The forgotten clade: the REAL proximal ancestors to Dinosauria

Ignored by Baron et al. 2017, and everybody else
the Junggarsuchus clade (including Pseudhesperosuchus, Carnufex and Trialestes in order of increasing quadrupedality, Figs. 1–4) nests as the proximal ancestors to Herrerasaurus (Fig. 1) and the rest of the Dinosauria (Fig. 5) in the large reptile tree (LRT). That cladogram tests a wider gamut of taxa in greater detail than any other reptile cladogram ever published, attempting to not overlook anything. The Junggarsuchia is a sister clade to the Crocodylomorpha with both arising from a taxon near Lewisuchus (Fig. 1). Traditional paleontology (see Wikipedia) nests this largely ignored clade with the sphenosuchian crocodylomorphs (Fig. 4)… and for two good reasons!

Figure 1. Members of the Junggarsuchus clade were derived from a sister to the basal crocodylomorph, Lewisuchus and produced one line that includes Pseudhesperosuchus and Trialestes. The other line produced dinosaurs. These taxa are shown to scale. Note the evolution from a bipedal configuration to a quadrupedal stance.

Figure 1. Members of the Junggarsuchus clade were derived from a sister to the basal crocodylomorph, Lewisuchus and produced one line that includes Pseudhesperosuchus and Trialestes. The other line produced dinosaurs. These taxa are shown to scale. Note the evolution from a bipedal configuration to a quadrupedal stance.

One: Paleontologists never seem to include Dinosauria
in their smaller gamut croc analyses because they’re looking at crocs!~. So once again, taxon exclusion is holding some workers back from seeing ‘the big picture’. ReptileEvolution.com and the blog you are currently reading is all about examining ‘the big picture.’

Figure 2. Skulls of the Junggarsuchus clade not to scale. Herrerasaurus is the basalmost dinosaur.

Figure 2. Skulls of the Junggarsuchus clade not to scale. Herrerasaurus is the basalmost dinosaur, closely related to Junggarsuchus.

Two: Junggarsuchians ALSO have elongate proximal wrist bones
Elongate proximal carpals are found in both sphenosuchian crocs and derived members of the Junggarsuchus clade. Paleontolgists wrongly assumed such odd wrist bones were homologous. It’s an easy mistake to make. However, the LRT makes clear that intervening taxa, including Junggarsuchus, do not have elongate wrist bones.

Among taxa that preserve the manus,
(Fig. 3) it is Junggarsuchus that nests closest to Herrerasaurus and the Dinosauria.

Figure 3. Hands of Lewisuchus, Herrerasaurus, Junggarsuchus, Pseudhesperosuchus and Trialestes. The proximal carpals (radiale and ulnare) were elongate by convergence with a line of crocodylomorphs. This has confused paleontologists and mentally removed them from possible ancestry to the Dinosauria. Note the very short proximal carpals in Junggarsuchus.

Figure 3. Hands of Lewisuchus, Herrerasaurus, Junggarsuchus, Pseudhesperosuchus and Trialestes. The proximal carpals (radiale and ulnare) were elongate by convergence with a line of crocodylomorphs. This has confused paleontologists and mentally removed them from possible ancestry to the Dinosauria. Note the very short proximal carpals in Junggarsuchus.

Like the basal members of the Crocodylomorpha
the Junggarsuchus clade (the Prodinosauria here) transition from bipedal basal members to quadrupedal derived members, with the longest forelimbs belonging to the most derived member, Trialestes (Fig. 3). Distinct from the others and contra the original interpretation, I think Trialestes may have had a larger ulnare than radiale, to match its larger ulna.

Figure 4. Crocodylomorph manus and carpus samples including Terrestrisuchus, Erpetosuchus, Hesperosuchus and Dibothrosuchus along with Scleromochlus documenting the elongate radiale and ulnare on derived taxa. Ticinosuchus is the closest example of an ancestral/plesiomorphic manus in the LRT.

Figure 4. Crocodylomorph manus and carpus samples including Terrestrisuchus, Erpetosuchus, Hesperosuchus and Dibothrosuchus along with Scleromochlus documenting the elongate radiale and ulnare on derived taxa. Ticinosuchus is the closest example of an ancestral/plesiomorphic manus in the LRT.

Let’s not forget
PVL 4597 (Fig. 6) which was mistakenly considered a specimen of Gracilisuchus by (Lecuona and Desojo 2011), but under phylogenetic analysis in the LRT, still nests as the proximal outgroup to Herrerasaurus. It is tiny specimen, supporting the hypothesis of phylogenetic miniaturization at clade origin. And it retains a small proximally oriented calcaneal tuber, as found in other Junggarsuchians.

Figure 1. Subset of the LRT focusing on the Archosauria (Crocodylomorpha + Dinosauria and kin). Gray areas document specimens with elongate proximal carpals (radiale and ulnare).

Figure 5. Subset of the LRT focusing on the Archosauria (Crocodylomorpha + Dinosauria and kin). Gray areas document specimens with elongate proximal carpals (radiale and ulnare).

We looked at
phylogenetic miniaturization at the origin of several pterosaur clades. Well, it happens here too, at the base of the Dinosauria (Fig. 1) with PVL 4597 (Fig. 6), easily overlooked, easily mistaken for something else.

One should not ‘choose’ outgroup taxa
based on paradigm, tradition, guessing, convenience or opinion. Rather outgroup taxa should ‘choose themselves’ based on rigorous testing of a large gamut of outgroup candidates in phylogenetic analysis. To minimize selection bias, the LRT provides 858 outgroup taxa the opportunity to nest close to dinosaurs.

Figure 6. The closest known taxa to the Dinosauria, PVL 4597, is a tiny taxon (phylogenetic miniaturization) with erect hind limbs, a large and deep pelvis and a tiny calcaneal tuber.

Figure 6. The closest known taxa to the Dinosauria, PVL 4597, is a tiny taxon (phylogenetic miniaturization) with erect hind limbs, a large and deep pelvis and a tiny calcaneal tuber.

 

References
Baron MG, Norman DB, Barrett PM 2017. A new hypothesis of dinosaur relationships and early dinosaur evolution. Nature 543:501–506.
Bonaparte JF 1969. 
Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.
Butler RJ. et al. 2014. New clade of enigmatic early archosaurs yields insights into early pseudosuchian phylogeny and the biogeography of the archosaur radiation. BMC Evol. Biol. 14, 128.
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.
Lecuona A and Desojo, JB 2011. Hind limb osteology of Gracilisuchus stipanicicorum(Archosauria: Pseudosuchia). Earth and Environmental Science Transactions of the Royal Society of Edinburgh 102 (2): 105–128.
Nesbitt SJ 2011. The early evolution of archosaurs: relationship and the origin ofmajor clades. Bull. Amer. Mus. Nat. Hist. 352, 1–292.
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.
Zanno LE, Drymala S, Nesbitt SJ and Schneider VP 2015. Early Crocodylomorph increases top tier predator diversity during rise of dinosaurs. Scientific Reports 5:9276 DOI: 10.1038/srep09276.

wiki/Pseudhesperosuchus
wiki/Junggarsuchus
wiki/Carnufex
wiki/Herrerasaurus
wiki/Sanjuansaurus

 

Diplovertebron and amphibian finger loss patterns

Diplovertebron punctatum (Fritsch 1879, Waton 1926; Moscovian, Westphalian, Late Carboniferous, 300 mya, Fig. 1) was considered an anthracosaur or reptile-like amphibian. That is confirmed by the large reptile tree (LRT, subset Fig. 2), where it nests with  Utegenia transitional between basal seymouriamorpha, like Kotlassia, and basal amphibians, like Balanerpeton (Fig. 3), yet close to the origin of stem reptiles, like Silvanerpeton. Based on the nesting of Tulerpeton in the LRT, Diplovertebron had origins in the Late Devonian.

Figure 1. Diplovertebron nests at the base of the lineage of amphibians, close to the base of the reptiles, all derived from seymouriamorphs. Note the retention of five fingers. Wish I had better data than this.

Figure 1. Diplovertebron nests at the base of the lineage of amphibians, close to the base of the reptiles, all derived from seymouriamorphs. Note the retention of five fingers. Wish I had better data than this.

In Diplovertebron,
the vertebral structure is primitive. The notochord persisted in adults. The ribs were long and slender as in basal taxa, not shortened as in lepospondyl amphibians. Five manual digits were preserved with a 2-3-3-3-4 formula, a formula similar to amphibians, not like reptiles (2-3-4-5-5). The ilium is bifurcate with a long posterior process. The pubis did not ossify, as in several basal tetrapods including Crassigyrinus and derived Amphibia. Small scutes covered the entire torso ventrally, as in basalmost tetrapods and basal reptiles.

Figure 2. The gradual loss of basal tetrapod fingers. Unfortunately fingers are not known for every included taxon.

Figure 2. The gradual loss of basal tetrapod fingers. Unfortunately fingers are not known for every included taxon. Odd Tulerpeton with 6 fingers may result from taphonomic layering of the other manus peeking out below the top one. See figure 6.

The presence of five manual digits
in Diplovertebron and Balanerpeton (Figs. 4, 5) sheds light on their retention in Acheloma + Cacops. There is a direct phylogenetic path between them (Fig. 2). Note that all other related clades lose a finger or more. Basal and stem reptiles also retain five fingers.

Figure 2. Utegenia nests as a sister to Diplovertebron.

Figure 3. Utegenia nests as a sister to Diplovertebron.

Note the narrow frontals,
on Diplovertebron distinct from the wide frontals in Utegenia and Kotlassia, but more similar to those in Balanerpeton (Fig. 4), another basal amphibian, and Silvanerpeton, a stem reptile. Yet none have the hourglass shape found in Diplovertebron.

Figure 4. The basal amphibian, Balanerpeton apparently has five fingers (see figure 5).

Figure 4. The basal amphibian, Balanerpeton apparently has five fingers (see figure 5).

As reported
earlier, finger five was lost in amphibians,while finger one was lost in temonospondyls. Now, based on the longest metacarpal in Caerorhachis and Amphibamus (second from medial), apparently manual digit one was lost in that clade also, distinct from the separate frog and microsaur clades. In summary, loss from five digits down to four was several times convergent in basal tetrapods.

Figure 5. DGS recovers five fingers in Balanerpeton with a Diplovertebron-like phalangeal pattern.

Figure 5. DGS recovers five fingers in Balanerpeton with a Diplovertebron-like phalangeal pattern. Two 5-second frames are shown here.

Finally, we have to talk about
Tulerpeton (Fig. 6). The evidence shows that the sixth manual digit is either a new structure – OR – all post-Devonian taxa lose the sixth digit by convergence, since they all had five fingers. Finger 6 has distinct phalangeal proportions, so it is NOT an exposed finger coincident rom the other otherwise unexposed hand in the fossil matrix.

Figure 2. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here.

Figure 6. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here. Digit 6 is either a new structure, or a vestige that disappears in all post-Devonian taxa.

References
Fritsch A 1879. Fauna der Gaskohle und der Kalksteine der Permformation “B¨ ohmens. Band 1, Heft 1. Selbstverlag, Prague: 1–92.
Kuznetzov VV and Ivakhnenko MF 1981. Discosauriscids from the Upper Paleozoic in Southern Kazakhstan. Paleontological Journal 1981:101-108.
Watson DMS 1926. VI. Croonian lecture. The evolution and origin of the Amphibia. Proceedings of the Zoological Society, London 214:189–257.

wiki/Diplovertebron

Better data for the manus of Eryops

Just found this reference
Dr. David Dilkes (2015) provides photo data (Fig. 1) on the carpus and manus of Eryops the giant temnospondyl. Earlier the best data I had was a decades old (Romer era) reconstruction and based on that manus and those of its sister taxa. With that data it appeared that the four digits preserved were 2–5, not 1–4 as traditionally considered. Dilkes likewise follows tradition in listing the fingers as 1–4.

Figure 1. Forelimb of Eryops from Dilkes 2005. Here freehand drawings of the manus cannot compete with a taking a tracing of the photo and restoring the digits and carpal elements to their in vivo positions. Note the subtle differences that happen in the freehand drawing by Dilkes and the Romer era illustrator.

Figure 1. Forelimb of Eryops from Dilkes 2005. Here freehand drawings of the manus cannot compete with  a tracing of the photo and restoring the digits and carpal elements to their in vivo positions (middle). Note the subtle differences that happen in the freehand drawing by Dilkes (above) and the Romer era illustrator (below).

The present data further cements
the hypothesis that the fingers of Eryops are 2–5, not 1–4.

And further cements
the hypothesis that freehand drawing is not as accurate as tracing a photo of the bones.

Today’s post also demonstrates
that better data, no matter where it comes from or makes your hypothesis go, must be incorporated. And finally…

Today’s post also demonstrates
that good Science can take place with second-hand data.

References
Dilkes D 2015. Carpus and tarsus of Temnospondyli. Vertebrate Anatomy Morphology Palaentology 1(1):51-87.

Temnospondyl fingers: 1-4? or 2-5?

If you’re going to have a good understanding
of reptile origins, you’re going to need a good topology of pre-reptiles (aka basal tetrapods). So today, tomorrow and the next day, we’ll cover temnospondyls and how they relate to their sisters and to reptiles.

According to Wikipedia
Temnospondyls have been known since the early 19th century, and were initially thought to be reptiles. They were described at various times as batrachiansstegocephalians, and labyrinthodonts, although these names are now rarely used. Animals now grouped in Temnospondyli were spread out among several amphibian groups until the early 20th century, when they were found to belong to a distinct taxon based on the structure of their vertebrae. Experts disagree over whether temnospondyls were ancestral to modern amphibians (frogssalamanders, and caecilians), or whether the whole group died out without leaving any descendants.” The large reptile tree (LRT) indicates that temnospondyls are now extinct and not related to lepospondyls (extant amphibians and their kin).

Figure 1. Manus of Sclerocephalus, a well-preserved temnospondyl with four fingers, here labeled 1-4 (traditional) and 2-5 (heretical). The carpus is not as fully ossified as in Eryops, figure 2.

Figure 1.Manus of Sclerocephalus, a well-preserved temnospondyl with four fingers, here labeled 1-4 (traditional) and 2-5 (heretical). The carpus is not as fully ossified as in Eryops, figure 2.

Pertinent to todays discussion
Wikipedia also reports, “most temnospondyls have small limbs with four toes on each front foot and five on each hind foot.” So, the question is: which finger (not toe) is missing from the temnospondyl manus? Traditionally temnospondyl fingers have been labeled 1-4, perhaps based on the hypothesis that the medial digit has two phalanges, as in pentadactyl taxa. That would mean digits 2 and 3 lose one phalanx and digit 4 loses two phalanges — IF you’re starting with a complete pentadactyl limb — but you’re not.

Figure 2. The manus of the temnnospondyl Eryops compared to those of the polydactyl, Acanthostega and the pentadactyl reptilomorphs, Seymouria and Proterogyrinus. Homologous digits and carpal element are colored the same. In Eryops the pollex or digit 1 is absent, leaving only a small bump on Centrale 1. Note distal carpal 3 is smaller than 2 and 4 in Eryops and Seymouria. Digit 4 is the longest in Acanthostega and temnospondyls.

Figure 2. The manus of the temnnospondyl Eryops compared to those of the polydactyl, Acanthostega and the pentadactyl reptilomorphs, Seymouria and Proterogyrinus. Homologous digits and carpal element are colored the same. In Eryops the pollex or digit 1 is absent, leaving only a small bump on Centrale 1. Note distal carpal 3 is smaller than 2 and 4 in Eryops and Seymouria. Digit 4 is the longest in Acanthostega and temnospondyls.

After comparing sister taxa
(and they are few at present) the digits have been relabeled here: 2-5. That means digit 1 is absent in temnospondyls. Thus, digits 2-5 lose one or two phalanges.

In the old days
paleontologists added a hallux to museum mounts and illustrations of Eryops (Fig. 3). So they were thinking the same thing with regard to digit numbers. The correct identification of the fingers is key to scoring these traits correctly in phylogenetic analyses that involve temnospondyls.

Figure 3. Old illustration of Eryops with five fingers. Hallux (in yellow) should not be there.

Figure 3. Old illustration of Eryops with five fingers. Hallux (in yellow) should not be there. See figure 2.

 

References
wiki/Temnospondyli

New insights into the ornithopod manus

Duckbills,
like Edmontosaurus, and their kin are the ornithopod ornithischian dinosaurs, a clade I have been ignoring until now. Wikipedia reports, “[they] started out as small, bipedal running grazers, and grew in size and numbers until they became one of the most successful groups of herbivores in the Cretaceous world, and dominated the North American landscape.” 

Dryosaurus, Camptosaurus, Iguanodon and Edmontosaurus are genera within this clade and each has an interesting manus (Fig. 1). When one works in phylogenetic analysis it is imperative to compare homologous digits (apples to apples). In ornithopods, those homologies appear to be masked and perhaps misinterpreted by the appearances of new phalanges and the disappearances of old phalanges. Putting them all in one image (Fig.1) clarifies all issues (even without traveling to visit the fossils firsthand!). Hopefully the data are accurate to start with.

This all started with a phylogenetic analysis
that appeared to indicate that Edmontosaurus had a manual digit 1 with an extra digit that made it look like manual digit 2. Comparisons to other ornithopods ensued. A quick look through the Internet brought B. Switek’s article (see below) to the fore.

Figure 1. Ornithopod manus. Here the hands of Dryosaurus, Camptosaurus, Iguanodon and Edmontosaurus are compared. Note the turquoise metatarsal homologies and the digit identification based on that.

Figure 1. Ornithopod manus. Here the hands of Dryosaurus, Camptosaurus, Iguanodon and Edmontosaurus are compared. Note the turquoise metatarsal homologies and the digit identifications based on that.

Science writer Brian Switek 
writing for Smithsonian.com reports,

  1. “…the great herbivore Iguanodon had prominent thumb spikes.
  2. “The peculiar false thumb of Iguanodon was originally thought to set into the dinosaur’s nose.”
  3. “But why should Iguanodon have a hand spike? “
  4. “Though my own suggestion is not any better than those I have been disappointed by, I wonder if the Iguanodon spike is a Mesozoic equivalent of another false thumb seen among animals today—the enlarged wrist bones of red and giant pandas…  the Iguanodon spike was rigid.” Unfortunately that’s as far as journalist Switek has allowed himself to go, rather than proposing the homologies and comparisons demonstrated here.

Giving credit where credit is due,
Switek may be the first to suggest the spike was not a digit. I don’t know and was not able to find out the history of the spike. Given the text from his blogpost, you can see Switek’s choice of words actually evolves from “thumb spikes” to “false thumb” to “hand spike” to “enlarged wrist bone”. Like Brian, I also lack a PhD, but that doesn’t stop us from making contributions. If I’m duplicating earlier academic efforts, please let me know so credit can be given.

Here we’ll show
that the spike is indeed a wrist element… that digit 1 in Iguanodon and related taxa have one more phalanx, making it look like digit 2.

We’ll start with
the right manus of Dryosaurus, a basal ornithopod (at least in the large reptile tree it is, where only one other ornithopod, Edmontosaurus, is currently represented). During the course of this, I want you to focus on the the homologies of metatarsals 2 and 3 (colored in turquoise). These, I think, will guide us to correct interpretations of the other elements of the various ornithopod manus.

Now back to the manus of Dryosaurus:

  1. Data comes form loose bones in a photo formed in the shape of a hand, not an in-situ articulated hand. Thus I do not know the identification or placement of the carpals
  2. Five metatarsals are present.
  3. Mt3 is the longest. Slightly shorter is mt2.
  4. Phalangeal formula is 2-3-4-3-2, but digit 1 does not appear to be tipped with a sharp ungual. Is it missing? If so, that adds a phalanx to the formula 3-3-4-3-2.
  5. Digit 3 is the longest. Slightly shorter is digit 2.
  6. Unguals are lost in digits 4 and 5.

The manus of Camptosaurus

  1. Is reduced (stumpy) by comparison to Dryosaurus
  2. Mt 1 is a disk. M1.1 is a disk
  3. M3.2 appears to fuse with m3.3
  4. m4.3 and m5.2 are lost
  5. The new phalangeal formula is 2-3-3-2-1

The manus of Iguanodon

  1. is more robust and highly modified by comparison to Dryosaurus
  2. Two wrist elements fill the wrist. Two others extend medially.
  3. Digit 1 is longer and now sports an ungual
  4. Ungual 1 is not sharp
  5. Ungual 2 is a round hoof
  6. Ungual 3 (m3.4) is lost along with m3.3
  7. Mt4 is shorter. Two tiny phalanges are added.
  8. Digit 5 is absent.
  9. The new phalangeal formula is 3-3-2-4-0

The manus of Edmontosaurus 

  1. is long and gracile by comparison to Dryosaurus.
  2. Again, digit 1 has 3 phalanges, matching digits 2–4.
  3. Digit 4 is a vestige
  4. Mt 5 is again absent
  5. As in Iguanodon, ungual 1 is not sharp and ungual 2 is a hoof
  6. The new phalangeal formula is 3-3-3-3-0.

Always interesting to 
uncover little paradigm busters like these. Now back to phylogenetic analysis…

Reconstructing the partial manus of Ceratosaurus and its bearing on Limusaurus

A recent paper
by Carrano and Choiniere 2016 excavated and described the metacarpus and forearm plus a few partial phalanges of the Ceratosaurus holotype  (USNM 4735).

FIguire 1. the partial manus of Ceratosaurus compared to that of Coelophysis, Dilophosaurus and Allosaurus.

FIguire 1. the partial manus of Ceratosaurus compared to that of Coelophysis, Dilophosaurus and Allosaurus. Restored areas are in gray.

Carrano and Choiniere reported,
“it is more parsimonious to identify the manus of Limusaurus as an autapomorphic condition instead of as primitive for Ceratosauria (Xu et al., 2009). This is particularly evidenced by the primitive metacarpal I in Ceratosaurus, nearly identical in morphology to that seen in other basal neotheropods and quite unlike that in Limusaurus.”

You may recall
that the putative digit 1 of Limusaurus is actually the reappearance of digit 0, a basal tetrapod digit medial to digit 1. We also looked at a possible digit 0 on a specimen of Coelophysis.

Although Ceratosaurus currently nests as a sister to Allosaurus
in the large reptile tree, the manus is more primitive (more like that of Dilophosaurus) with a smaller digit 1 and the retention of a vestigial digit and mc4. Limusaurus nests with the oviraptorid, Khaan.

Carrano and Choiniere correctly conclude:
“Therefore, extreme manus reduction occurred twice in ceratosaurs—once in Limusaurus (and possibly in closely related taxa) and once in derived abelisaurids—and differently in each.”

References
Carrano MT and Choiniere J 2016. New information on the forearm and manus of Ceratosaurus nasicornis Marsh, 1884 (Dinosauria, Theropoda), with implications for theropod forelimb evolution, Journal of Vertebrate Paleontology, DOI:10.1080/02724634.2015.1054497

Did Coelophysis have a digit ‘zero,’ too?

Ran across this image of a Coelophysis manus
(Figs. 1, 2) in the SuppData to Xu et al. 2009 (the paper that introduces us to Limusaurus. Fig. 4), which we looked at earlier here. The fossil appears to be one of the best preserved examples of a Coelophysis manus. Every bone is laid out without disturbance.

I count
6 possible phalanges here. Three are vestiges.

Xu et al. (2009)
introduced the concept of a “phase shift” in theropod digits to account  for the medial bud in Limusaurus, which they counted as digit 1 and the similar bud in chicken embryos. In their hypothesis the other digits changed their appearance to resemble the digit that was medial to each one. One through Three became Two through Four.

Figure 1. Coelophysis manus with digit 0, as in Limusaurus, medial to digit 1. This image also includes mt5 with phalanges, identified by Colbert as distal carpal 4.

Figure 1. Coelophysis manus with digit 0, as in Limusaurus, medial to digit 1. This image also includes mt5 with phalanges, identified by Colbert as distal carpal 4. IF that is not digit zero, then it must be a slipped carpal element, but I think I’ve identified them all here. Note the PILs align even withe vestigial digits.

Unfortunately
Xu et al. 2009 did not clarify, but only muddied avian digital homologies. As we learned earlier, that medial digit is actually a ‘throwback” to basal tetrapods that once had an extra digit medially and is lost on almost all other taxa after hatching. Embryos retained it for a short time.

In this revised hypothesis
the appearance of the bud does not represent a “phase shift” of phalanges, but rather the appearance of a very old medial digit, digit “0”. The other digits retain their original pentadactyl numbers 1-5.

That brings us to
a specimen of Coelophysis (Figs. 1,2) that appears to preserve the manus without disturbance and with that rare medial bud. Perhaps this is another digit “0” — and this time with vestigial phalanges — if interpreted correctly.

Figure 3. Carpus of Coelophysis GIF animation. I'm a little confused about the medial bones here. If they don't represent digit '0', then perhaps some carpals have shifted medially here.

Figure 2. Carpus of Coelophysis GIF animation. I’m a little confused about the medial bones here. If they don’t represent digit ‘0’, then perhaps some carpals have shifted medially here. The 1+2 carpal could be the medial centrale (Ce1) and the Ce1 could be the 1+2 carpal. Also, this specimen appears to have more carpals than Colbert 1989 illustrated (below).

Apparently
Digit 0 on theropods appears to be retained as a fused medial element in some modern birds (Fig. 3). It’s that little bump medial to metacarpal 1.

Figure 2. A selection of pre-bird and bird hands/wings including Haplocheirus, Limusaurus, Velociraptor, Archaeopteryx, Anser, Passer and two versions of the Hoatzin , Opisthocomus, adult and juvenile. Click to enlarge. Not to scale. Note the medial digit of the outlier, Limusaurus, which is a product of neotony, retained from embryonic tissue recapitulating the seven-finger manus of basal tetrapods (figure 3). Note the return of digit 0 fused to the anterior rim of Anser, Passer and the adult Opisthocomus.

Figure 3. A selection of pre-bird and bird hands/wings including Haplocheirus, Limusaurus, Velociraptor, Archaeopteryx, Anser, Passer and two versions of the Hoatzin , Opisthocomus, adult and juvenile. Click to enlarge. Not to scale. Note the medial digit of the outlier, Limusaurus, which is a product of neotony, retained from embryonic tissue recapitulating the seven-finger manus of basal tetrapods (figure 3). Note the return of digit 0 fused to the anterior rim of Anser, Passer and the adult Opisthocomus.

Vestigial digits 
I presume, are lost during excavation every so often. Are they worth scoring in phylogenetic analysis? I score them.

There goes that hypothesis
Earlier I thought digit 0 appeared on Limusaurus because it was an embryological artifact retained on a vestigial manus retained after hatching and maturation  (Fig. 4). As everyone knows, the manus is not vestigial in Coelophysis, so… there goes that hypothesis – if that medial bud is indeed the ephemeral digit “0” and not something else.

Figure 2. Limusaurus also has four fingers and a scapula with a robust ventral area, like Majungasaurus, but those four fingers are not the same four fingers found in Majungasaurus.

Figure 4. Limusaurus also has four fingers and a scapula with a robust ventral area, like Majungasaurus, but those four fingers are not the same four fingers found in Majungasaurus.

References
Cope ED 1889. On a new genus of Triassic Dinosauria. American Naturalist 23: 626
Late Triassic Norian
Colbert E. 1989. The Triassic Dinosaur Coelophysis. Museum of Northern Arizona Bulletin 57: 160.
Xu X, et al 2009. A Jurassic ceratosaur from China helps clarify avian digital homologies. Nature 459, 940-944. doi:10.1038/nature08124