Brontomerus mcintoshi, Taylor & Wedel & Cifelli, 2011

Taylor, Michael P., Wedel, Mathew J. & Cifelli, Richard L., 2011, A new sauropod dinosaur from the Lower Cretaceous Cedar Mountain Formation, Utah, USA, Acta Palaeontologica Polonica 56 (1), pp. 75-98 : 78-86

Brontomerus mcintoshi sp. nov.

Figs. 1 , 2 , 5–8 , 10 , 12 ; Table 3.

Etymology: In honor of veteran sauropod worker John S. McIntosh, whose seminal paleontological work, done mostly unfunded and on his own time, has been an inspiration to all of us who follow.

Holotype: OMNH 66430 , a left ilium.

Tentatively referred material: OMNH 66429, crushed presacral centrum; OMNH 61248, mid−to−posterior caudal vertebra; OMNH 27794, partial distal caudal centrum; OMNH 27766, anterior right dorsal rib; OMNH 27761, nearly complete left scapula missing anterior portion; OMNH 66431 and 66432, two partial sternal plates; other fragments as detailed in Table 3.

Type locality: Hotel Mesa Quarry (OMNH locality V857), Grand County, eastern Utah.

Type horizon: Top of the Ruby Ranch Member of the Cedar Mountain Formation (Lower Cretaceous, Aptian–Albian).

Diagnosis.—Preacetabular lobe 55% of total ilium length, longer than in any other sauropod; preacetabular lobe directed anterolaterally at 30° relative to the sagittal plane, but straight in dorsal view and vertically oriented; postacetabular lobe reduced to near absence; ischiadic peduncle reduced to very low bulge; ilium proportionally taller than in any other sauropod—height is 52% of total length, compared with a maximum of 45% in other sauropods. If the tentatively referred elements do belong to the same species as the holotype, then the following additional characters also diagnose the new taxon: presacral vertebrae camellate; mid−to−posterior caudal vertebrae with elongate pre− and postzygapophyseal rami, having the postzygapophyseal facets hanging below the level of the ramus; first dorsal rib with expanded, dorsally oriented articular facets, laterally curving shaft, and ventrally directed pneumatic foramen in head; acromion expansion of scapula pronounced and steep, but not forming acromion fossa; dorsal and ventral margins of scapular blade “stepped”; sternal plates crescentic, and three times as long as broad.

Unambiguous autapomorphies distinguishing Brontomerus from the root of the polytomy in which it is recovered in the strict consensus of most parsimonious trees in the phylogenetic analysis below: character 184, ratio of centrum length:height in middle caudal vertebrae Ẑ 2.0; 185, sharp ridge on lateral surface of middle caudal centra at arch−body junction absent; 212, posterior end of scapular body racquet−shaped (dorsoventrally expanded); 261, in lateral view, the most anteroventral point on the iliac preacetabular lobe is also the most anterior point (preacetabular lobe is pointed); 264, projected line connecting articular surfaces of ischiadic and pubic peduncles of ilium passes ventral to ventral margin of postacetabular lobe of ilium.

Description

This taxon is based on a collection of elements all from the same quarry, all of them consistent with assignment to a single taxon ( Fig. 1 ). However, the elements were not found articulated, and their differing sizes do not permit interpretation as belonging to a single individual. For example, the partial scapula is 98 cm long. Reconstruction after the scapula of Giraffatitan brancai ( Janensch 1914) suggests that the complete element was about 121 cm long. In Rapetosaurus Curry Rogers and Forster, 2001 , the scapula and ilium are about the same length ( Curry Rogers and Forster 2001: fig. 3) but the ilium of Brontomerus is only one third the reconstructed length of the scapula. The quarry therefore contains at least two individuals of widely differing sizes. The smaller, providing the holotype ilium and the referred presacral centrum and sternal plates, is interpreted as a juvenile; the remaining elements probably belonged to the larger, mature individual.

Among the sauropod elements recovered from the Hotel Mesa site, at least three (the caudal vertebra, scapula and ilium) have characters not known in any other sauropod. Since it is not possible to conclusively demonstrate that all three elements belong to the same taxon (and size differences preclude their belonging to a single individual), it is therefore possible that the quarry contains as many as three new taxa. However, since all the informative elements have characters that indicate a titanosauriform identity, we conservatively consider it more likely that only a single new taxon is present. Groups of sauropods of mixed ages are known for several taxa (e.g., Bird 1985; Coria 1994; Bader 2003), so there is a precedent for finding adults and juveniles together. Although we diagnose the new taxon solely on the type ilium, we also tentatively refer the other elements to this taxon and note the additional characters that pertain if this referral is correct; we hope that subsequent work in the Hotel Mesa Quarry or elsewhere in the Cedar Mountain Formation will yield specimens that allow us to confirm or refute this referral.

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The assignment of specimen numbers to the material described here is complex ( Table 3). Specimen number OMNH 27773 comprises three elements; OMNH 27784 consists of 21 small fragments of bone, none of them informative; all other elements have their own specimen numbers, in the range 27761–27800 apart from the mid−caudal vertebra OMNH 61248 and the reassigned numbers 66429–66432, for elements extracted from OMNH 27773.

Ilium.—The most informative element is OMNH 66430, a left ilium ( Fig. 2 ), which has therefore been selected as the holotype. The ilium was preserved complete, but lay hidden beneath the scapula, and so was damaged in the field (James Kirkland, personal communication, March 2008). The ilium is preserved in three parts: one provides most of the bone, including the well preserved preacetabular lobe, pubic and ischiadic peduncles and acetabular margin, and the other two provide most of the dorsal margin, giving a good indication of the degree of curvature. The relative positions and orientation of the two smaller fragments can not be definitely ascertained, but they appear to be parts of a single large fragment broken at the point where we have reconstructed them as touching; and if this interpretation is correct then the curvature of the pair indicates which side must be oriented laterally.

The ilium is remarkable in that the preacetabular lobe is relatively larger than in any other sauropod ( Fig. 3 , Table 4; measurement protocol is illustrated in Fig. 4 ), the postacetabular lobe is reduced almost to the point of absence, and the ilium is proportionally taller than in any other sauropod. The ischiadic peduncle is reduced to a very low ventral projection from almost the most posterior point of the ilium. The near absence of the ischiadic peduncle cannot be attributed to damage as the iliac articular surface is preserved. Immediately posterodorsal to this surface is a subtle notch between the peduncle and the very reduced postacetabular lobe. This notch and the areas either side of it are composed of finished bone, demonstrating that the great reduction of the postacetabular lobe, too, is a genuine osteological feature and not due to damage. In regard to the proportionally large preacetabular lobe, the ilium of Brontomerus resembles that of Rapetosaurus ( Fig. 3E , Table 4). However, that taxon has a normal postacetabular lobe and is not proportionally tall. In overall proportions, the ilium of Brontomerus is more similar to the left ilium HMN J1 assigned to Giraffatitan brancai ( Janensch 1961: pl. E: 2) which also has a reduced postacetabular lobe —see also Fig. 3D . However, the ilium of Brontomerus is proportionally taller than that of G. brancai , and its anterior margin comes to a point rather than being smoothly rounded as in that taxon. In Brontomerus , the maximum height of the ilium above the acetabular margin, when measured perpendicular to the longest axis, is 52%; in other sauropods examined this proportion does not exceed 45%, and averages 40% ( Table 4).

As with many sauropods, the preacetabular lobe of the ilium flares laterally. However, in most sauropods this flaring is progressive, so that in dorsal or ventral view the most posterior part of the preacetabular lobe is nearly parallel with a line drawn between the pubic and ischiadic peduncles, and smooth lateral curvature inclines the more anterior parts increasingly laterally, so that the more anterior part is almost at right angles to this line and the ilium appears smoothly doi:10.4202/app.2010.0073

curved in dorsal or ventral view—for example, Apatosaurus Marsh, 1877 ( Upchurch et al. 2004b: pl. 4: D, E), Haplocanthosaurus Hatcher, 1903b ( Hatcher 1903a: pl. 5: 1) and Saltasaurus Bonaparte and Powell, 1980 ( Powell 1992: fig. 17). In Brontomerus , by contrast, the blade of the ilium appears to be “hinged”—deflected laterally at a point directly anterior to the public peduncle—so that the preacetabular lobe is straight in dorsal or ventral view, and directed anterolaterally by an angle of about 30° to the sagittal. In this respect, it more closely resembles the ilium of Camarasaurus Cope, 1877 ( Osborn and Mook 1921: fig. 49) although it differs in other respects.

The Brontomerus ilium is laterally compressed, and unlike most sauropod ilia the dorsal margin is not deflected laterally relative to the more ventral part, so that in ventral view it appears very thin ( Fig. 2B ). It is well established that the long bones of sauropods grow isometrically through ontogeny ( Carpenter and McIntosh 1994; Wilhite 1999, 2003; Ikejiri et al. 2005; Tidwell and Wilhite 2005; Taylor 2009) while their vertebrae undergo significant changes in proportions, lamination, pneumatic excavations, and neurocentral fusion ( Wedel 2003a: 248, b: 352–354). Ontogenetic changes in limb−girdle elements such as the ilium are less well understood due to a paucity of sufficiently well preserved specimens (Ray Wilhite, personal communication, October 2007). Therefore the lateral compression of the Brontomerus ilium may be a juvenile character, with the ilium thickening through ontogeny to support the growing weight of the animal, or it may in fact be phylogenetically significant.

Presacral centrum.—A single presacral centrum, OMNH 66429, was recovered ( Fig. 5 ). Unfortunately, preservation is very poor: the neural arch and all processes have been lost, and the centrum has been greatly crushed dorsoventrally so that the remaining part is essentially flat: the element is 14 cm in both anteroposterior length and transverse width, but no more than 3 cm in dorsoventral depth. The small size indicates that this element belonged to a juvenile. The internal structure of the centrum is visible, however, and consists of fine septa dividing a hollow internal space irregularly into many small camellae. This morphology is characteristic of titanosauriforms ( Wedel 2003b: 354–355). Highly camellate internal structure has not previously been observed in juvenile sauropod vertebrae, but this may be due to sampling bias: so far, all juvenile sauropod vertebrae that have been studied for internal structure have been those of Camarasaurus and diplodocoids, which follow an ontogenetic trajectory in which large, shallow lateral fossae develop into camerae from which smaller accessory camerae and camellae de−

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velop ( Wedel et al. 2000a: fig. 11; Wedel 2003b: 349). The Hotel Mesa presacral suggests that camellate vertebrae may have developed differently in titanosauriforms, possibly by in−situ formation of camellae during pneumatization, as occurs in the camellate vertebrae of birds (personal observation, MJW).

Caudal vertebrae.—OMNH 61248 is a distinctive caudal vertebra with elongated pre− and postzygapophyseal rami ( Fig. 6 ). Apart from the tip of the right prezygapophysis, the element is complete and well preserved. While the centrum is only 11 cm in length, the distance from the prezygapophysis to postzygapophysis is 14.5 cm. The centrum is slightly broader than tall (6 cm compared with 5.5 cm anteriorly, 6.5 cm compared with 5 cm posteriorly) and gently waisted. The neural arch is set forward on the centrum but does not reach the anterior margin. The neural spine is so reduced and so strongly inclined posteriorly as to be all but indistinguishable, and is apparent only as a very low eminence above the postzygapophyses. The postzygapophyseal facets themselves are set on the posterolateral faces of a low process that hangs below the main postzygapophyseal ramus. Chevron facets are weakly present on the posterior margin of the ventral surface of the centrum, but not on the anterior margin. The elongation index of 2.2 indicates a mid−to−posterior position in the caudal sequence for this element, as similar centrum proportions do not appear until about caudal 30 in Giraffatitan brancai ( Janensch 1950: pl. 3).

This vertebra most closely resembles the indeterminate sauropod vertebra BMNH 27500 from the Wessex Formation of the Isle of Wight, figured by Naish and Martill (2001: pl. 33). The Barremian age of that specimen places it about 15 Ma earlier than OMNH 61248. Its neural arch is less elevated than that of the Hotel Mesa specimen, its postzygapophyses project yet farther posteriorly and its prezygapophyses less far anteriorly, and it is very mildly biconvex rather than procoelous; but in other respects, including absolute size, it is a good match for the Brontomerus caudal.

Also included in the Hotel Mesa material is OMNH 27794, a partial distal caudal centrum figured by Wedel (2005: fig. 7.7). This centrum is approximately round in cross−section, about 4 cm in diameter, and internally consists of apneumatic cancellous bone.

Ribs.— The Hotel Mesa material contains several dorsal ribs in various states of preservation but no readily identifiable cervical ribs. The dorsal rib elements include the shaft of a large, flat rib ( OMNH 27762 ), portions of several smaller rib shafts ( OMNH 27763–27765 , 27768 , and others), a flattened rib head ( OMNH 27767 ) and most informatively a small complete rib ( OMNH 27766 , Fig. 7 ). Despite its excellent preservation and apparent lack of distortion, this element is difficult to interpret. Its shaft is straight for almost its whole length and both articular facets are directed dorsally rather than being inclined medially. The tuberculum is directly in line with the main part of the shaft of the rib, and the capitulum is at an angle of about 30° to it. In these respects the rib resembles the most anterior dorsal rib of the Diplodocus carnegii holotype CM 84 (personal observation, MPT; this element was not figured by Hatcher [1901]). We therefore interpret this rib as having probably belonged to the right side of the first dorsal vertebra .

The rib is unusual in other respects, however, most notably that the ventral part of the shaft curves laterally rather than medially. Careful inspection of the bone reveals no indication of distortion or of incorrect reconstruction. It may be possible that in life the thorax was transversely compressed so that the dorsal part of the rib shaft was directed ventromedially and the more ventral part was vertical. Both articular facets are subcircular in dorsal view, and significantly expanded compared with the rami that bear them. On the anterior face of the head, a low ridge arises just below the capitulum and extends down the medial edge of the rib for about 40% of its length.

The head of the rib is also unusual in that a thin sheet of bone connects the rami that support the articular facets, and this sheet extends much farther proximally than in most sauropod ribs. Its precise extent cannot be ascertained due to breakage. The sheet of bone is perforated close to the capitular ramus, and from this perforation a pneumatic cavity invades the shaft of the rib, extending ventrally from within a shallow fossa in the posterior face. Pneumatization of the dorsal ribs is a synapomorphy of Titanosauriformes ( Wilson and Sereno 1998), although pneumatic dorsal ribs are also infrequently present in diplodocids ( Gilmore 1936; Lovelace et al. 2003, 2008).

Pectoral girdle.—OMNH 27761 is a partial scapula, consisting of the blade and part of the anterior expansion, but missing the glenoid region and the remainder of the anterior expansion ( Fig. 8 ). As preserved, the element is nearly flat; but this may be due to post−mortem distortion, and in any case the most strongly curved part of most sauropod scapulae is the anterior part that is missing from this specimen. The gentle curvature preserved in the posterior part of the blade indicates that the element was from the left side. The bone is surprisingly thin in all preserved parts, never exceeding a few cm, in contrast to for example the scapula of Camarasaurus supremus , which is thick even in mid−blade ( Osborn and Mook 1921: fig. 74b). This suggests that the glenoid thickening and the acromial ridge may have been located some distance anterior to the preserved portion, and a reconstruction after the proportions of Giraffatitan brancai ( Fig. 8 ) suggests that about 80% of the scapula’s full length is preserved. The posterior part of the acromion expansion is preserved, however, and is sufficient to show that this expansion was pronounced, so that the maximum dorsoventral height of the scapula was more than two and half times its minimum height, at the midpoint of the blade. The dorsal margin slopes up towards the anterior expansion rather than forming a posteriorly directed “hook” or a distinct acromion fossa between the blade and the acromion process.

The posterior expansion of the scapula is distinctive. In some sauropods, the posterior part of the scapular blade is expanded not at all or only slightly: for example in Omeisaurus Young, 1939 ( He et al. 1988: fig. 41), Apatosaurus ( Upchurch et al. 2004b: fig. 4) and Rapetosaurus ( Fig. 9E ). In others, the ventral margin of the scapular blade is straight or nearly so while the dorsal margin is deflected dorsally to create an asymmetric expansion: for example in Camarasaurus ( Fig. 9C ) and Giraffatitan brancai ( Fig. 9D ). In a few sauropods, however, the ventral margin of the blade is also deflected ventrally, to form a “racquet−shaped” posterior expansion. This is seen in rebbachisaurids and some titanosaurians, e.g., some specimens of Alamosaurus Gilmore, 1922 ( Gilmore 1946: fig. 6). In Brontomerus , the posterior part of the scapular blade is expanded in a characteristic manner: the ventral margin is straight except for a posteroventral doi:10.4202/app.2010.0073

excursion two thirds of the way along the preserved portion, after which the margin continues parallel to its original trajectory, so that the excursion appears as a gentle “step”. The dorsal margin is also “stepped” in this manner, though with two distinct steps rather than one, of which the more anterior is most strongly pronounced. The net result of these features is that the dorsal and ventral borders of the scapula are both straight near the posterior extremity, and that they are subparallel, diverging by only about five degrees in the region just anterior to the rounded end of the posterior expansion. The step in the ventral border is not known in any other sauropod; however, the scapula of Neuquensaurus Powell, 1992 has a stepped dorsal border similar to that of Brontomerus ( Huene 1929 via Glut 1997: 275).

These characters of the scapula must be treated with some caution, however, since this bone appears subject to more variation than any other in the sauropod skeleton: see for example the range of shapes in scapulae of Giraffatitan brancai ( Janensch 1961: pl. 15: 1–3) and in Camarasaurus supremus ( Osborn and Mook 1921: figs. 74–80).

Two small, flat elements OMNH 66431 and 66432 are interpreted as partial sternal plates ( Fig. 10 ). The medial edge of each is identifiable due to its rugose texture which formed the attachment site for cartilage joining the plates to each other and to the sternal ribs. The sternals are anteroposteriorly elongate and mediolaterally narrow: when complete, they were probably at least three times as long as broad, as in “ Saltasaurus ” robustus Huene, 1929 ( Huene 1929 via McIntosh 1990: fig. 16.9L) and proportionally longer than in any other sauropod including Saltasaurus loricatus ( McIntosh 1990: fig. 16.9; Powell 1992: fig. 30). The sternals are crescentic in shape, the anterior and posterior extremities curving laterally away from the midline. This state was considered a titanosaurian synapomorphy by Wilson (2002: 268) but its distribution is more complex in the current analysis, being synapomorphic for Neosauropoda with losses in Flagellicaudata and Camarasaurus .

Figures

Fig. 1. Skeletal inventory of the camarasauromorph sauropod Brontomerus mcintoshi gen. et sp. nov. from the Lower Cretaceous Cedar Mountain Formation of Utah, in left lateral view. Preserved elements are white, missing elements are reconstructed in gray. After a Camarasaurus grandis reconstruction kindly provided by Scott Hartman.

Fig. 2. Left ilium of the camarasauromorph sauropod Brontomerus mcintoshi gen. et sp. nov. from the Lower Cretaceous Cedar Mountain Formation of Utah, type specimen OMNH 66430 in lateral view reconstructed from the three fragments (A), and ventral view (B).

Fig. 5. Damaged presacral vertebra of the camarasauromorph sauropod Brontomerus mcintoshi gen. et sp. nov. from the Lower Cretaceous Cedar Mountain Formation of Utah, OMNH 66429, in dorsal view, as photograph (A) and interpretive drawing (B). Shading indicates air spaces.

Fig. 6. Mid−caudal vertebra of the camarasauromorph sauropod Brontomerus mcintoshi gen. et sp. nov. from the Lower Cretaceous Cedar Mountain Formation of Utah, OMNH 61248 in dorsal (A), anterior (B), left lateral (C), posterior (D), and ventral (E) views.

Fig. 7. First right dorsal rib of the camarasauromorph sauropod Brontomerus mcintoshi gen.et sp. nov.from the Lower Cretaceous Cedar Mountain Formation of Utah, OMNH 27766 in posterior view:head of rib, showing pneumatic invasion of shaft (A) and complete rib, showing laterally directed curvature of shaft (B).

Fig. 8. Partial left scapula of the camarasauromorph sauropod Brontomerus mcintoshi gen. et sp. nov. from the Lower Cretaceous Cedar Mountain Formation of Utah, OMNH 27761, in lateral view, tentatively reconstructed after Giraffatitan brancai HMN Sa 9 (Janensch 1961: pl. 15: 1).

Fig. 10. Partial paired sternal plates of the camarasauromorph sauropod Brontomerus mcintoshi gen. et sp. nov. from the Lower Cretaceous Cedar Mountain Formation of Utah, OMNH 66431 and 66432, in?ventral view.

Fig. 12. Speculative life restoration of the camarasauromorph sauropod Brontomerus mcintoshi gen. et sp. nov. from the Lower Cretaceous Cedar Mountain Formation of Utah. Adult individual (sized according to the referred scapula) protects juvenile (sized according to the holotype ilium) from a Utahraptor: the enlarged femoral protractors may have enabled a powerful kick. Executed by Francisco Gascó under direction from MPT and MJW, reproduced with permission.

Fig. 3. Ilia of sauropod dinosaurs, scaled to same total length. A. Mamenchisaurus hochuanensis Young and Zhao, 1972, holotype CCG V 20401, right ilium reversed, modified from Young and Zhao (1972: pl. 6: 1a). B. Diplodocus carnegii Hatcher, 1901, CM 94, right ilium reversed, modified from Hatcher (1901: pl. 10: 1). C. Camarasaurus supremus Cope, 1877, AMNH 5761 Il. 1, left ilium, modified from Osborn and Mook (1921: fig. 87). D. Giraffatitan brancai (Janensch, 1914), HMN J1, left ilium, modified from Janensch (1961: pl. E: 2). E. Rapetosaurus krausi Curry Rogers and Forster, 2001, holotype FMNH PR 2209, left ilium, modified from Curry Rogers (2009: fig. 39B). F. Brontomerus mcintoshi gen. et sp. nov. holotype OMNH 66430, left ilium.

Fig. 4. Measurement protocol for sauropod ilia as illustrated in Fig. 3 and shown in Table 4. Total length is measured along the longest axis of the ilium; lengths of preacetabular and postacetabular lobes are measured parallel to this axis, and extend from the extremity of the lobe to the anterior margin of the pubic peduncle and posterior margin of the ischiadic peduncle respectively. Supracetabular height is measured perpendicular to the longest axis, and extends from the highest point of the acetabulum to the point level with the highest part of the ilium.

Fig. 9. Scapulocoracoids and scapulae of sauropod dinosaurs, scaled to same length of scapular blade from posterior point of glenoid to posterior margin of blade. A. Mamenchisaurus youngi Young and Zhao, 1972, holotype ZDM0083, left scapulocoracoid, modified from Ouyang and Ye (2002: fig. 22). B. Diplodocus longus Hatcher, 1901, USNM 10865, right scapulocoracoid reversed, photograph by MPT. C. Camarasaurus supremus Cope, 1877, AMNH 5761 Sc. 1, left scapula, and AMNH 5761 Cor. 1, left coracoid, probably associated, modified from Osborn and Mook (1921: figs. 75, 81a). D. Giraffatitan brancai (Janensch, 1914), HMN Sa 9, left scapula, modified from Janensch (1961: pl. 15: 1). E. Rapetosaurus krausi Curry Rogers and Forster, 2001, holotype FMNH PR 2209, right scapula reversed, modified from Curry Rogers (2009: fig. 32). F. Brontomerus mcintoshi gen. et sp. nov. OMNH 27761, left scapula, tentatively reconstructed after Giraffatitan brancai.