoral elements, pelvic elements, and dermal armor. Phylogenetic analyses also indicate that taxa with cylindrical teeth and strongly procoelous posterior caudal vertebrae (or opisthocoelous in the case of Opisthocoelicaudia) are more derived than those with broad teeth and platycoelous middle and distal caudal vertebrae.

Skull Shape and Morphological Diversity in Malawi Sauropods. Cranial material attributed to titanosaurians includes one or two partial braincases of Titanosaurus indicus from the Late Cretaceous of India (Berman and Jain 1982; Chatterjee and Rudra 1996), a maxilla from India (von Huene and Matley 1933), a partial braincase and partial skull roof of Saltasaurus (PVL 4017-161) from the Late Cretaceous of Patagonia (Powell 1986, 2003), a premaxilla of Titanosauridae indet. from the Late Cretaceous of Patagonia (Scuitto and Martinez 1994), a premaxilla (PVL 3670-12) that Powell (1979) identified as Laplatasaurus but later referred to as Titanosauridae indet. (Powell 1986, 2003), a fragmentary braincase from the Late Cretaceous of France (Le Loeuff et al. 1989), the braincase, quadrate, quadratojugal, squamosal, and the lower jaws of Antarctosaurus wichmannianus (MACN 6904) from the Late Cretaceous of Patagonia (von Huene 1929; Powell 1986, 2003), two partial braincases of Antarctosaurus septentrionalis from the Late Cretaceous of India (von Huene and Matley 1933; Chatterjee and Rudra 1996), a nearly complete disarticulated cranium of Rapetosaurus from the Late Cretaceous of Madagascar (Curry Rogers and Forster 2001, 2004), and the specimens of Malawisaurus from the Early Cretaceous of Malawi.

There are two basic morphs of sauropod skulls: one high and short, the other low and elongate (Wilson 2002). The high, short morph is referred to as a macronarian skull, whereas the low, elongate morph is generally referred to as a diplodocoid skull (Coombs 1975; McIntosh and Berman 1975; Berman and McIntosh 1978; Salgado and Calvo 1997; Wilson 2002). The macronarian skull is present in Brachiosaurus, Camarasaurus, Datousaurus, Euhelopus, Mamenchisaurus, Omeisaurus, Shunosaurus, and most prosauropods, whereas the diplodocoid skull is present in Apatosaurus, Diplodocus, and dicraeosaurids (Salgado and Calvo 1997; see also Tidwell and Carpenter, 2003).

Comparison of cranial features of Malawisaurus with macronarian and diplodocoid skulls suggests that Malawisaurus had a high, short macronarian skull (Figure 31). The anterior section of the suture between premaxilla and maxilla appears to have been nearly vertical as suggested by the highly angled articular surface for the maxilla on the premaxilla. The high premaxilla also suggests that Malawisaurus had a high, short, and blunt snout, and separate nares positioned rostrally and facing laterally. The tooth row extends more than half the length of the dentary. The mandibular symphysis is oblique to the long axis of the mandible. The occipital condyle projected posteroventrally and the basipterygoid processes projected ventrally. The quadrate axis was nearly vertical, the pterygoid process of the quadrate was approximately perpendicular to the long axis of the pterygoid, the pterygoid process was directed anteriorly as in Brachiosaurus (Janensch 1935) and Camarasaurus (Osborn and Mook 1921), and the mandibular articulation was placed posteriorly beneath the level of the occipital condyle. Thus, this study demonstrates that at least some titanosaurians, including Malawisaurus, had high and short crania, as compared to others such as Rapetosaurus, which had low and elongate crania (Curry Rogers and Forster 2001, 2004).

In addition, the titanosaurians Nemegtosaurus (Nowinski 1971), and Quaesitosaurus (Kurzanov and Bannikov 1983; Curry Rogers and Forster 2001) had slender teeth restricted to the anterior portion of the mandible, and also had low and elongate crania. If that association of characters is general within titanosaurians, by implication Karongasaurus would also have had a low and elongate cranium. Further, phylogenetic analyses indicate that titanosaurians with slender teeth and which have strongly procoelous middle and posterior caudal vertebrae are derived relative to basal titanosaurians (Upchurch 1995; Salgado et al. 1997; Curry Rogers and Forster 2001). Thus, both Karongasaurus and Titanosauridae indet. from the Dinosaur Beds are derived relative to Malawisaurus. If those characters are linked, the vertebra assigned to Titanosauridae indet. may belong to Karongasaurus.

In any case, Malawisaurus and Karongasaurus are two distinct titanosaurian taxa that coexisted in the Early Cretaceous of Malawi and exhibited quite different morphological features, certainly in their lower jaws and teeth, and probably also in their skull shapes. Cylindrical teeth, an anteriorly restricted tooth row, and a long, low skull shape evolved as a complex at least twice, once within diplodocoids and once within titanosaurians. This character complex is functionally and adaptively important for feeding. Its multiple origins, and its variance from the macronarian skull pattern, implies that Malawisaurus and Karongasaurus were ecologically distinct. If so, differences seen in the lower jaw, teeth, and probably the skull of these herbivores were significant in the ecological partitioning of their Early Cretaceous environment. Although the macronarian and diplodocoid skull morphs are well known to occur together, for example in the Jurassic of Africa, in the Early Cretaceous of Malawi approximately equally divergent skull morphs are exhibited at a lower systematic level among titanosaurians alone.