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)


    • 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.

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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

The origin of dinosaur hands

Dinosaurs have a variety of hands.
Everyone knows T-rex had but two fingers. Birds have fused fingers derived from fingers 1-3 that are not fused in the basal bird, Archaeopteryx. Basal sauropodomorphs had five fingers, but later giants reduced this to a U-shaped column of metacarpals.

Figure 1. the manus and carpus of several basal bipedal crocs and basal dinosaurs showing the similarities and differences. Click to enlarge.

Figure 1. the manus and carpus of several basal bipedal crocs and basal dinosaurs showing the similarities and differences. Click to enlarge.

The currently known basalmost dinosaur
Herrrerasaurus, had five metacarpals, but digit 4 was a vestige and digit 5 was absent on a vestigial metacarpal (Fig. 1).

Proximal outgroups to the Dinosauria
among basal bipedal crocs appear to have had four digits and a vestigial metacarpal 5 (Fig. 1), but this may be due to poor preservation and excavation more than in vivo reality.

Among other basal crocs,
digits 1-4 are sometimes known, sometimes partly known (distal phalanges tend to disappear first) and sometimes not known. In all basal crocs, metacarpals 1-3 appear to increase in length laterally and are aligned distally.

The carpus in bipedal basal crocs
includes an elongate ulnare and an even longer radiale (both are proximal wrist bones). No dinosaurs have elongate proximal wrist bones, a key difference. But proto-dinosaurs, like Junggarsuchus and Trialestes, did.

A transitional croc/dino taxon with slightly longer proximal carpals has not been identified.

Like dinosaurs,
most crocs reduce metacarpals 4 and 5, both in length and diameter. But there are certain exceptions (Fig. 1), including Terrestrisuchus and the SMNS 12352 specimen. Junggarsuchus may also have a reduced digit 5, as it appears from the photo, distinct from the Clark et al. 2004 interpretation (Fig. 1) which includes a large metacarpal 5 and lacks a metacarpal 1, distinct from all known sister taxa.

Compared to dinos
the basal bipedal crocs appear to have shorter distal phalanges and smaller hands, as if they were more often in contact with the ground and not used to snare prey.

By contrast, in basal dinos
like Herrerasaurus and Tawa the distal phalanges were trenchant claws and the penultimate phalanges were likewise elongated. Among dino and croc ancestors only tiny Lewisuchus has such long dangerous claws and fingers, perhaps developed by convergence.

The hands of more ancient rauisuchians
are not often found and what is known suggests the hands were small with short distal phalanges. The hands of Eoraptor have fingers longer than metacarpals and relatively longer lateral metacarpals, but the fingers are not so long as in therpods. In Massospondylus there’s a combination of short fingers and large trenchant claws, probably for grabbing tree trunks for feeding high in the boughs.

The difference in manual claw size between basal dinos and basal bipedal crocs may be one of the key differences that enabled dinosaurs to rise to greatness in the Mesozoic.

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What Drives the Elongation of the Metacarpus in Pterosaurs?

In several clades of pterosaurs the metacarpus became elongated. We looked at this phylogenetically earlier in an 8-part series on pterosaur fingers starting here. Generally an elongated metacarpus is seen as a synapomorphy of the old “Pterodactyloidea,” but it is more complicated than that. We’ll look at each elongation event clade by clade. I don’t think anyone has tackled this subject yet, hence the lack of references below.


Figure 1. Protoazhdarchids

Clade 1 – The Proto-azhdarchids
In Pterodactylus? spectabilis, n1, the metacarpus was not elongated. Rather the forearm was shortened with respect to the ancestral sister Dorygnathus (SMNS 50164). In the next taxon, Beipiaopterus, the metacarpus was genuinely elongated. (We looked at this yesterday.) So what happened here?

Beipiaopterus was several times larger overall with a longer tibia, as long as the glenoid-acetabula length.  Beipiaopterus was the first of the stork-like taxa with an elongated neck and more gracile proportions. Still the metacarpus was only half the tibia length. The next taxon after Beipiaopterus, n44, was much smaller overall, but had the proportions of an azhdarchid with a hyper-elongated metacarpus, slightly longer than the tibia. It is likely that Beipiaopterus was a sideline in the lineage between n1 and n44. That means the hyperelongation of the metacarpus occurred in relatively small pterosaurs with very tiny and likely grounded hatchlings.


Figure 2. Protoctenochasmatids

Clade 2 – The Proto-ctenochasmatids
In St/Ei I  the pattern of metacarpal elongation was similar. Overall St/Ei I was much smaller than its phylogenetic predecessor, Dorygnathus R156, which also had a much longer forearm. Two intervening taxa, D. purdoni and Angustinaripterus, are known only by skulls. The ctenochasmatid clade achieved its greatest metacarpal length with Gegepterus, which had stork-like proportions overall. The total length of the tibia is unknown. Other ctenochasmatids, like Ctenochasma and Pterodaustro, did not greatly elongate the metacarpus. Neither became subequal to the tibia. The Pterodaustro embryo, interestingly enough, had a relatively shorter metacarpus than the adult with a longer humerus and more robust forearm, but smaller fingers. Is this a clue that forelimb proportions changed slightly during ontogeny?

Tiny scaphognathids

Figure 3. Tiny scaphognathids

Clade 3 – The Post-Scaphognathids
The lineage of cycnorhamphids and ornithocheirids includes several tiny pterosaurs derived from a series of ever smaller Scaphognathus specimens. The first in this lineage to sport an elongated metacarpus was Gmu-10157 with a metacarpus just short of the tibia and ulna length. So, once again metacarpal elongation occurred first with the tiny pterosaurs and their grounded hatchlings. In this clade, Cycnorhamphus had the longest metacarpus and tibia. The ornithocheirids never developed a hyper-elongated metacarpus. Basal ornithocheirids  had a relatively longer metacarpus compared to the ulna. The forearm elongated in derived forms, so the metacarpus appeared to be relatively shorter. The length of the tibia was shorter in succeeding ornithocheirids, then became more elongate, relative to the metacarpus in the most derived ornithocheirds. Matching its adult sisters, the hatchling ornithocheirid had a metacarpus to match its forearm length, but a much longer tibia was present.


Figure 4. Click to enlarge. Scaphognathus and its descendants demonstrating the elongation of the metacarpus immediately following the smaller specimens of Scaphognathus.

Clade 4 – More Post-Scaphognathids: Ornithocephalids
The lineage of Pterodactylus and Germanodactylus (with all of its many descendants) originated with a sister to Gmu-10157, the tiny pterosaur, Ornithocephalus. Thus clades 3 and 4 likely had a single common ancestor as yet undiscovered. In this clade the hyperelongation of the metacarpus occurred with Eopteranodon and Eoazhdarcho, two stork-like taxa often mistaken for azhdarchids, and perhaps convergently, Nyctosaurus and Pteranodon. I wonder if these two were derived from long-legged taxa, like Eopteranodon, that reduced their hind limb length? Or did they never have long legs and simply developed elongated metacarpals from the likes of Muzquizopteryx and the SMNK-PAL specimen of Germanodactylus? The skulls seem to point to the latter hypothesis, which would make Eopteranodon and Eoazhdarcho offshoot cousins of these two lineages. The juvenile Pteranodon had a hyperelongated metacarpus.

The Evidence in Summary
Apparently there are two stages to metacarpal elongation. The first occurs among tiny pterosaurs in which the metacarpus elongates to 3/4 or so of the tibial length. The second sometimes, but not always, occurs in larger pterosaurs, in which the metacarpus elongates to at least the tibial length, which is sometimes, but not always also elongated.

That’s evolution for ya.

An elongated metacarpus and tibia impart a stork-like appearance to the pterosaurs that have these traits and the best guess is a stork-like lifestyle was their niche. In Pteranodon and Nyctosaurus this was not the case as these albatross-like soarers developed their long wings by elongating their metacarpus. The first appearance of an elongated metacarpus in tiny pterosaurs and presumably their hatchlings might have provided limbs that had more similar proportions, perhaps for added efficiency during terrestrial locomotion. This efficiency would have been lost in Pteranodon and Nyctosaurus in which the metacarpus was much longer than the tibia.

Nothing about metacarpal elongation in the literature, hence no references.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.