The archosaurs or "ruling reptiles" were the dominant group of vertebrates for most of the Mesozoic era, diversifying over a quarter of a billion years to span an incredible variation of body plans, sizes an ecologies - from sauropods to hummingbirds. Inevitably, this means that the group had a great diversity in limb morphologies and functions.
A lot of aspects of the functional morphology of archosaur limbs are important in establishing aspects of their appearance, behaviours and palaeoecologies. The range of any given joint is a particularly important factor in reconstructions of limb motion, posture and ecology. An example of this was White et al (2015), which studied the forelimb in the megaraptoran Australovenator - the study found that Australovenator had a humeral ROM much greater than other theropods its size and unguals that were capable of hyperextension. From this, they hypothesised the forelimbs were principally used in prey capture - unusually among large theropods.
However, these studies are complicated by the fact that soft tissue, which greatly affects range of motion is mostly lost in the fossil record (aside from occasional discoveries of calcified articular cartilage).
A lot of inaccurate interpretations of the appearance and behaviours of fossil archosaurs have arisen from not taking soft tissues into account. A particularly notorious example is the inaccurate hypothesis that sauropod dinosaurs lived in swamps - as seen in every old children's book about dinosaurs (an example seen below). How did it come about, however?
Y'see, early observations of the seemingly poorly ossified ends of sauropod limb bones were used as evidence that the limbs weren't able to support the body's weight on land for extended periods of time - this erroneous notion was held until the 1970s, where various aspects of anatomical and sedimentological data were used to surmise that sauropods were fully terrestrial. Still, it took until 2007 for an articular capsule to be identified on a sauropod limb joint when one was found on the end of the humerus of a specimen of the basal eusauropod Cetisauriscus - it was likely that a similar capsule was present on the limbs of all sauropods, increasing the forelimb length by 6-10%.
This is a case study on how taking soft tissues into account can change dinosaur reconstructions - this discovery revealed that the shoulder height and hip height of sauropods had been previously underestimated, indicating that they were taller than previously thought.
In recent years, there's been a much-needed move into including estimates of missing soft tissue - Thulborn (1982) used a correction factor of 9% when studying speeds and gaits in dinosaur taxa, derived from the bone spacing in a hindlimb fossil in a small ornithopod dinosaur and Hutchinson et al (2005) used correction factors of roughly 5-10% when studying running speed in Tyrannosaurus. However, there still isn't any definitive correction factor, nor have potential problems associated with ignoring this have been identified - it's therefore important to find out whether osteology alone can be used to figure out these constraints.
Fortunately, the extant phylogenetic bracket (EBP) of fossil archosaurs - crocodylians and birds - and osteological correlates on fossil remains are rather useful in this regard. An area of research where this has been pretty helpful is reversions to quadrupedality in at lest four non-avian dinosaur lineages (once in sauropodomorphs and three times in ornithiscians) and the osteological changes associated with its evolution. Whilst initially, a bunch of features, based on comparisons with large mammals, were used to indicate quadrupedality, phylogenetic distribution analysis found that many of them were useless.
From dissection of crocodillian pelvic muscles and the development of thigh muscles in chick embryos, Romer concluded that transverse broadening of the illium and reduction of the postpubis in some ornithiscians were related to the evolution of quadrupedality. It turned out Romer was pretty ahead of his time - as more recent studies have concurred with his assessments that the evolution of quadrupedality involved significant reorganisation of pelvic musculature. Many of these changes were acquired convergently at different times and had different functions - for instance, ankylosaurs and hadrosaurs had laterally projecting acromial processes, but used their forelimbs in very different ways.
So... does soft tissue increase or decrease ROM at any given joint? The answer varies - and extant analogies can produce conflicting results. A particular example of this is the debate about the posture, elevation and flexibility of the necks of sauropodomorph dinosaurs.
Early works depicted sauropod necks as very mobile, with an avian-like flexibility - however, in the 1980s, there was a growing argument that sauropod necks were largely inflexible (in particular, Kent Stevens and Steven Parrish's Dino-Morph studies in the 1990s claimed that Diplodocus could barely raise its neck above the horizontal, which influenced the depiction of the animal in the BBC series Walking with Dinosaurs), with the presence of soft tissue and cartilage limiting neck ROM.
However, Stevens and Parrish's results have been criticised in recent years for setting arbitrary zygapophyseal safety limits for disarticulation and underestimating the role of soft tissue. Mike Taylor and Matt Wedel in 2013, using CT scans of fossils and cartilage:bone ratio in extant birds to extrapolate the anteroposterior length of sauropod zygapophyseal facets with cartilage found that including cartilage increased dorsoventral flexibility, meaning that sauropod necks were longer, more elevated an more flexible than bones alone would indicate.
Further complicating matters, a 2020 study into the neck ROM of the basal eusauropod Spinophorosaurus (and comparing it to modern giraffes) found that, contrary to previous assertions bone articulation in the ONP (osteologically neutral pose) can be used to estimate intervertebral space accurately.
Another problem is whether articular cartilage faithfully represents the bone underneath - whilst adult birds have relatively thin layers of articular soft tissues that match the bone morphologies underneath, crcodiles have thick layers of articular soft tissues throughout life that do not match the bone morphology underneath. Based on osteological correlates (and incongruent, rugose bony joints in fossils) for thick soft tissue in fossil archosaurs, the latter condition is the most likely for fossil archosaurs
So what can we conclude? Whether or not soft tissue increases or decreases ROM at any given joint is still somewhat of an unclear question - and the answer is likely to vary. Despite these pitfalls, the practice of manipulating archosaur limbs to obtain ROM is a growing area of interest. So what happened when I studied this? See below...
My own research
As part of my mPhil research, I recorded the ROM of the third digit of an ostrich foot at progressive stages of dissection. The aim of this was twofold - a) to assess the effectiveness of my workflow and b) to evaluate the effects of the removal of soft tissue on pedal ROM and possible inferences for fossil bipedal archosaurs.
The results found that, overall, ROM increased as soft tissues were removed - with removal of underlying musculature having a much larger effect on ROM than removing integument (both increases were statistically significant). Significant variation was observed within the phalanges - phalange 1 had the highest ROM compared to the more distal ones (with a greater increase in flexion as opposed to extension), with the digit 3 claw having the lowest ROM.
Ostriches are among the most cursorial of living birds - they are recognised as both the fastest extant bipeds and the ones with the highest capacity for long-distance running. Many aspects of these results - particularly the similarities and differences between the individual phalanges can be explained as adaptations for high cursoriality, with phalange 1 having a higher ROM due to the role of metatarsophalangeal joint (and various ligaments and tendons used in elastic energy storage) in energy storage and release, whilst the digit 3 claw serves as a rigid element to provide forces at push-off and serves as a positional anchor during high-speed locomotion.
This opens up an interesting avenue of research - using pedal ROM (based on both the EPB and osteological correlates on fossil remains) as a means of assessing degrees of cursoriality in fossil bipedal archosaurs. Of course, a broader comparison of pedal ROM in a broader range of avian taxa (including cursorial and non-cursorial taxa) would be useful in this regard, but that is sadly beyond the scope of this study.
Comments
Post a Comment