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

Interested in whale carpals?

Gavazzi et al. 2020 bring us their views
on whale and pre-whale carpal elements. They used the invalidated artiodactyls, pig (Sus) and  (Diacodexis) as outgroup taxa. The latter is incompletely known, slender and appears to be not far from the more completely known Cainotherium.

By contrast
here  (Figs. 1–3, 5, 7, 8) competing whale and pre-whale carpal elements are presented along with tree shrew (Ptilocercus) carpals (Fig. 4) as an example of the plesiomorphic condition in placentals.

Bottom line:
Due to taxon exclusion, actual evolutionary patterns were overlooked by Gavazzi et al., but that didn’t matter much. About the same story can be told with the wrong outgroup taxa. Carpals don’t change much across these members of the placental clade.

Figure 1. Odontocete flipper and ancestral taxa manus. Homologous wrist elements are colored the same. Green is the pisiform, missing in the dolphin, Tursips.  Ambulocetus image from Gavazzi et al. 2020 and repaired here. Dorudon image from Cooper et al. 2007 and repaired here.

Figure 1. Odontocete flipper and ancestral taxa manus. Homologous wrist elements are colored the same. Green is the pisiform, missing in the dolphin, Tursips.  Ambulocetus image from Gavazzi et al. 2020 and repaired here. Dorudon image from Cooper et al. 2007 and repaired here.

From the Gavazzi et al. 2020 abstract:
“During the land-to-water transition in the Eocene epoch, the cetacean skeleton underwent modifications to accommodate life in the seas. These changes are well-documented in the fossil record. The forelimb transformed from a weight-bearing limb with mobile joints to a flipper with an immobile carpus.”

Unfortunately Gavazzi et al. still consider ‘Cetacea’ a monophyletic clade. That hypothesis of interrelationships became invalid in 2017 with a blogpost here and an online paper here.

Figure 2. The manus of Eubaelana, an extant right whale (bottom) compared to two desmostylians.

Figure 2. The manus of Eubaelana, an extant right whale (bottom) compared to two desmostylians. Note the similarities between the two unrelated clades. Missing parts are restored based on phylogenetic bracketing.

Mammal carpal homologs:

  1. Distal carpal 1 = Trapezium – red
  2. Distal carpal 2 = Trapezoid – cyan
  3. Distal carpal 3 = Capitum – lavender
  4. Distal carpal 4+5 = Hamatum – yellow green
  5. Radiale = Scaphoideum – pink
  6. Intermedium = Lunatum – yellow
  7. Ulnare = Triquetrum – pale orange
  8. Centrale = Centrale – indigo
  9. Pisiform = Pisiform – green
Figure 2. Hippos manus. Pisiform in green.

Figure 3. Hippos manus. Pisiform in green.

Continuing from the Gavazzi et al. 2020 abstract:
“We used micro-CT imaging to assess evolutionary changes in carpal size, orientation, and articulation within Eocene cetacean taxa associated with the transition from a terrestrial to amphibious niche. We compared Ambulocetus natans (Fig. 1), a well-preserved amphibious archaeocete, with other archaeocetes, and with Eocene terrestrial artiodactyls, the sister group to Cetacea.”

Figure 3. Ptilocercus is a tree shrew closer to the plesiomorphic wrist condition. Compare to taxa in figures 1 and 2.

Figure 4. Ptilocercus is a tree shrew closer to the plesiomorphic wrist condition. Compare to taxa in figures 1 and 2.

Figure 4. X-ray of Tursiops flipper showing no trace of a pisiform.

Figure 5. X-ray of Tursiops flipper showing no trace of a pisiform. But look at digit 5. Compare to figure 4.

Eocene terrestrial artiodactyls are not the sister group to Cetacea, which is not a monophyletic clade in the LRT. Adding overlooked taxa resolves this issue.

“A cylindrical carpus in terrestrial taxa evolved into a mediolaterally flattened, cambered carpus in the semi-aquatic and fully aquatic cetaceans.

Gavazzi et al. chose the wrong terrestrial taxa.  Tenrec carpals (Figs. 1, 7) are not as cylindrical as those of pigs and their ancestors, though the pisiform does orient itself ventroposteriorly. All flippers are flattened with a lateral pisiform, which turns out to be a reversal back to the tree shrew orientation (Fig. 4).

“Specifically, the pisiform bone shifted from a ventral [= posterior, Fig. 3] orientation in terrestrial taxa to a lateral orientation, in plane with the carpus, within semi-aquatic and fully aquatic taxa.”

A ventroposterior pisiform is also found in Tenrec (Fig. 1)  and Hippopotamus (Fig. 3) among the actual ancestors of odontocetes and mysticetes respectively.

“Flattening of the carpus, including lateral rotation of the pisiform, likely relates to functional shifts from weight-bearing terrestrial locomotion to aquatic locomotion. This laterally projecting pisiform morphology is retained in all extant cetaceans.”

Gavazzi et al. used the wrong outgroup taxa. In both Tenrec and Hippopotamus the carpus is less cylindrical than in the artiodactyls, Sus and Diacodexis.

By the way, the pisiform is absent in the dolphin Tursiops (Figs. 1, 5), but look at the lateral orientation of digit 5 in comparison to the lateral orientation of the pisiform in the plesiomorphic wrist of Ptilocercus (Fig. 4). One wonders if Gavazzi et al. mistook digit 5 in Tursiops for a pisiform, given their statement.

“Our results suggest this shift, along with other modifications to the carpus, predominantly occurred during the middle Eocene and facilitated an obligatorily aquatic lifestyle in late Eocene cetaceans.”

Without the real ancestors, the Gavazzi et al. paper and its conclusions would be a waste of time, except that carpals are largely interchangeable in quadrupedal placentals. So their conclusions remain largely valid. Thirteen years ago Cooper et al. 2007 also looked at whale carpals and compared them to predecessor taxa (see below).

Figure 6. Ambulocetus carpals from Gavazzi et al. (left) here restored to their original positions and colored.

Figure 6. Ambulocetus carpals from Gavazzi et al. (left) here restored to their original positions and colored.

Try to always restore scattered elements
to their original positions. Don’t leave the work half done. Colors help to make this process easier and aid in making confident comparisons between taxa.

Figure 7. Manus and wrist of the extant tenrec Hemicentetes (from Digimorph.org and used with permission). Colors added. Compare to related taxa in figures 1 and 6.

Figure 7. Manus and wrist of the extant tenrec Hemicentetes (from Digimorph.org and used with permission). Colors added. Compare to related taxa in figures 1 and 6. Pisiform in bright green.

The traditionally overlooked overall resemblance of tenrecs
to Indohyus (Fig. 8) and later odontocete ancestors (Fig. 8) extends to the manus and carpus.

Whale workers who refereed the manuscript
The Triple Origin of Whales‘ declined to allow it to be published. In that light, one wonders why whale workers prefer to cherry-pick pigs and pig ancestors rather than adding valid taxa to their cladograms. Keeping their blinders on is a continuing problem in paleontology, which is why this blogpost exists. For those readers hoping someday to make discoveries in this field of science, beware. Many professors will attempt to suppress your work to  keep the discoveries for themselves.

Figure 8. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.

Figure 8. Odontoceti (toothed whale) origin and evolution. Here Anagale, Andrewsarchus, Sinonyx, Hemicentetes, Tenrec Indohyus and Leptictidium precede Pakicetus. Maiacetus and Orcinus are aquatic odontocetes.

Figure 1. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

Figure 9. Taxa in the lineage of right whales include Desmostylus, Caperea and Eubalaena. The tiny bit of jugal posterior to the orbit (in cyan) is found in all baleen whales tested so far. The frontals over the eyes are just roofing the eyeballs in Desmostylus, much wider in Caperea and much, much longer in Eubalaena.

Cooper et al. 2007
dissected extant whale flippers and compared them to Ambulocetus (Fig. 1) and Dorudon (Fig. 1) without correcting for disarticulation problems in fossils. Cooper et al. report, “Most odontocetes also reduce the number of phalangeal elements in digit V, while mysticetes typically retain the plesiomorphic condition of three phalanges.Perhaps Cooper et al. did not notice there are four phalanges on digit five in Eubaelana (Fig. 2). The unguals are tiny.


References
Cooper LN, Berta A, Dawson SD and Reidenberg JS 2007. Evolution of hyperphalangy and digit reduction in the cetacean manus. The Anatomical Record Special Issue: Anatomical Adaptations of Aquatic Mammals 290(6): https://doi.org/10.1002/ar.20532
Gavazzi LM, Cooper LN, Fish FE, Hussain ST and Thewissen JGM 2020.
Carpal Morphology and Function in the Earliest Cetaceans. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2020.1833019

https://www.researchgate.net/publication/328388746_The_triple_origin_of_whales

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

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.

References
Bonaparte JF 1969. Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.
Clark JM et al. 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20 (4): 683–704.
doi:10.1671/0272-4634(2000)020[0683:ANSOHA]2.0.CO;2.
Clark JM, Xu X, Forster CA and Wang Y 2004. A Middle Jurassic ‘sphenosuchian’ from China and the origin of the crocodilian skull. Nature 430:1021-1024.
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
Reig OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.
Sereno PC, Forster CA, Rogers RR and Moneta AM 1993. Primitive dinosaur skeleton form Argentina and the early evolution of the Dinosauria. Nature 361, 64-66.
Sereno PC, Martínez RN and Alcober OA 2013. Osteology of Eoraptor lunensis (Dinosauria, Sauropodomorpha). Basal sauropodomorphs and the vertebrate fossil record of the Ischigualasto Formation (Late Triassic: Carnian-Norian) of Argentina. Journal of Vertebrate Paleontology Memoir 12: 83-179 DOI:10.1080/02724634.2013.820113

wiki/Eoraptor
wiki/Herrerasaurus
wiki/Sanjuansaurus
wiki/Pseudhesperosuchus

Darwinopterus carpus and another 5th manual digit

I appreciate it when authors provide close-ups of the pterosaur carpus. It gives me a chance to once again document the near universal presence of a vestigial manual digit 5 and other ptero traits missed by other workers.

Figure 1. The carpus of Darwinopterus linglongtaenis. Vesitigial digit 5 is scattered on metatarsal 4. The pteroid articulates in the saddle of the radiale. The preaxial carpal articulates on the first distal carpal now fused to the other distal carpals in a syncopal.

Figure 1. Click to enlarge. The carpus of Darwinopterus linglongtaenis. Vesitigial digit 5 is scattered on metatarsal 4. The pteroid articulates in the saddle of the radiale. The preaxial carpal articulates on the first distal carpal now fused to the other distal carpals in a syncopal.

Here digit 5 is scattered, but all the elements are there. In red: distal carpal 5. In green: metacarpal 5. In blue: two proximal phalanges. In amber: a sharp ungual. This matches the pattern seen in basal fenestrasaurs in which manual digit 5 is not a vestige.

Note the pteroid is located in the saddle of the radiale (Peters 2009) and disconnected from the preaxial carpal (both former centralia, having migrated to the medial wrist, convergent with the mammalian prepollex).

References
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29:1327-1330.
Wang X, Kellner AWA, Jiang S-X, Cheng X, Meng Xi and Rodrigues T 2010
. New long-tailed pterosaurs (Wukongopteridae) from western Liaoning, China. Anais da Academia Brasileira de Ciências 82 (4): 1045–1062. pdf online

wiki/Kunpengopterus

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