Astragalus and Tibia
FIGURE 10.2 Important characters for Clade Sauropodomorpha: distal part of the tibia covered by an ascending process of the astralagus, short hind limbs in comparison to the torso length, spatula-like teeth with bladed and serrated crowns, 10 elongated cervical vertebrae along with 15 dorsal vertebrae (25 presacrals), large digit I on manus.
■ Thin and flat (spatula-like) teeth with bladed and serrated crowns.
■ Minimum of 10 cervical vertebrae that are typically elongated and 25 pre-sacral vertebrae.
Sauropodomorphs had small skulls in relation to their postcranial skeletons, with the skull being less than half the femur length. This disparity is especially apparent in larger members of the clade.
Modifications to the limbs and torsos of most sauropodomorphs reflect adaptations to load-bearing, whether it was as bipedal or quadrupedal animals. Most sauropodomorph species tended toward sizes of at least several metric tons, so such adaptations were necessary for the more massive species. None approached the smaller sizes exhibited by some theropods, such as the troodontids, compsognathids, and feathered coelurosaurs (Chapter 9). Accordingly, femurs, tibia, humeri, radii, ulnas, metatarsals, metacarpals, and phalanges were normally robust, although with some exceptions. In some cases the proximal bones reflect sites for the attachment of huge muscles (see Fig. 5.11). Those sauropodomorphs that were obligate quadrupeds should have had stout metacarpals and phalanges on the manus adapted for bearing weight. In contrast, if any of the phalanges on the manus seem more delicate and adapted for grasping, then a sauropodomorph was more likely bipedal or at least facultatively bipedal.
The relatively puny teeth and jaws of sauropodomorphs were definitely not adapted for much grinding or other oral processing of plant material. The small skulls, with their lack of evidence for attachments of large masticatory musculature, corroborate their inability to chew much food. Instead, the teeth and jaws seemed more suited for raking and shearing foodstuff before sending it down often-long necks to the rest of the alimentary canal. The long torso of most sauropodomorphs relative to their hind-limb lengths also provided more room for a gut that digested large amounts of plant material. This adaptation was reflected by prodigious body sizes beginning in the Middle Jurassic and lasting until the end of the Cretaceous.
Because sauropodomorph skulls were disproportionately the smallest in comparison to body size among all dinosaurs, the observation that they had the lowest EQs should also come as no surprise. Sauropodomorphs also had an anatomical predisposition to a taphonomic bias. Because their skulls were only held in place by a small cervical vertebra, the atlas, they were prone to detaching and becoming separated from the rest of their voluminous bodies. This resulted in numerous headless sauropodomorph skeletons. One hypothesis proposed previously for the rarity of sauropodomorph skulls was that predators preferentially ate them for their brain matter. However, this assertion is belied by the extremely small amount of brain matter that would have been gained from such discriminatory eating. A more likely explanation is that both physical and biological processes disaggregated the small, relatively delicate bones of a typical sauropodomorph skull once it was separated from the rest of the body. This left sturdy vertebrae and limb bones as the most likely candidates for burial, permineralization, and subsequent preservation (Chapter 7). Indeed, skull parts are represented in only 24 of the more than 100 genera of sauropods.
Pleurocoels were lateral spaces on the vertebrae that lessened the density of these already weighty bones, thus lightening their skeletal structure. These served a function similar to pneumatization in theropods (Chapter 9) and they similarly may have been filled with air sacs. Regardless, these structures aided in decreasing the mass per unit volume of the vertebrae. Transverse processes, neural spines, and chevrons, which were elongate processes on the ventral surfaces of the caudal vertebrae, added to the ornate appearance of many sauropodomorph vertebrae. These features provided attachment sites for musculature but in the case of chevrons also prevented damage to veins and arteries in the tail. With such complex and variable forms, sauropodomorph vertebrae are useful for identifying species, especially with regard to their centra. For example, a sauropodomorph vertebra that has the "positive" (ball) part of the centrum anterior and the "negative" (socket) part posterior is termed opisthocoelus. The opposite situation is called procoelous. If both parts are sockets, then it is called amphicoelous. Simple identification of such conditions in vertebrae, with no other body parts present, can immediately help paleontologists narrow down the possible choices of sauropodomorphs to which the bones belong.
An interesting logistical problem associated with the study of sauropodomorphs is related to how they are normally the largest of dinosaurs, either as solitary skeletons or in an assemblage. Consequently, they are the least likely dinosaurs to be recovered from their discovery site and studied later in more detail. When given a choice between transporting a 1-meter long theropod skeleton and a 20-meter long sauropod from a remote field area, a paleontological crew will have no difficulty coming to their decision. So, although sauropod bones are relatively common in a few areas of the world, sauropodomorphs have not received as much detailed
description as theropods. Fortunately, some spectacular sauropodomorph skeletons were recovered from sites in remote areas of Argentina, Tanzania, Egypt, Mongolia, and China, and these finds were brought back to laboratories and museums. As a result, paleontologists have a better understanding of sauropodomorph evolutionary history and paleobiology than would have been possible from only field descriptions.
A combination of body and trace fossil evidence unique to sauropodomorphs can aid in further exploration for their remains and can better illuminate their lifestyles. Sauropodomorph trace fossils, particularly tracks, are well documented, even from areas where their bones are uncommon (Chapter 14). In fact, both prosauropod and sauropod tracks have been reliably identified. Some of these tracks are the largest ones ever left by an animal in any environment in the history of the Earth. Wide, deep, and semi-circular pothole-like features in Mesozoic rock are difficult-to-miss clues to the former presence of a sauropodomorph (see Fig. 14.4B). In places where large numbers of them walked, the disturbed sediment was permanently deformed. Other trace fossils of sauropodomorphs include gastroliths, which are potentially very abundant when recognizable. Gastroliths have been found in at least one sauropod skeleton and are also associated with some prosauropod skeletons. Where bones or tracks were not preserved, the concentration of gastroliths in some deposits may be the only sign that sauropodomorphs were in an area. Sauropodomorph toothmarks are unknown, and some coprolites, which are appropriately rather large, have been only tenuously attributed to this group. Hence, neither of these trace fossils are currently useful for studying sauropodomorphs. In contrast, spectacular examples of sauropodomorph nests, allied with body fossils such as eggs, bones of hatchlings, and embryos, have been described recently. Such trace fossils can thus provide evidence of a former sauropodomorph presence in areas where adult bones may be absent.
Details of their fossil record show that size is not all that matters in the definition of sauropodomorphs. We gain a better picture of sauropodomorphs as real animals through the fossil remains of their:
1 often-large bodies coupled with small heads;
2 tooth and jaw arrangements;
3 limb structures;
4 complex vertebral anatomy; and
5 tracks, gastroliths, nests, eggs, and remains of young offspring.
In many instances, whether dealing with prosauropods or sauropods, paleontologists have enough material to study. Nonetheless, the discovery of new sauropodomorph species, especially with attached skulls, is always welcomed and provides yet more insight into their uniqueness in the history of animals.
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