Our computational models of ontogeny revealed novel information regarding the life history of Mussaurus patagonicus and its shift in locomotor stance during ontogeny. Our a priori prediction that reduced pectoral appendages and head/neck vs. enlarged pelvic limbs drove the CoM caudally across the ontogeny of Mussaurus was only partly supported. The main segmental influences on CoM, as Fig. 3’s FMM data show, were indeed from a reduced head/neck but more from an enlarged tail than hindlimbs (or reduced pectoral appendages). We inferred that the caudad shift of the CoM from hatchling toward adulthood involved a shift from quadrupedal to bipedal stance for three reasons. First, the CoM of hatchlings was too far forward for the short hindlimbs to place the feet under the CoM in a statically stable bipedal pose (except in the most extreme, more implausible “maximal caudal” model, where the hatchling CoM was <1 femur length craniad to the acetabula; see Table 2). Second, the position of the CoM of Mussaurus hatchling (about halfway along the trunk; and more than one femur length craniad to the acetabula) corresponded to the CoM recently estimated for several quadrupedal sauropods, showing a similar position to that of Camarasaurus4 (Fig. 4) and far from any bipeds, supporting quadrupedal stance in Mussaurus’s early ontogenetic stages. Third, prior analyses have shown that adults, at least, were unlikely to be able to use their forelimbs in locomotion8 as inferred for other early sauropodomorphs (e.g.37,41).

Figure 4 Comparative analysis of whole-body CoM position vs. body mass in Sauropodomorpha, with Mussaurus ontogenetic trajectory data (Fig. 3; Table 2) mapped onto interspecific convex hull-based modelling data from4 (see Supplementary Table S3). All sauropods included clearly were quadrupedal, whereas Plateosaurus and possibly Lufengosaurus are considered bipedal as adults (and have CoM values close to that of the Mussaurus adult). Three theropods (unambiguous bipeds) of varying masses (spline-based model data from4,32) are also included for comparisons. Mussaurus spline-based model data points are shown for the maximal cranial and maximal caudal models (Table 2), which refer to the modifications of body segments in sensitivity analyses representing (as relatively implausible extremes) maximum caudal and cranial CoMs. Full size image

Whilst our study is not a test of the two volumetric methods used, it is useful to discuss the two methods used herein and technical issues that underlay our motivation for using both methods. Both convex hull modelling and spline-based approaches are commonly used for estimating the mass of extant and fossil species, and tend to produce roughly similar body mass estimates; as in this study. Convex hull modelling is quick in comparison to spline-based models and is relatively more objective42. However, we have concerns that convex hull models, especially for sauropsid tetrapods with large tails and pelvic limbs, can produce CoM estimates that are inaccurate for those segments and for the whole body (e.g. if hulled too tight to the tail, an estimated CoM that is too far craniad than a reconstruction based on fleshy rather than skeletal tail anatomy). To date, validation studies of extant tetrapods using convex hulls have focused on mammals and birds40,43,44 that have relatively small tails, not on modelling thick tails that connect to the pelvic limb. One study of an extinct lion also has cautioned that the mass and CoM of the proximal limb are error-prone in convex hull models45 due to the inherent limits of wrapping the bones vs. bulky proximal limb muscles. Prior studies of sauropsid segment dimensions have noted the wide deviation of the actual tail (and pelvic limb) from the actual skeleton38,46,47. Some convex hull-based modelling of tails in dinosaurs seems to have implicitly attempted to compensate for this deviation by extending the hulls from the pelvis to the tail, but methods vary (e.g.39,40). Here we used a different method that hulls the tail as a single entity extending from the pelvis (bounded by the ischium ventrally, transverse processes laterally, ilium and neural spines dorsally and distally the terminal caudal vertebra), which seems to help our overall CoM patterns overlap (for convex hull vs. spline-based models), by producing tail morphologies that are more anatomically plausible in light of existing data from phylogenetic bracketing. Further validation work should be carried out to test and refine best practices for convex hull modelling in such species.

A vexing problem for assessing quadrupedal stance is that the possibility of forelimb usage in hatchlings, in particular, is difficult to evaluate because of the poor ossification of joint surfaces used to infer mobility in adults8. Such poor ossification of joint surfaces is a widespread condition among preserved dinosaur remains of early ontogenetic stages20,30. The close association of multiple articulated skeletons of hatchlings along with eggs and eggshells26 suggests that these individuals may have been altricial and therefore a quadrupedal stance need not have involved efficient quadrupedal locomotion moving far from the nest. There are two more reasons that the inference of quadrupedal stance in the hatchlings is reasonable. First, the preserved craniomedial position of the radius relative to the ulna in the articulated skeleton PVL 4068 (Fig. 1) is congruent with, at least, a semi-pronation of the forelimb8,48. Second, considering the appreciable morphological disparity between Mussaurus’s hatchlings and adults, and assuming a rapid rate of morphological change across ontogeny34, even if adult Mussaurus were incapable of full manus pronation, as previously proposed for some early sauropodomorphs41, that does not mandate that hatchlings were similarly incapable of pronation. Hence overall we contend that our conclusion is the most plausible one in light of all available evidence. Because of uncertainty in the models’ estimates, inferences on the locomotor stance in the yearling are ambiguous. Nonetheless, we speculate that the yearlings at least had a facultatively bipedal stance, based on the ~3–11% closer CoM distance to the acetabula, bringing the CoM within 1 femur length of the hips; unlike in the hatchlings (Figs 2, S1).

Although a recent analysis conducted on ornithischian dinosaurs concluded that a caudally positioned CoM does not necessarily imply bipedal locomotion49, it is still the case that a CoM too far cranial to the hips will mandate quadrupedal stance. Multiple lines of evidence support a bipedal stance in adult specimens of Mussaurus, coincident with its more caudally positioned CoM, including proportionally shorter forelimbs (shifting from forelimb/hindlimb length ratios of 0.76 to 0.55 from hatchlings to adults; Supplementary Table S2), an arched metacarpus, a recurved and medially divergent first digit of the manus, and absence of a fully pronated manus (evidenced by an interlocking radius and ulna)8,41. Moreover, the ontogenetic changes described herein for Mussaurus complement previous studies in which Mussaurus’s adult specimens were inferred as bipeds based on minimum limb bone shaft circumferences10.

The pattern estimated across ontogeny in Mussaurus reveals that at least some early sauropodomorphs experienced a transition from quadrupedal to bipedal within their life history. Although previous studies based on limb proportions suggested that Massospondylus also experienced a locomotor shift during ontogeny28, novel information on inner ear morphology does not support such a locomotor shift31. Other studies have questioned the ability of inner ear morphology to infer head, neck or body orientation in tetrapods50,51, but further analysis of this problem and data on ontogenetic changes of inner ear morphology in Mussaurus would be valuable. Consequently, and considering the available evidence to date, it is not possible to elucidate if the locomotor shift estimated for Mussaurus corresponded to a trend or an exception among early sauropodomorphs. However, if it was actually a trend it contrasts with the available evidence in sauropods indicating a single stance (quadrupedal) across all known ontogenetic stages52. Nonetheless, multiple lines of evidence support an evolutionary change of stance along sauropodomorph phylogeny, from bipedal (in early sauropodomorphs) to quadrupedal (in sauropods/Eusauropoda; Fig. 4), ranging from different studies that concluded adult forelimbs were unable to have acted in a major locomotor role in early sauropodomorphs8,37,41 to an analysis that reconstructed a progressively caudal to cranial shift of CoM through sauropodomorph phylogeny4. One exception to this pattern is the clade Lessemsauridae (either sauropods or close-to-Sauropoda, depending on the Sauropoda definition), whose members’ body masses are estimated at over 10 tonnes, with putatively obligate quadrupedal stance involving heavily built, moderately flexed forelimbs9,10. Biomechanical studies of the latter clade could be particularly informative.

While Fig. 4 shows that all bipedal taxa had a CoM <40% of gleno-acetabular distance craniad to the hips, and no quadrupeds had a CoM <25% of gleno-acetabular distance craniad to the hips, there is an interesting zone of overlap between unambiguous bipeds, quadrupeds, and taxa of ambiguous locomotor stance between these zones. Such ambiguity and overlap of CoM “morphospace” should be more closely examined, especially in the context of limb lengths and other important parameters (e.g. femur length as discussed here), and where feasible across ontogeny and phylogeny.

The ontogenetic shift from quadrupedal to bipedal stance in early sauropodomorphs was previously regarded as providing evidence that the quadrupedal stance in sauropods evolved through paedomorphosis28. Our analyses based on 3D skeletal models revealed a more complex evolutionary scenario. Despite the CoM analysis suggesting a quadrupedal to bipedal ontogenetic shift in Mussaurus that superficially appears as inverse to the bipedal to quadrupedal shift in the phylogeny of Sauropodomorpha, the skeletons of adult sauropods and juvenile early sauropodomorphs are built in different ways and cannot be equated. In particular, our analysis of FMMs revealed major differences in how the relative dimensions of certain body segments of the body plan of Mussaurus developed (Fig. 3). Even though the proportional reduction of the head/neck region in Mussaurus adults, and its correlation with a shift of locomotor stance throughout ontogeny, fit those previously proposed for Massospondylus28, we found that the enlarged tail had the dominant influence on moving CoM caudally in the late ontogeny of Mussaurus, constituting a previously unreported key factor (Fig. 3). Furthermore, our study is the first to analyse the effects of all major body segments on CoM (i.e. using FMMs) and thus locomotor stance during the ontogeny of a non-avian dinosaur. Our results somewhat parallel those of4,32, who uncovered evidence that a reduced tail in theropods on one hand, and an enlarged head/neck in sauropodomorphs on the other, were correlated with craniad shifts of CoM across macroevolutionary transitions. It is noteworthy that the hatchling’s tail was reconstructed from the juvenile’s tail and scaled proportionally. This resulted in a maximal tail size for this ontogenetic stage (with a FMM similar to the yearling’s; Fig. 3) and, despite this, the CoM was strongly cranially positioned. Any smaller tail size would have resulted in an even more cranial CoM, bolstering our finding that a caudal body CoM was largely driven by increasing tail size (along with decreasing head and neck sizes). The tail’s important role in this transition fits its increasing importance in supporting bipedal locomotion via the tail-based caudofemoralis muscle (e.g.53). Thus, although previous studies have emphasized the influence of hindlimb/forelimb length proportions in sauropodomorph stance20,28, our study indicates that the relative development of tail (and neck) was more influential in constraining/determining the locomotor stance in this clade, in both ontogenetic and phylogenetic changes.

The ontogenetic patterns of body shape and CoM in Mussaurus are somewhat counter to those estimated for the theropod Tyrannosaurus (albeit across a narrower ontogenetic spectrum). In the latter taxon, the torso enlarged whereas the limbs reduced relative to body size, concurrent with an approximate craniodorsal shift of the body’s CoM54; as in extant archosaurs38. This contrast indicates hitherto unappreciated diversity in ontogenetic patterns of body shape and CoM in Archosauria. More such studies of biomechanically-linked parameters across growth series of archosaurs would be valuable to infer how much diversity exists.