Carpus evolution in human ancestry back to basal reptiles

Out of 3400 prior posts
only two prior posts focused on carpals. One looked at the prepollux (radial sesamoid) of pandas and the pteroid + preaxial carpal of pterosaurs. Two looked at whale carpals here.

At present
the large reptile tree (LRT, 1825+ taxa) includes relatively few carpal traits, and none related to the migration of the pisiform and carpal 4 in mammals (see below). Crocodylomorphs elongate the proximal carpals. Many taxa do not ossify the carpals. As mentioned above, fenestrasaur centralia migrate  to become the pteroid and preaxial carpal in pterosaurs. So some carpals are more interesting than others.

FigFigure 1. Diplovertebron right manus dorsal view. Carpal elements colored.

Figure 1. (Left) Diplovertebron right manus dorsal view. Carpal elements colored. (Right) Thrinaxodon right manus dorsal view. Some elements rotated to fit reconstruction. Some phalanges are reduced to discs in Thrinaxodon on their way to disappearing in mammals.

I was also interested
in the origin of the styliform process on the human ulna. It is located where the pisiform is located in Diplovertebron (Fig. 1) a basal archosauromorph amphibian-like reptile. And thus began a look at sample taxa in the lineage of humans.

The next step
was the basal cynodont, Thrinaxodon (Fig. 1). Here the elements are larger, link closer to one another and are better ossified. Some phalanges are reduced to discs in Thrinaxodon on their way to disappearing in mammals.

Figure 2. Right manus of the platypus, Ornithorhynchus and early therian, Eomaia. Carpal elements colored.

Figure 2. Right manus of the platypus, Ornithorhynchus (left) and early therian, Eomaia (right). Carpal elements colored. Note the disappearance (or fusion) of distal tarsal 4 in Eomaia along with the centralia.

The next step in carpal evolution is represented by the basalmost mammal,
Ornithorhynchus (Fig. 2), the platypus. Here distal tarsal 5 is ventral to the lateral centralia. The pisiform is tiny. The radiale and ulnare completely cap the radius and ulna. The platypus is a highly derived monotreme, not a basal taxon.

The enlargement of the distal radius width
relative to the distal ulna width begins with Eomaia (Fig. 2), a basal therian. So does the enlargement of distal carpal 5, taking the place of distal carpal 4.

The migration of tiny distal 4 to the palmar surface
is documented in the evolution of human carpals (Fig. 4), but probably originated with Eomaia (Fig. 2) where distal tarsal 4 is not diagrammed.

At this point it is worth noting
that mammal carpals have different names than those of other tetrapods. Here are the mammal homologs (which we will ignore):

Proximal Tarsals:

    • Radiale = Scaphoid (lavendar)
    • Intermedium = Lunate (tan)
    • Ulnare = Triquetrum (dull pink)
    • Pisiform = Pisiform (yellow green)

Centralia

    • Medial Centralia = Prepollex (blue gray)
    • Lateral Centralia = Lateral Centralia (blue gray)

Distal Tarsals:

    • DT1 = Trapezium (yellow)
    • DT2 = Trapezoid (orange)
    • DT3 = Capate, magnum (green)
    • DT4+5 = Hamate, unciform (4= blue, 5=purple)
Figure 3. Right manus dorsal view of basal tree shrew, Ptilocercus (left), and basal lemur, Indri (right). Carpal elements colored.

Figure 3. Right manus dorsal view of basal tree shrew, Ptilocercus (left), and basal lemur, Indri (right). Carpal elements colored.

The next step in carpal evolution is represented by a basal placental,
Ptilocercus (Fig. 3), a tree shrew close to the base of the gliding and flying mammals. The fusion of distal tarsal 3 to the medial centrale is seen in Ptilocercus and its descendants. The ulna has a styloid process and the pisitorm extends laterally. Distal tarsal 1 is medially elongate to support a diverging thumb, further supported by the medial centralia.

Turns out the styloid process of the ulna
is not a fused carpal, but a novel outgrowth of the distal ulna appearing in basal placentals. The styloid process may have something to do with the ability of basal placentals to laterally rotate the manus for tree climbing in any orientation, including inverted, and to create a stop to prevent further rotation. Bats take this ability to its acme during wing folding.

Figure 4. Manus of human (Homo) in dorsal (left) and ventral/palmar (right) views. Carpal elements colored.

Figure 4. Manus of human (Homo) in dorsal (left) and ventral/palmar (right) views. Carpal elements colored. Carpal 4 and pisiform palmar only. Compare to Diplovertebron (Fig. 1) in which so little has changed, including relative finger length.

The final step in carpal evolution
takes us from the lemur, Indri (Fig. 3) to the human, Homo (Fig. 4). Here a ventral (palmar) view of the manus is also provided so we can finally see the ultimate destination of distal tarsal 4.

Before finishing this blog post
scroll back and forth between figures one and four to see how close the human hand and all of its proportions so greatly resembles that of a very basal ampibian-like reptile. Even the relative finger length is the same. This is probably the most important takeaway today. The LACK of change is the news story here. Dinosaurs, horses and snakes cannot make the same statement.

There is no reason to continue using
the mammal specific identification of the carpals in paleontology when those bones are homologs to tetrapod wrist bones going back to the Devonian. Medical communities should also start using tetrapod homologs and let the analog identities fade into history.

Simply put:
There are five distal carpals named one through five in tetrapods. Some of them fuse with other carpals. There are three centralia. Some of these fuse with other carpals. Tetrapods have three proximal carpals. Their names are easy. The radiale is on the radius. The ulnare is on the ulna. The intermedium is intermediate between them. These tend not to fuse with other carpals, at least in basal placentals. And finally the pisiform appears by itself on the lateral margin sometimes in contact with the distal ulna sometimes not.

On a similar note,
we supported earlier efforts to provide tetrapod homologs for fish skull bones here. Make things simple. There is enough hard work out there without needlessly translating bone identities.


References
Hamrick MW and Alexander JP 1996. The Hand Skeleton of Notharctus tenebrous (Primates, Notharctidae) and Its Significance for the Origin of the Primate Hand. American Museum Novitates 3182, 20pp.
Kielan-Jaworowska Z 1977. Evolution of the therian mammals in the Late Cretaceous of Asia. Part n. Postcranial skeleton in Kennalestes and Asioryctes. In: Z. Kielan-Jaworowska (ed.) Results Polish Mongolian Palaeont. Expeds. VIII. – Palaeont, Polonica, 37, 65-84.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Salesa MJ, Antón M, Peigné S and Morales J 2005. Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas. Proceedings of the National Academy of Sciences of the United States of America. abstract and pdf

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

Is the prepollex (radial sesamoid) analogous to the pteroid?

The migration of the central carpals
(the medial and lateral centralia) is today’s topic. We’re going to wonder if these carpals migrated to the medial wrist twice, in mammals and fenestrasaurs (including pterosaurs, Peters 2009).

In basal reptile, like Haptodus (Fig. 1), the two centralia are entirely within the wrist and articulate between the distal and proximal carpals. So that forms our evolutionary baseline.

Figure 1. Carpal evolution from the basal (plesiomorphic) condition represented by Haptodus, to the transitional taxon, Biarmousuchus, to the human carpus where all the bones are renamed. Also note the reduction and disappearance of manual 3.2, m4.2 and m4.3. Left and right images from Peters 1991. Central image refigured.

Figure 1. Carpal evolution from the basal (plesiomorphic) condition represented by Haptodus, to the transitional taxon, Biarmousuchus, to the human carpus where all the bones are renamed (see text). Also note the reduction and disappearance of manual 3.2, m4.2 and m4.3. The two centralia are in gray, absent in humans.

If you’re like me,
the carpal bones are the last ones you learn. They’re small. They’re round. They used to be not that interesting. However, while I’m writing and illustrating this, I’m learning my carpals in a primary fashion and am finding them fascinating.

As you already know…
the carpus evolves in distinct ways in various lineages.

In birds,
a derived arced carpus permits wing folding.

In crocodylomorphs,
the proximal carpals (radiale and ulnare) become greatly elongated.

In tritosaur lizards
The carpus is unossified in Huehuecuetzpalli, likely as the two centralia (in pterosaurs renamed “pteroid” and “preaxial carpal”) were migrating to the medial surface. More on this below.

In legless taxa
While you might think the wrist bones would be among the first to disappear in taxa that have vestigial or absent limbs, that is not so in Adriosaurus, a sister to the ancestor of many snakes.

The synapsid/mammal carpus
In most therapsids (Fig. 1) distal tarsals 4 and 5 become fused. Exceptions include Titanophoneus and Galechirus. The pisiform (in red) becomes enlarged in certain therapsids and mammals (Fig. 4), but is reduced in humans. The medial centralia either disappears (and the prepollex appears de novo) or the medial centralia becomes the prepollex (aka radial sesamoid). The lateral centralia disappears in humans, but not in other primates.

In frogs
The mammal (mole, lemur, elephant, panda) prepollex is not homologous with the amphibian prepollex found in certain frogs. More on that here.

In mammals the carpals change names.
Perhaps this is one other reason why they are the last bones to be learned. Coloring them really helps. Here are the translations.

Proximal Tarsals:

  • Radiale = Scaphoid (colored green in all figures here)
  • Intermedium = Lunate (amber)
  • Ulnare = Triquetrum (pink)
  • Pisiform = Pisiform (red)

Centralia

  • Medial Centralia = Prepollex (colored gray)
  • Lateral Centralia = Lateral Centralia (colored gray)

The question is: is the medial centralia the same bone as the prepollex? The medial centralia of Asioryctes (Fig. 2, a basal mammal) appears to be split in two (which may not be the case in reality), but it retains the configuration seen in Haptodus and Biaromosuchus (Fig.1)

Distal Tarsals:

  • DT1 = Trapezium (all colored lavender here)
  • DT2 = Trapezoid
  • DT3 = Capate
  • DT4+5 = Hamate
Figure 2. Asioryctes (basal placental mammal) carpus. Prepollex in gray between scaphoid (=radiale) and trapezium (distal tarsal 1).

Figure 2. Asioryctes (basal placental mammal) carpus. Prepollex in gray between scaphoid (=radiale) and trapezium (distal tarsal 1). The ventral scaphoid (= radiale in green) has a ventral tuberosity (upper left). Red oval is the presumed, but missing, pisiform. From Kielen-Jaworowska 1977.

Basal Primate
The hand of Notharctus (Fig. 3), an extinct lemuroid, is transitional between that of Asioryctes and Homo (with many other taxa in-between both!). Note the presence of the prepollex (in gray) and the absence of a medial centralia (like Clark Kent and Superman, you never see them in the same room together). Here the prepollex (medial centralia) is separated from the lateral centralia by a distal tarsal 1 process.

Figure 3. Notharctus (basal primate) wrist elements. The prepollex extends medially.

Figure 3. Notharctus (basal primate) wrist elements. The prepollex extends medially (in gray). Otherwise only one centralia appears here (also in gray), if the prepollex is not the medial centralia.

Pre-panda and panda thumbs
Simocyon (Salesa et al. 2005, a Miocene pre-panda) has a prepollex (radial sesamoid) and Ailuropoda (giant panda, Fig. 4) expands this to create a “false thumb” extending outside of the wrist proper. The similarity in morphology to the carpus of Notharctus (Fig. 3) is notable. Apparently in pre-pandas the lateral centralia became lost or fused to the radiale (= scaphoid, compare to Figure 3). However, the medial centralia (prepollex, radial sesamoid, “rs,” in gray) is radically enlarged in the red panda (Ailuropoda). This may not be a sesamoid, but a centralia after migration, as in pterosaurs.

Figure 4. Metacarpals and carpals of Simocyon (Miocene) and Ailuropoda (Recent) from Salesa et al. 2005. Note the enlargement of the prepollex (radial sesamoid, gray) and otherwise the lack of centralia. The enlargement of the pisiform is interesting and potentially confusing, but not pertinent to the present discussion as it emerges from the lateral wrist.

Figure 4. Metacarpals and carpals of Simocyon (Miocene) and Ailuropoda (Recent) from Salesa et al. 2005. Note the enlargement of the prepollex (radial sesamoid, gray) and otherwise the lack of centralia. The enlargement of the pisiform is interesting and potentially confusing, but not pertinent to the present discussion as it emerges from the lateral wrist. Not sure what this means, but Simocyon does not have a distal tarsal 1.

Comparisons to fenestrasaurs (including pterosaurs)
Earlier we discussed how the medial centralia becomes the pterosaur pteroid and the lateral centralia becomes the preaxial carpal after these migrate to the medial wrist rim from their central origins (Fig. 5, Peters 2009). This is heretical thinking, of course, but now this migration hypothesis has support in that it may be convergent with the appearance of the prepollex in Notharctus and other mammals.

pterosaur wings

Figure 5. Click to enlarge. The origin of the pterosaur wing. Note the disappearance of the centralia and the reappearance of the pteroid and preaxial carpal.

I learned something while writing this.
Let me know your thoughts, especially if I missed any interesting configurations or evolutions.

References
Hamrick MW and Alexander JP 1996. The Hand Skeleton of Notharctus tenebrous (Primates, Notharctidae) and Its Significance for the Origin of the Primate Hand. American Museum Novitates 3182, 20pp.
Kielan-Jaworowska Z 1977. Evolution of the therian mammals in the Late Cretaceous of Asia. Part n. Postcranial skeleton in Kennalestes and Asioryctes. In: Z. Kielan-Jaworowska (ed.) Results Polish Mongolian Palaeont. Expeds. VIII. – Palaeont, Polonica, 37, 65-84.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Salesa MJ, Antón M, Peigné S and Morales J 2005. Evidence of a false thumb in a fossil carnivore clarifies the evolution of pandas. Proceedings of the National Academy of Sciences of the United States of America. abstract and pdf