Long carpals on crocodylomorphs = quadrupedal stance?

Figure 1. Terrestrisuchus is a bipedal basal crocodylomorph with elongate proximal carpals.

Figure 1. Terrestrisuchus is a bipedal basal crocodylomorph with elongate proximal carpals.

Long proximal carpals,
like the radiale and ulnare in Terrestrisuchus (Figs. 1, 2; Crush 1984), distinguish most crocodylomorphs from all basal dinosaurs (Fig. 2).

The question is:
why did long carpals develop? A recent comment from a reader suggested they enabled quadrupedal locomotion. But looking at the proportions of Terrestrisuchus does not inspire great confidence in that hypothesis. Terrestrisuchus has elongate carpals AND it seems to be comfortably bipedal with hands that only descend to the knees. And the pectoral girdle is relatively gracile.

Figure 2. Manus of several crocodylomorphs compared to the basal dinosaur, Herrerasaurus. Not sure what those two bones are on Junggarsuchus as they were cut off as shown when published.

Figure 2. Manus of several crocodylomorphs compared to the basal dinosaur, Herrerasaurus. Not sure how long those two proximal carpals are on Junggarsuchus. They were cut off as shown when published. Since the distal carpals are labeled (dc) I assume  proximal carpals are cut off below them. Oddly the radiale is much smaller than the ulnare if so, or rotated beneath it, unlike the other crocs.

Looking back toward more primitive taxa
provides only one clue as to when the proximal carpals first started elongating: with Terrestrisuchus. The following basal and often bipedal croc taxa unfortunately do not preserve carpals.

  1. Lewisuchus
  2. Gracilisuchus
  3. Saltopus
  4. Scleromochlus
  5. SMNS 12591
  6. Litargosuchus
  7. Erpetosuchus

Phylogenetic bracketing suggests that all
were bipedal or facultatively bipedal. Post-crania is missing or partly missing in several of these specimens.

Gracilisuchus

Figure 3. Gracilisuchus does not preserve the hands or carpals, but was possibly experimenting with bipedal locomotion based on its proximity to taxa that were obligate bipeds. Note the tiny pectoral girdle.

The distal carpals,
wherever preserved (Figs. 2, 3), appear to be small, scarce and flat, the opposite of a supple flexible wrist. So the proximal carpals of crocs comprise the great majority of the wrist, distinct from dinosaurs (Fig. 2).

Figure 3. Alligator carpals.

Figure 3 Alligator carpals. Of course, this is a quadruped that has inherited long carpals from bipedal ancestors in the Triassic.

So… what do other bipedal taxa do with their hands?
Cosesaurus, a bipedal ancestor to pterosaurs, probably flapped, based on the shape of its  stem-like coracoid and other traits. Herrerasaurus, a bipedal ancestor to dinosaurs had elongate raptorial unguals (claws) lacking in any basal crocodylomorph (Fig. 2). Such claws were probably used in grasping prey in dinos… not so much in crocs.

The elongate proximal carpals in crocodylomorphs
appear to extend the length of the slender antebrachium (forearm) of Terrestrisuchus for only one reason at present. The offset lengths of the shorter radius and longer ulna become subequal again with the addition of the longer radiale and shorter ulnare. So there is no simple hinge joint at the antebrachium/proximal carpal interface. So that joint was relatively immobile. The lack of deep distal carpals also suggests a lack of mobility at the metacarpal/distal carpal interface in basal taxa. However in extant crocs, that hinge appears to be more flexible.

Figure 5. Trialestes parts. Note the much larger ulna relative to the radius and the much longer forelimb relative to the bipedal basal crocs.

Figure 5. Trialestes parts. Note the much larger ulna relative to the radius and the much longer forelimb relative to the bipedal basal crocs.

In Trialestes
(Fig. 5) the elongate fore limbs more closely match the hind limbs. So the elongate carpals in Trialestes do appear to enhance a secondarily evolved quadrupedal stance.

Also take a look at
Hesperosuchus, Dromicosuchus, Protosuchus. Saltoposuchus, Dibrothrosuchus, Baurusuchus, Simosuchus, and Pseudhesperosuchus. After long carpals first appeared in Terreistrisuchus, they do not change much despite the many other changes in the morphology of derived taxa. Bipeds have them. Quadrupeds have them. Long-bodied taxa have them, Short-bodied taxa have them.

Some thoughts arise
when considering the first crcoc with elongate carpals, Terrestrisuchus.

  1. At some point in the day Terrestrisuchus probably rested on its elongate pubis bone (the first in this lineage), flexing its long hind limbs beneath itself to do so. In that pose elongate carpals may have been useful in steadying the animal as it balanced on the pubis tip and whenever it rose to a bipedal stance.
  2. A male Terrestrisuchus may have used its hands to steady itself while riding on the back of a female while mating. The carpals were elongated as part of the balancing act performed during this possibly awkward bipedal conjugation.
  3. Coincidentally, the coracoids in crocodylomorphs begin to elongate in this taxon. So freed from quadrupedal locomotion duties, basal crocs may have done some early form of flapping as part of a secondary sexual behavior, long since lost in extant taxa.

So, in summary
I think the elongate carpals developed in crocs with a really long pubis to steady it while resting. Very passive. Not sure what other explanation explains more.

Did I miss anything?
Has anyone else promoted similar or competing hypotheses?

References
Crush PJ 1984. A late upper Triassic sphenosuchid crocodilian from Wales. Palaeontology 27: 131-157.

wiki/Terrestrisuchus

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2 thoughts on “Long carpals on crocodylomorphs = quadrupedal stance?

  1. This is an interesting topic. :-)

    why did long carpals develop? A recent comment from a reader suggested they enabled quadrupedal locomotion. But looking at the proportions of Terrestrisuchus does not inspire great confidence in that hypothesis. Terrestrisuchus has elongate carpals AND it seems to be comfortably bipedal with hands that only descend to the knees.

    It does look pretty comfortable as a biped. You show a strongly flexed shoulder, though, and a pretty strongly flexed elbow. How far could they be extended?

    You also show digitigrade feet which lift the hands even farther from the ground. Why is that?

    And the pectoral girdle is relatively gracile.

    It is, but so is the pelvic girdle.

    Figure 2. Manus of several crocodylomorphs

    Most of them don’t leave any space for distal carpals. This is a common error in published reconstructions of dinosaur hands, too.

    Not sure how long those two proximal carpals are on Junggarsuchus. They were cut off as shown when published. Since the distal carpals are labeled (dc) I assume proximal carpals are cut off below them. Oddly the radiale is much smaller than the ulnare if so, or rotated beneath it, unlike the other crocs.

    Evidently the hand isn’t in strict palmar view, but largely edge-on, so the radiale is almost entirely hidden behind the ulnare.

    Figure 3 Alligator carpals. Of course, this is a quadruped that has inherited long carpals from bipedal ancestors in the Triassic.

    I don’t agree that Alligator “of course” had bipedal ancestors. It’s not impossible, but currently it doesn’t look like any of its ancestors spent much time in bipedal mode; its closest clearly bipedal relatives – some or all of the poposaurs – are nested inside another clade, and so is Terrestrisuchus/Saltoposuchus which may have been facultatively bipedal.

    The elongate proximal carpals in crocodylomorphs
    appear to extend the length of the slender antebrachium (forearm) of Terrestrisuchus for only one reason at present. The offset lengths of the shorter radius and longer ulna become subequal again with the addition of the longer radiale and shorter ulnare.

    I don’t understand what the advantage of this would be, especially to a biped. In a quadruped I can see the advantage of elongate proximal carpals: they allow you to be functionally digitigrade without actually being digitigrade – there’s an extra joint between the shoulder and the ground like in a digitigrade forelimb, which adds elasticity, springiness, for agile locomotion. But in a biped?

    In Trialestes
    (Fig. 5) the elongate fore limbs more closely match the hind limbs.

    Much like in Gracilisuchus, actually.

    At some point in the day Terrestrisuchus probably rested on its elongate pubis bone (the first in this lineage), flexing its long hind limbs beneath itself to do so. In that pose elongate carpals may have been useful in steadying the animal as it balanced on the pubis tip and whenever it rose to a bipedal stance.

    1) Wouldn’t the feet do that anyway?
    2) Why elongate the carpals and not simply the forearm?

    The carpals were elongated as part of the balancing act performed during this possibly awkward bipedal conjugation.

    Why the carpals, and not the fingers and perhaps metacarpals, where the actual gripping happened?

    Coincidentally, the coracoids in crocodylomorphs begin to elongate in this taxon. So freed from quadrupedal locomotion duties, basal crocs may have done some early form of flapping as part of a secondary sexual behavior, long since lost in extant taxa.

    The fact that the coracoid is still so long in extant crocodylians strongly suggests that flapping isn’t the only use for it. Off the top of my head, I suggest stability during quadrupedal locomotion, compensating for the rather narrow sternum and interclavicle.

  2. Re: How far could they be extended? — Don’t know the limits from available drawings.

    Re: digitigrady — metatarsals without expanded ends able to be more tightly bound than in basal dinos + simple hinge ankle joint + limb length disparity (cursorial) further extended by elevating metatarsals + short dino-like torso + relationship to obligate biped and cursor, Scleromochlus. No matching tracks yet and I have not made the soda-straw model yet.

    re: but so is the pelvic girdle. — often no one factor proves anything, but as part of a suite of traits the argument is strengthened.

    re: space for distal carpals. — Maybe they were on their way out. Let’s keep looking for them.

    re: almost entirely hidden — perhaps you are right. Someday hope to do a walk around it.

    re: bipedal crocs — https://pterosaurheresies.wordpress.com/2011/10/23/ten-bipedal-crocs-blog-100/ also I don’t see any quads at the base of the clade.

    re: advantage — I don’t know either. No doubt, like theropods, they were doing something else with their forelimbs once no longer needed for locomotion. Display? Fighting? Mating? Not climbing. Not grasping.

    re: 1) Wouldn’t the feet do that anyway? 2) Why elongate the carpals and not simply the forearm? — I’m speculating. Feel free to jump in. The water’s murky here.

    re: flapping isn’t the only use for it…. stability during quadrupedal locomotion, compensating for the rather narrow sternum and interclavicle. — We should look at the genesis of the long carpals in Terrestrisuchus and maybe earlier (pertinent body parts are missing) for our answer. Afterwards, as in living crocs, its a vestige or a legacy of an earlier competitive advantage that works just as well. Why questions are fun when they click, not when they don’t.

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