Sauropodomorpha, Huene, 1932

Norman, David B, 2021, Scelidosaurus harrisonii (Dinosauria: Ornithischia) from the Early Jurassic of Dorset, England: biology and phylogenetic relationships, Zoological Journal of the Linnean Society 191 (1), pp. 1-86 : 35-38

Sauropodomorpha

Triassic sauropodomorphs (prosauropods) are generally small-headed, large-bodied facultative bipeds with large, muscular, cantilevering tails and an essentially herbivorous diet ( Fig. 26A ), although, as pointed out by Barrett (2000), omnivory cannot be excluded. Cropped vegetation was orally pulped, after which food was swallowed and then further processed in a gastrolith-laden gizzard ( Attridge et al., 1985). The enlarged herbivore-adapted gut was positioned anterior to the propubic pubes; these latter bones meet in the midline and form a curtain-like wall at the back of the abdominal cavity ( Norman & Weishampel, 1991). The gut and gastrolith-laden gizzard, and its forward position relative to the centre of balance at the acetabulum, was counterbalanced by the massive tail. The abdominal floor was lined by well-developed gastralia ( Fig. 26A ), implying that cuirassal aspiration supplemented costal ventilation and, furthermore, that the raising and lowering of the mass of the gut during cuirassal aspiration could not have been at an overwhelming energetic cost.

Jurassic and Cretaceous sauropodomorphs (sauropods) are extremely large, pillar-limbed quadrupeds with long tails and necks ( Fig. 26B ). They were microcephalous herbivores that raked and/or cropped food into the mouth before swallowing after minimal oral treatment ( Barrett & Upchurch, 1994). The pelvis was mesopubic and the pubes formed a bony wall at the rear of an abdominal cavity that lay in front of the acetabulum. Quadrupedality, an arched, dorsal vertebral column and pillar-like limbs created bridge-like support for a massive gut. Food passed into a stomach that included a substantial gastrolithfilled gizzard and, judged by the space available in the torso, a voluminous (probably multichambered) gut. No gastralia are preserved in sauropods ( Claessens, 2004; Fig. 24 ) and cuirassal aspiration is considered unlikely because it would have involved the raising and lowering of an exceptionally massive gut ( Carrier & Farmer, 2000a). The aspiratory mechanics of sauropods are not well understood, although it has been inferred (because of the presence of postcranial pneumatism) that sauropods had an avian-style flow-through respiratory system (e.g. Sander et al., 2011). Dorsal vertebrae have conventional synovial articulations for their ribs, suggesting that costal aspiration was possible.

Theropoda

Triassic and Early Jurassic theropods are generally small to medium-sized (2–5 m long) bipeds with large, muscular tails; they are considered (by all) to be carnivores and had, as a correlate, much smaller guts than typical herbivores of equivalent size ( Fig. 27A ). The pelvis is propubic but the gut would have been positioned anterior to the centre of balance and was comparatively small, so was unlikely to impair balance or mobility: the tail was an effective cantilever. The thoracic rib articulations were mobile and the abdomen was floored by gastralia; this implies that early theropods/theropod-like dinosauromorphs were capable of using both costal and cuirassal forms of aspiration. From the mid-Jurassic onward, the size-range and variety of theropods increased substantially.

Larger tetanuran theropods ( Fig. 27B ): For example Allosaurus retains the classical body proportions and carnivorous adaptations (large skull, sharp recurved teeth, raptorial forelimbs) of basal dinosaurs, and also displays a well-developed set of gastralia. Other subclades (see below) diversified their body forms to a greater extent:

Ornithomimosaurs: Generally lightly built with small heads and toothless beaks/bills ( Fig. 27C ), resemble living omnivore-carnivore derivatives (ratites). In the case of the structurally similar and closely related alvarezsaurs, their jaws are lined by small teeth, instead of a beak/bill, and some authors have speculatively linked their dental features to those in animals with a myrmecophagous (ant-based) diet ( Longrich & Currie, 2009). Both of these groups were scored as herbivores by Macaluso & Tschopp (2018), yet both groups have lightly built cursorially adapted skeletons, have long arms and grasping hands, are propubic and have long cantilever-like tails. This body configuration is more readily explained if they were comparatively smallgutted, pursuit adapted, carnivore/omnivores. Both groups retain a well-developed set of gastralia.

Therizinosaurs ( Fig. 27D ): Exhibit comparatively small skulls with jaws lined by small leaf-shaped teeth. However, their body proportions include a capacious abdominal cavity, broadly flared iliac blades, an opisthopubic pelvis and a much-reduced tail ( Zanno et al., 2009). Their body form bears a passing resemblance to that seen in herbivorous xenarthrans (ground sloths). Therizinosaurs were scored, entirely appropriately, given their overall cranial and body form, as herbivores by Macaluso & Tschopp (2018). Gastralia have been reported in therizinosaurs.

Oviraptorosaurs ( Fig. 27E ): Are characterized by having medium sized head equipped with short, powerful toothless beaks. The pelvis is mesopubic, so the gut was positioned anterior to their centre of balance; they also have comparatively short tails and the femur lacks a prominent 4 th trochanter. These two latter features are strong indicators of a bird-like alteration to their limb mechanics to compensate for the reduced cantilever effect of the tail. Furthermore, the length and raptorial structure of the forelimbs of oviraptorosaurs ( Norell et al., 2018) represent clear adaptations associated with prey capture (and hence carnivory). Oviraptorosaurs have short, powerful jaws (indicating a strong bite – which could be interpreted either way in relation to diet), and there is a report of gastroliths (in Caudipteryx ). These animals were scored as herbivores by Macaluso & Tschopp (2018).

The discovery of lizard remains in the body cavity of an Oviraptor , as well as those of juvenile troodontid skulls in association with a nest of Citipati , are both arguably suggestive of carnivory in these animals ( Bever & Norell, 2009). Equally, crocodiles also have gizzards with gastroliths and can hardly be argued to be herbivores and, in the absence of teeth, gastroliths may have been important bone fragment processors in the gut of carnivorous oviraptorosaurs. As a general observation, the presence of a gastrolith-laden gizzard associated with an expansive and heavy gut (necessary, if these animals were indeed herbivores), which would have been positioned anterior to the centre of balance, is incompatible with the build, mechanics of balance and indications of locomotor style seen elsewhere in their bodies. Common sense suggests that oviraptorosaurs were carnivores. Oviraptorosaurs also possess welldeveloped gastralia.

Stem-lineage avians ( Fig. 27F ): Finally, among the paravian–avialian (stem-lineage birds), of which Deinonychus ( Fig. 27F ) is a well-known example, the predatory adaptations seen in the skull, as well as those of the fore- and hindlimbs, are self-evident ( Ostrom, 1969). The tail is long, but is thin and light, the femur lacks a 4 th trochanter and the pubis is fully retroverted. The balance and pose of this animal would have been bird-like and necessitated a ‘neoacetabular’ knee-joint; an adequate locomotor stride would have been achieved by lengthening the tibia–fibula and metatarsus. The posture depicted in Fig. 27F (a common style of reconstruction of this animal) is not accurate to these principles because it indicates that the femur swung through a pendulum-like arc, which it could not have done because it was ‘suspensory’. Gastralia are well developed in dromaeosaurs. This general paravian body pose would have been reproduced in Archaeopteryx [the so-called ‘first bird’ – which also exhibits gastralia, even though these bones are lost in true, flight-capable (ornithothoracine) birds].

A broader consideration of the morphofunctional organization and fossil evidence that can be applied to a diversity of theropods suggests that the dietary assignments that have been proposed in the recent past are, in many instances, open to doubt. Furthermore, in each of these theropod taxa, gastralia are known to be present, indicating that these animals had the potential to use cuirassal aspiration as a component of their respiratory repertoire. It is only among the more derived avialians that a large thoracic keel evolves, gastralia are lost, the ventral pelvic bones separate along the midline and the tail becomes so abbreviated that it forms a pygostyle – a suite of structural modifications that allow true birds to retain a bipedal pose and locomotor capacity in the complete absence of a cantilever tail.

CONCLUSIONS

The range of anatomical configurations exhibited by the entire dinosaurian clade includes obligate bipedality, facultative bipedality and quadrupedality, and obligate quadrupedality. These locomotor postures are co-dependent on the positioning of the gut (and its mass), as well as general pelvic construction, irrespective of the respiratory system. The structural adaptations associated with the feeding apparatus have a direct bearing on diet and gut structure in these animals, which in turn influences the balance and pose of the body. Inferences about the dietary preferences of these animals require a holistic approach that incorporates jaw morphology, tooth shape, skull size, body proportions, locomotor mechanics, limb functionality and, rarely, the fortuitous discovery of fossilized gut contents. Using this range of criteria, there is justification to doubt the scoring of the diets assigned to the various theropod subclades considered by Macaluso & Tschopp (2018). There is also clear anatomical evidence that contradicts the assignment of respiratory mechanisms among theropod dinosaurs proposed by Macaluso & Tschopp (2018).

Drawing broad physiological and functional comparisons between such disparate body forms as ornithischians, sauropodomorphs and theropods risks conflations and/or misunderstandings. Even among closely related and persistently bipedal theropod dinosaurs that all possess gastralia (and by implication cuirassal aspiration), taxa are variously specialized. Some (e.g. ornithomimids) reduce their dentitions, leading to the evolution of a bird-like keratinous beak/bill; some (e.g. therizinosaurs) shorten and reduce the mass of the tail and, consequently, partially or completely retrovert the pubis; some (e.g. ‘paravialians’) modify the pose of the hindlimb through the evolution of a suspension-style femur and alter the musculature that protracts and retracts the legs. However, these configurations are not consistent across all taxa and instead indicate a suite of adaptive morphologies that require explanation in light of the total body plan and a range of additional evidence that enhances the interpretation of the putative biology of each subclade ( Fig. 28 ). The evidence available cannot be used to support the notion that there is a consistent, phylogenetically mappable, pattern implying that the aspiratory mechanism was the sole ‘evolutionary driver’ of pelvic morphology among dinosaurs, as argued by Macaluso & Tschopp (2018).

Figures

Figure 26. Sauropodomorph skeletal forms. A, Plateosaurus (a prosauropod). B, Nigersaurus (a sauropod). Gastralia are present in the prosauropod, but no sauropods possess gastralia. Images kindly provided by Scott Hartman who retains the copyright of each. Scale bar in centimetres.

Figure 27. Theropod skeletal forms. A, Herrerasaurus (a dinosauriform, stem-dinosaur or a basal theropod – according to various analyses). B, Allosaurus a tetanuran theropod – see Fig. 28). C, Ornithomimus a coelurosaur. D, Nothronychus, a coelurosaur. E, Oviraptor, a maniraptoran. F, Deinonychus, a paravian. Herrerasaurus and all other theropods possess gastralia. Images kindly provided by Scott Hartman who retains the copyright of each. Scale bar in centimetres.

Figure 28. Simplified theropod phylogeny. All tetanuran theropods have gastralia; these are only lost in true birds (n.gast). The orientation of the pubis varies across these clades and is dependent upon multiple factors: dietary preference (herbivory, omnivory, carnivory); body proportions (notably the reduction in length and mass of the tail); and more specific locomotor adaptations and habits of individual theropods within each subclade. There is no simple and unambiguous correspondence between pelvic structure and posture, habit, locomotor style or putative diet and respiratory mechanics. Dinosaur images kindly provided by Scott Hartman, who retains the copyright of each. As illustrated here these animals are not strictly to the same scale. The raptor silhouette was made available through the following website: http://clipart-library.com/bird-silhouette.html

Figure24. Asimplified phylogenyofdinosaurs(afterBaron et al., 2017b). Note that gastralia are lost independently in sauropods and ornithischians but retained in all other clades [being also lost independently in the derived, powered flight-capable, Theropoda (= birds)].