Thursday, May 30, 2019

Walk This Way

The right manus of Camarasaurus. Note the columnar arrangement and virtual absence of fingers.
Late last year, I introduced you all to the Lessemsauridae, a group of near-sauropod "prosauropods" that grew unreasonably large—up to 12 tons in Ledumahadi mafube. There’s some disagreement about the posture of these enormous animals: in true, blue Sauropoda, the forelimbs are columnar, the hands are pronated, the weight is bore on the fingertips, and there is a reduction in phalanges and claws. This arrangement is present to some degree even in the earliest true sauropods like Melanorosaurus and Barapasaurus.

The right hand of Plateosaurus. Note the long digits II & III and surprisingly large claws.
By contrast, prosauropods have forelimbs that could not pronate, and their hands are a bit more theropod-like, with elongate digits I-III followed by much shorter, vestigial digits IV & V. The thumb (digit I) is interesting in that the first phalange articulates to its metacarpal at an angle—a “twist” that seems to be the standard configuration in prosauropods going back even as far as Eoraptor. Also popular among prosauropods is an extremely large, recurved thumb-claw. If you go by their hand morphology, it’s clear that prosauropods were still actively using their forelimbs for non-weight-bearing activities. What kind of hands did lessemsaurids have? Unfortunately, that remains a mystery.

But wait, you say, if normal prosauropods weren’t walking around on their hands that must mean they were bipedal…?

Plateosaurus was a perfectly capable biped, from Mallison (2010a).
I know, this is a concept I’m still trying to wrap my head around, even though Bonnan & Senter (2007) and then Mallison (2010a; 2010b) deemed these big herbivores incapable of quadrupedality. Certainly my ingrained mental image of any given prosauropod is basically a mini-sauropod with all four extremities used for walking around, palms flat on the ground. Turns out that would’ve been more or less impossible for somebody like Plateosaurus: the wrist was inflexible, the arm couldn’t pronate, only digits II & III would’ve reached the ground anyway, and the limb proportions would’ve led to some interesting compromises as a quadruped. Thus, I’ve had to say goodbye to that classic prosauropod posture, but old habits die hard, and I have a weirdly difficult time seeing them as bipeds.

(I have a more difficult time understanding how those pot-bellied therizinosaurs got around on two legs, but that’s a topic for another day.)

But somewhere along the line—well, specifically around the origin of Sauropoda—they must have figured out how to walk on all fours. Surprisingly, it’s looking like they did this through paedomorphosis—that is, the retention of juvenile characteristics.

It's so cute!
In 2005, Reisz et al. published on gorgeous, articulated, near-term embryos referable to poster-child prosauropod Massospondylus carinatus. As an adult, Massospondylus was—like its cousin Plateosaurus—an obligate biped. The hatchlings, however, must have been obligate quadrupeds: they have a proportionately enormous skull, horizontally-held neck seemingly incapable of a proper S-curve, proportionately long forelimbs, and weak hind limbs. The baby’s center of mass would have been too far forward for a bipedal posture anyhow—attempting to walk on its hind legs would have caused it to, tragically but I have to assume hilariously, topple forward.

Reisz et al. (2005) also suggest that, due to the large head, weak limbs, and nearly complete absence of teeth in these hatchlings, that Massospondylus babies were altricial and had to be cared for by adults for some amount of time after hatching. 

This idea is reinforced when Reisz et al. revisited the embryos in 2010, noting that in near-term hatchlings of later sauropods and hadrosaurs already have teeth. Notably, proper sauropod hatchlings are assumed to have been precocial in the manner of sea turtles—digging their way out of the nest after hatching and able to survive on their own. Well, you know, those that survived the immediate population bottleneck created by every available carnivore on Hatching Day.* One wonders if altriciality in Massospondylus was derived or represents the basal condition for Sauropodomorpha as a whole.

Restoration of a Massospondylus hatchling from Reisz et al. (2010)
At some point during their lives, though, the juvenile Massospondylus would have developed stronger hind limbs, less-forgiving forelimbs, and switched to a bipedal posture. But how old were the animals when this shift occurred?

Mussaurus growth series from Otero et al. (2019)
A close relative has provided the answer: very young. Otero et al. (2019) just published a paper that specifically addresses this question in Mussaurus patagonicus. The authors studied a group of hatchlings, a yearling, and an adult. Brief aside: the hatchlings were closely associated and found with eggs and eggshells, suggesting they stayed together in the nest. Their joint surfaces are also poorly ossified, which may be more evidence for post-hatching parental care in prosauropods. Anyway, Otero et al. (2019) note that, in one of the hatchling skeletons, the position of the radius and ulna indicate “at least, a semi-pronation of the forelimb.” By the time Mussaurus celebrated its first birthday, however, the center of mass had shifted caudally enough that it could assume a bipedal posture, and the forelimbs begin to take on their adult proportions. At the very least, yearlings were facultative bipeds. 

The biggest reason for this center of mass shift is the development of a long, muscular tail which, in turn, supported a growing M. caudofemoralis muscle anchored to a large fourth trochanter, allowed the hindlimbs to take over. The semi-pronated forelimb of the hatchlings is intriguing, though, and suggests that true sauropods evolved their columnar, pronated forelimbs via retention of juvenile (in this case, hatchling) characters.

It’s funny to think, then, that the largest land animals to ever exist might really be oversized babies.

Dreadnoughtus, a giant baby, by the incomparable Mark Witton.

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