SYSTEMATIC PALEONTOLOGY
(continued)

Cervical Vertebrae. Presacral vertebrae are composed of highly pneumatized camellate bone (Wedel 2003). The centra are strongly opisthocoelous (sensu Romer 1956) with the anterior convexity centrally placed on the centrum. Eighteen cervical vertebrae (Figure 8, Figure 9, Table 2), including nine isolated vertebrae and two articulated sets (Mal-278, three vertebrae; Mal-280, six vertebrae), of at least two individuals, are attributed to Malawisaurus. The atlas-axis complex is not represented. Mal-180 (Jacobs et al. 1996, figure 5a), Mal-243, Mal-245, the Mal-278 set, the first four of the Mal-280 set, and Mal-301 are fairly well preserved while Mal-187-1, Mal-193-1, Mal-244, Mal-246, the last two of the Mal-280 set, and Mal-291 (= 89-78, Jacobs et al. 1993, figure 1e) are poorly preserved.

In sauropod cervical vertebrae, the interprezygapophyseal distance, development of laminae and fossae on the lateral surface of the neural arch, and the length of the diapophyses increase posteriorly (Osborn and Mook 1921; Gilmore 1936; Powell 1986; McIntosh et al. 1996a, 1996b). In Haplocanthosaurus (Hatcher 1903), the penultimate cervical vertebra is wider than high and the last cervical vertebra is shorter than the preceding cervical vertebra. In Camarasaurus (Osborn and Mook 1921) and Apatosaurus (Gilmore 1936), the lengths of distal cervical vertebrae are less than the lengths of medial cervical vertebrae. Among titanosaurians, articulated cervical vertebrae are known in “Titanosauridae indet. DGM Series A” from Brazil, (12 cervical vertebrae, except the atlas, preserved in articulation with three proximal dorsal vertebrae; Powell 1986, 1987a) and in “Titanosauridae indet., DGM Series B” from Brazil (five cervical vertebrae articulated with 10 dorsal vertebrae; Powell 1986, 1987a). Thus, titanosaurians are considered to have 13 (Powell 1987a) cervical vertebrae. In “Titanosauridae indet. DGM Series A,” the neural spines are low and lean posteriorly in the second to sixth cervical vertebrae (Powell 1986). Based on comparison of the interprezygapophyseal distance, orientation of the neural spines, development of the posterior centrodiapophyseal and postzygodiapophyseal laminae and neural arch fossae, length of the diapophyses, and width/height ratio of centra with “DGM Series A and B,” Apatosaurus (Gilmore 1936), Camarasaurus (Osborn and Mook 1921; McIntosh et al. 1996a, 1996b), and Haplocanthosaurus (Hatcher 1903), the positions of Malawisaurus cervical vertebrae were estimated.

The anterior ball of each medial and distal cervical vertebra of Malawisaurus is medially depressed. The planes of the posterior cups are inclined anterodorsally and not perpendicular to the axes of the vertebrae, suggesting an anteriorly rising neck as in Camarasaurus (McIntosh et al. 1996a). The ventral surfaces of the centra are concave medial to the parapophyses but convex posteriorly. Undivided pleurocoels (sensu McIntosh 1990) are represented by small fossae directly ventral to the diapophyses as opposed to multiple large anteroposteriorly aligned fossae on the centra of other sauropods (Amargasaurus [MACN-15, Salgado and Bonaparte 1991]; Apatosaurus [Gilmore 1936]; Brachiosaurus [Janensch 1929a, 1950]; Camarasaurus [BYU 9047; Osborn and Mook 1921; McIntosh et al. 1996a]; Dicraeosaurus [Janensch 1929b]). The parapophyses are directed slightly ventrolaterally. Cranial and caudal peduncle fossae are small. Diapophyses project laterally and become progressively longer posteriorly.

The neural arches are low, long, and medially attached anteroposteriorly on the centra. The laminae on the lateral surfaces of the neural arches are rudimentary (sensu Wilson and Sereno 1998). There are only two laminae on the posterolateral surface of a neural arch, termed the posterior centrodiapophyseal and postzygodiapophyseal laminae by Wilson (1999). These laminae enclose the infrapostzygapophyseal fossa. The infrapostzygapophyseal fossae enlarge and deepen posteriorly along the cervical column. The supradiapophyseal fossae (posterolateral to the spinoprezygapophyseal laminae and dorsal to the diapophyses) are shallow anteriorly but deepen and widen posteriorly along the cervical column.

The prezygapophyses are small and ellipsoidal in anteriorly cervical vertebrae, but are large and rounded in middle and posterior cervical vertebrae. They extend beyond the anterior end of the centrum in all cervical vertebrae. The prezygapophyses become more divergent posteriorly. Intraprezygapophyseal and intrapostzygapophseal laminae are present in the middle and posterior cervical vertebrae. The spinoprezygapophyseal laminae are sharp and thin in anterior cervical vertebrae but thicken posteriorly. Medial to each spinoprezygapophyseal lamina, at the base, is an ovoid depression in the anterior cervical vertebrae.

The neural spines are single in all cervical vertebrae. They are rounded and cant posteriorly in the anterior cervical vertebrae but become progressively more anteroposteriorly compressed and more vertical posteriorly. In anterior cervical vertebrae, the neural spines are medial to and are restricted towards the spinopostzygapophyseal laminae. In the posterior cervical vertebrae, the neural spines are supported by and rise directly from the spinoprezygapophyseal and the spinopostzygapophyseal laminae. Mal-245 has well-developed pre-spinal laminae. Short pre-spinal and long, deep post-spinal fossae occur medial to the spinozygapophyseal laminae in all cervical vertebrae.

Cervical Ribs. The cervical ribs (Figure 9) are coossified to the centra (Mal-243, Mal-180, Mal-245, Mal-278, Mal-280, and Mal-301) or are isolated fragments (Mal-64, Mal-146, Mal-147, Mal-149, Mal-162, and Mal-187-2). The ribs are nearly parallel to the long axes of the centra. The heads terminate at the anterior limits of their associated centra in proximal cervical vertebrae (Mal-180), but terminate posterior to the anterior ends of their associated centra in medial and distal cervical vertebrae. This is in contrast to “Titanosauridae indet. DGM Series A” where the heads of the ribs extend beyond the anterior ends of their associated centra (Powell 1986, 1987a). The ribs are thin and dorsoventally compressed, relatively broad transversely in their proximal halves, but thin and rounded rods in their distal halves. The shafts of the ribs in the proximal and medial cervical vertebrae extend up to 320 mm beyond the posterior ends of their associated centra, and even extend beyond the ends of the next succeeding centra. In the distal cervical vertebrae, the shafts of the ribs do not extend beyond the centra (Mal-245).

Dorsal Vertebrae. Ten isolated dorsal vertebrae (Figure 9, Figure 10, Figure 11, Table 3) are attributed to Malawisaurus. There are no hyposphene-hypantrum articular surfaces in any of the vertebrae. The posterior ends of the centra are flared. The neural arches are low and possess fossae that vary in size and shape on either side of each vertebra. Undivided and short neural spines are supported anteriorly by the spinodiapophyseal laminae, not by the spinoprezygapophyseal laminae as in Apatosaurus (Gilmore 1936), and posteriorly by the spinopostzygapophyseal laminae. All, except Mal-242 (where it cannot be observed), have well-developed pre-spinal and post-spinal laminae. The pre-spinal laminae are bifid at the base and are more prominent than the post-spinal laminae.

Among titanosaurians, the most complete articulated dorsal series known are in a specimen identified as “Titanosauridae indet. DGM Series B” from Brazil (10 dorsal vertebrae; Powell 1986, 1987a) and in Opisthocoelicaudia (10 or 11 dorsal vertebrae; Borsuk-Bialynicka 1977). The description and discussion of Powell (1986) imply that “DGM Series B,” Epachthosaurus (MACN-CH 1317, cast), and Neuquensaurus (Titanosaurus) australis have 10 dorsal vertebrae, whereas the illustrations of Neuquensaurus (von Huene 1929, figure 10; Powell 1986, plate 56) suggest the presence of 11 dorsal vertebrae. Wilson and Sereno (1998, figure 47) indicate that although the number of dorsal vertebrae in titanosaurians varies between 10 and 12, 10 is the standard number. Thus, Malawisaurus is predicted to have 10 dorsal vertebrae.

In “DGM Series B,” as well as other sauropods (Hatcher 1903; Osborn and Mook 1921; Powell 1986, 1987a; McIntosh and Williams 1988), the parapophyses move in position from being on the centrum in proximal dorsal vertebrae to being high on the neural arch in posterior dorsals. The transverse processes project laterally in the proximal dorsal vertebrae but dorsolaterally in the distal vertebrae. The interprezygapophyseal distance progressively decreases posteriorly along the dorsal series. Comparison with “DGM Series B” and other sauropods (Hatcher 1903; Riggs 1903; Osborn and Mook 1921; Gilmore 1936; Powell 1986, 1987a; McIntosh et al. 1996a; Jain and Bandyopadhyay 1997) using the positions of the parapophyses, the orientation of transverse processes, and the interprezygapophyseal distances, indicates that Mal-181 (Jacobs et al. 1996, figure 5b, c), Mal-236, Mal-238, Mal-239, and Mal-283 occur in the proximal half of the dorsal series in positions one to five, while Mal-182 (Jacobs et al. 1996, figure 5d), Mal-237, Mal-240, Mal-241, and Mal-242 occur in the distal half of the dorsal series, in positions six to 10.

All the dorsal vertebrae possess small, anteroposteriorly elongate, eye-shaped (sensu Calvo and Salgado 1995) pleurocoels that are restricted to the dorsal half of the centrum. The pleurocoels face laterally in dorsal vertebrae one to three and dorsolaterally in the other dorsal vertebrae. In Mal-239 and Mal-283, the parapophyses are subrounded and situated on the centrum. Their ventral limit is 20 mm below the ventral limit of the pleurocoels. Mal-283 is interpreted as the first dorsal of Malawisaurus, because its parapophyses are immediately anterior to the pleurocoels, and the diapophyses face ventrolaterally as in the first dorsal vertebra of other sauropods (Hatcher 1903; Osborn and Mook 1921; Powell 1986, 1987a; McIntosh and Williams 1988). The centrum is shorter and the zygapophyses are smaller than in the posterior cervical vertebrae but are similar to the first dorsal vertebra of “Titanosauridae indet. DGM Series A and B” and of Neuquensaurus (Powell 1986, 1987a).

Parapophyses are anterior to the pleurocoels in Mal-239. Although the parapophyses are on the centrum in the second dorsal vertebrae of “Titanosauridae indet. DGM Series A” from Brazil, the centrum is much shorter than in the first dorsal vertebra (Powell 1986). The centrum of Mal-239 is much shorter than that of Mal-283 (Table 3) and might be the second dorsal vertebra of Malawisaurus. In Mal-236, the parapophyses are crescentic and extend from the centrum to the lower portion of the neural arch. In “DGM Series B,” the parapophyses on the third dorsal vertebra are on the neural arch and the centrum is wider than high. The centrum of Mal-236 is similarly wider than high. Mal-236 is also similar to third dorsal vertebrae of Apatosaurus (Riggs 1903, Gilmore 1936) and Camarasaurus (Osborn and Mook 1921) in that half of its parapophysis arises from the centrum. Thus, Mal-236 might be the third dorsal vertebra of Malawisaurus.

In all other dorsal vertebrae, the parapophyses are high on the neural arch. In Mal-238, the parapophyses are crescentic, and their dorsal limits are slightly lower than the dorsal limits of the prezygapophyses. In Mal-181 (Jacobs et al. 1996), the parapophyses are at the same level as the prezygapophyses. Parapophyses on the neural arch, but below or at the same level as the prezygapophyses, occur in dorsal vertebrae three to five of “DGM Series B.” In Mal-237 and Mal-240, the parapophyses are subtriangular with medial depressions. The dorsal limits of the parapophyses are 40 mm higher than the prezygapophyses. In Mal-241, Mal-182, and Mal-242 the neural arches are not complete, and the parapophyses and diapophyses are not preserved. However, the decrease in interprezygapophyseal distance suggests a posterior progression from Mal-181, Mal-237, Mal-241, Mal-240, and Mal-182 in that order. Mal-242 is considered the most posterior and may be the last dorsal vertebra of Malawisauru,s because its centrum is the least excavated among the dorsal vertebrae, similar to the last dorsal vertebrae of Camarasaurus lewisi (BYU 9047; McIntosh et al. 1996a) and of Brachiosaurus (Riggs 1904). Thus, based on parapophyseal position, interprezygophyseal distance, and size of lateral fossae on the centrum, the anatomical position of Malawisaurus dorsal vertebrae were approximated.

The prezygapophyses are at approximately the same level as the postzygapophyses in Mal-283 and in Mal-239, and are lower than the postzygapophyses in dorsal vertebrae posterior to the second, Mal-239. Neural arches are attached to the anterior half of centra in Mal-283, Mal-239, Mal-236, Mal-238, and Mal-181, interpreted anatomical positions one to five, and are centrally attached to centra in Mal-237, Mal-241, Mal-240, Mal-182, and Mal-242, interpreted anatomical positions six to 10. In dorsal vertebrae one to three (Mal-283, Mal-239, and Mal-236), the posterior limits of the neurocentral junction are anterior to the posterior limits of the pleurocoels. The neural arch attachments extend the entire lengths of the pleurocoels in dorsal vertebrae four to nine (Mal-238, Mal-181, Mal-237, Mal-241, Mal-240, and Mal-182), and extend beyond the posterior limits of the pleurocoels in dorsal 10 (Mal-242).

The centrodiapophyseal laminae are dorsally broad and ventrally forked into wide anterior and posterior centrodiapophyseal laminae in all the dorsal vertebrae except in Mal-242. The fossae between the anterior and posterior centrodiapophyseal laminae are deep in Mal-283, Mal-239, Mal-236, Mal-238, and Mal-181, dorsal vertebrae one to five, shallow in Mal-237, Mal-241, Mal-240, and Mal-182, dorsal vertebrae six to nine, and have completely disappeared in Mal-242, dorsal 10, so that the centrodiapophyseal laminae in Mal-242 are massive and not forked. The posterior centrodiapophyseal laminae and the centropostzgapophyseal laminae are separate in Mal-283, Mal-239, and Mal-236, dorsal vertebrae one to three, but merge in all dorsal vertebrae posterior to Mal-236.

The transverse processes are directed laterally in Mal-238, Mal-239, and Mal-236, and are slightly dorsolaterally inclined in Mal-283, Mal-181, Mal-237, and in Mal-240. Diapophyses are ellipsoidal, longer dorsoventrally than anteroposteriorly in Mal-283, Mal-239, Mal-236, and Mal-238, but longer anteroposteriorly than dorsoventrally in Mal-181, Mal-237, and Mal-240. The diapophyses face ventrolaterally in Mal-283, Mal-239, Mal-236, and Mal-238, and laterally in Mal-181, Mal-237, and Mal-240.

The neural spine is paddle shaped in anterior view in each of the anterior dorsals and taper dorsally in the posterior dorsals. The neural spine lacks lateral pendant processes. It is inclined posteriorly in any anterior dorsal vertebrae and becomes vertical posteriorly. Narrow longitudinal fossae occur lateral to the pre-spinal laminae in Mal-181, Mal-237, and Mal-182, and lateral to the post-spinal laminae in Mal-238. In other dorsal vertebrae, the presence of longitudinal fossae on the neural spines cannot be determined because the spines are incomplete.

Dorsal Ribs. Twelve dorsal ribs (Figure 12A-B, Table 4) are attributed to Malawisaurus. Comparison with Apatosaurus (Riggs 1903), Camarasaurus (BYU 9047; Osborn and Mook 1921; McIntosh et al., 1996a; McIntosh et al. 1996b), Brachiosaurus (Janensch 1950), and Opisthocoelicaudia (Borsuk-Bialynicka 1977) suggests that Mal-295, Mal-296, Mal-297, Mal-298, and Mal-308 occur in the proximal half of the series, from the first to the fifth position, whereas the remainder occur in the distal half of the series, posterior to the fifth rib. The posterior surfaces of Mal-282-1 and Mal-282-2, around positions seven and eight, have quadrangular pneumatic cavities 30 mm from the capitulum-tuberculum split (Gomani et al. 1999, figures 1d, e) indicating that pneumatization extends beyond the anterior one third of the dorsal series mentioned for the Mendoza titanosaurian by Wilson and Sereno (1998, p. 52, character 97). The shafts are flattened in all the ribs.

Sacrum. The sacrum (Figure 13 A-B; anteroposterior length = 574 mm; half width across sacral two = 310 mm; narrowest half width across sacral three = 266 mm; half width across sacral six = 395 mm) consists of six vertebrae that are completely fused. The posterior end of sacral six is not completely preserved. The prezygapophyses and postzygapophyses are fused. Neural spines are entirely coossified and form a plate that overhangs the neural arches as in Titanosauridae indet. n. sp. C from Brazil (Campos and Kellner 1999) and in Epachthosaurus (Powell 1986). The dorsal surface is roughened with longitudinal grooves. The lateral walls of the neural spines are thin and strengthened by well-developed laminae as in Opisthocoelicaudia (Borsuk-Bialynicka 1977). Except for the ribs of the dorsosacral, all the ribs are medially thin, and the laminae are fused to the parapophyses and diapophyses as in Titanosauridae indet. n. sp. B from Brazil and other sauropods. The sacral ribs of the dorsosacral are higher on the neural arch while the ribs of the other sacral vertebrae rise from the ventral surfaces of the associated sacral vertebrae. The ribs of sacral vertebrae two to five have subcircular fenestrae proximally as in Saltasaurus (PVL 4017; Powell 1986). Sacral rib two has an expanded distal ventral plate that overhangs part of sacral rib three. Ribs two and three lean posteriorly, ribs four and five lean anteriorly in ventral view, and ribs one and six are vertical. The ventral openings that separate rib three from ribs two and four are large relative to other openings. Unlike in the Brazilian sacra (Campos and Kellner 1999), the distal ends of all sacral ribs in Mal-277-1 are completely coalesced to form the sacricostal yoke as in Opisthocoelicaudia (Borsuk-Bialynicka 1977).

Caudal Vertebrae. Fifty-one caudal vertebrae (Figure 14, Figure 15, Figure 16, Table 5) of at least three individuals of Malawisaurus have been described by Jacobs et al. (1993, figure 2b, c; Gomani 1999b, figure 1). The centra are strongly procoelous (sensu Romer 1956) in the most anterior caudal vertebrae (Mal-191, Mal-225-1, Mal-232-1, Mal-279-1, and Mal-279-2), becoming slightly procoelous to platycoelous (sensu Romer 1956) in more the posterior of the anterior caudal vertebrae (Mal-225-2, Mal-225-3, Mal-225-4, Mal-227, and Mal-228), and platycoelous in the medial and distal caudal vertebrae (sets of Mal-197, Mal-196, and Mal-192, and isolated vertebrae [Mal-2, Mal-198, Mal-206, Mal-223, Mal-224, Mal-226, Mal-229, Mal-231, Mal-232-1, and Mal-233]). In the procoelous tail vertebrae, the posterior ball is restricted to the dorsal half of the centrum. For a complete description and additional measurements of Malawisaurus caudal vertebrae see Gomani (1999b, table 2).

Chevrons. Among titanosaurians, only Alamosaurus (25 chevrons; Gilmore 1946) and Opisthocoelicaudia (at least 19 chevrons; Borsuk-Bialynicka 1977) have complete series of chevrons preserved. Based on comparison with Alamosaurus, 24 recovered chevrons of three different morphologies are considered to belong to Malawisaurus (Figure 17, Figure 18, Table 6). The chevrons belong to at least two individuals. Mal-192-22, Mal-197-24, Mal-197-25, Mal-197-26, Mal-197-27, Mal-197-28, and Mal-197-29 were found in natural articulation, and Mal-194, Mal-197-23, Mal-197-30, and Mal-197-31 were found in close association with an articulated tail of 21 vertebrae (Mal-197-1 to Mal-197-21; Gomani 1999a, 1999b) of Malawisaurus. Mal-195, Mal-277-2, Mal-277-3, and Mal-310 were associated with the sacrum (Mal-277-1), Mal-255, and the Mal-186 articulated caudal sets. Malawisaurus is interpreted as having at least 25 chevrons based on comparison with Alamosaurus (Gilmore 1946). The anteroposterior length of chevrons decreases to the fifth chevron in Alamosaurus (Gilmore 1946), to the sixth or seventh chevron in Brachiosaurus (Janensch 1950, figures 109-136), and to the seventh chevron in Opisthocoelicaudia (Borsuk-Bialynicka 1977). In these taxa and in Camarasaurus (BYU 9047; McIntosh et al. 1996a) the length of the haemal canal relative to the length of the spine increases posteriorly. In Alamosaurus, a V-shaped chevron first appears between caudal vertebrae 16 and 17 and is referred to as chevron 16 (Gilmore 1946). Thus, in one articulated caudal series (Mal-197) of Malawisaurus (Gomani 1999a, 1999b), the first V-shaped chevron (Mal-197-26) is interpreted as the sixteenth chevron. Hence, based on comparison with these taxa and this interpreted position of Mal-197-26, the morphology, proximodistal length, and the length of the haemal canal relative to the length of the spine, the anatomical positions of isolated chevrons of Malawisaurus were estimated. All chevrons lack a bridge of bone so that the haemal canal is open, unlike the completely enclosed haemal canals or forked chevrons in diplodocids and dicraeosaurids (Osborn 1899; Hatcher 1901; Gillette 1991).

The first morph (Y-shaped; Figure 17A-F and Figure 18A-E) has two arms that unite below the haemal canal to form a spine. All Y-shaped chevrons except Mal-195 have laterally compressed spines and arms with subequal steep anteroventrally sloping and gentle anteriorly facing articular facets. In Mal-195 the spine is dorsoventrally flattened. The heads on the arms curve medially and have a single articular facet that faces dorsomedially so that when articulated with vertebrae, it would make an acute angle with the vertebrae column. The spine has a low, broad median ridge on the posterior surface that divides into two smaller ridges posteroventrally. The arms are thicker medially than laterally as opposed to being thicker posteriorly than anteriorly in other Y-shaped chevrons. The spine is slightly depressed on the anterior surface. Based on these features, Mal-195 is interpreted as the first chevron. This matches the less developed chevron articular facet on caudal vertebra Mal-200 as compared to more posterior vertebrae. In Opisthocoelicaudia (Borsuk-Bialynicka 1977), the spines of the first four chevrons are dorsoventrally flattened as in Mal-195. A first chevron that is slightly different from the posterior chevrons also occurs in Brachiosaurus (Janensch 1950, figure 109) and Mamenchisaurus hochuanensis (Yang and Zhao 1972).

The second morph (V-shaped; Figure 17G-J and Figure 18F-H) has two arms but lacks the spine. The arms have small articular facets that face anterodorsally as opposed to the anteriorly facing articular facets of the Y-shaped chevrons. The third morph (rod shaped; Figure 17K-L) has two separate arms. The arms are laterally compressed and have anterodorsal facing articular facets in Mal-152-2 and Mal-197-31. Mal-197-23 is elongate and cylindrical.

Alamosaurus has 15 Y-shaped chevrons in positions 1 to 15, three V-shaped chevrons in positions 16 to 18, and seven rod-shaped chevrons in positions 19 to 25. The articulated series of Malawisaurus (Mal-197) has five V-shaped chevrons, the last (Mal-197-30) of which is larger than Mal-152-1. This suggests that Mal-152-1 occurs distal to Mal-197-30 or belongs to a smaller individual. Although the actual position of Mal-152-1 relative to Mal-197-30 cannot be determined definitely, Mal-152-1 is considered to be distal to Mal-197-30. It is clear that Malawisaurus has at least five V-shaped chevrons, more than the three in Alamosaurus (Gilmore 1946).

Sternal Plate. Left (Mal-188-1; Figure 19A-B; length = 490 mm; width = 300 mm; depth range = 5 mm to 50 mm) and right (Mal-188-2) sternal plates were found articulated (Jacobs et al. 1993, figure 1f). They are semilunar, typical of titanosaurians (Salgado et al. 1997), and their medial articular surfaces are irregularly ridged. The distal end of the sternal plate is indented probably for the attachment of the cartilaginous sternal ribs as in Alamosaurus (Gilmore 1946).