A 2020 primer on reptile tarsals

This blogpost had its genesis 
with a new paper by Blanco, Ezcurra and Bona 2020, who studied archosaur and stem archosaur ankles. They reported, “Here, we integrate embryological and palaeontological data and quantitative methodologies to test the hypothesis of fusion between the centrale and astragalus, or the alternative hypothesis of a complete loss of this element.”

More on the results of that paper
after this short primer.

Not sure how much readers know about this subject.
Ankles used to be ‘the thing’ back in the 1980s when terms like “crocodile normal” and “crocodile reversed” were common and influential. Today, with over 230 tested traits in the large reptile tree (LRT, 1658+ taxa), a few ankle traits fade to insignificance. Tiny details, like pegs and sockets, are ignored here. Instead this primer will start with broader, readily visible patterns of presence, absence and fusion.

Figure 1. Basal tetrapod ankles with tarsal elements identified by color.

Figure 1. Basal tetrapod ankles with tarsal elements identified by color.

The origin of carpals preceded the origin of tarsals.
The most primitive (but late appearing) tetrapods, like Trypanognathus, had poorly ossified tarsals and tiny limbs and digits. The most primitive appearances of tarsals in the LRT comes tentatively with Early Carboniferous Pholidogaster (Fig. 1) and completely with Early Carboniferous Greererpeton (Fig. 1). Both are more primitive in the LRT than the traditional fin-to-finger transitional Late Devonian taxa, Acanthostega and Ichthyostega, with their robust limbs and supernumerary digits. We don’t have fossils yet, but we have tracks of Middle Devonian tetrapods. Five is the primitive number of pedal digits in Pholidogaster. Four remains the primitive number of manual digits.

Remember, the first reptile,
Silvanerpeton, is from coeval Early Carboniferous strata. That means we’re missing many intervening taxa from earlier (Late Devonian) layers.

Starting with the sub-equal distal tarsals of Greererpeton
the medial distal tarsals of Gephyrostegus shrink, matching the smaller diameters of the medial metatarsals. The medial centrales also shrink. Two proximal tarsals fuse to become the astragalus and together with the new calcaneum (Pholidogaster lacks one) form the largest tarsal elements, strengthening the tarsus into a tighter, stronger set.

Note the slight rotation of the hind limb of Gephyrostegus
(Fig. 1) relative to the axis of the toes, corresponding to the increased asymmetry of the digit lengths. Distinct from the more fish-like Greererpeton, short-bodied, big-footed Gephyrostegus was able to clamber about on a myriad of landforms, stems and branches with its belly raised off the substrate (Fig. 2).

Gephyrostegus in anterior view

Figure 2. Gephyrostegus in anterior view demonstrating the need for shorter medial toes in tetrapods with a sprawling gait. This insures the toes to not scrape the substrate during the recovery phase and also assures that all the toes contribute to the propulsive phase.

Immediately following Silvanerpeton in the LRT
the Reptilia (=Amniota) split to form two major clades, the Archosauromorpha and the Lepidosauromorpha. At first, both were amphibian-like reptiles (laying amnion-layered eggs) with many traditional amphibians in their number here transferred to the Reptilia based on the last-common ancestor in the LRT method of classification, rather than a list of traditional skeletal traits.

Archosauromorpha step one: Gephyrostegus to Petrolacosaurus
Gephyrostegus (Fig. 3) is the proximal outgroup to the clade Reptilia. Five distal tarsals are present. 4 and 5 are larger than 1, 2 and 3. Four centrale are present. Two proximal tarsals are present, the astragalus (= tibiale) and calcaneum (= fibulae). No intermedium is present.

Petrolacosaurus (Fig. 3) is a basal diapsid. Here Two medial centrale fuse together. Another centrale fuses to the astragalus. The lateral centrale fuses to distal tarsal 4 doubling its size. Distal tarsal 5 shrinks to half. That makes pedal 5 not line up with the other four tarsals.

We’ll return to archosauromorphs below
after dealing with the lepidosauromorphs in order. But first compare the minor differences between the two major reptile clades following Gephyrostegus (Figs. 3, 4).

Figure 1. Gephyrostegus, a reptile outgroup, compared to Petrolacosaurus, a Late Carboniferous archosauromorph basal to archosauriforms and archosaurs.

Figure 3. Gephyrostegus, a reptile outgroup, compared to Petrolacosaurus, a Late Carboniferous archosauromorph basal to archosauriforms and archosaurs. Compare to figure 2.

Lepidosauromorpha step one: Gephyrostegus to Nyctphruretus
Compared to Gephyrostegus, in the owenettid Nyctiphruretus (Fig. 4) two medial centrale fuse together by convergence. Two lateral centrale fuse to distal tarsal 4 tripling its size. Distal tarsal 5 enlarges. Distinct from Petrolacosaurus (Fig. 3), all five metatarsals remain aligned proximally and the hind limb becomes more aligned with the axis of the toes again.

Figure 2. Gephyrostegus compared to the basal lepidosauromorph. Note the fusion of the some centrales into distal tarsal 4.

Figure 4. Gephyrostegus compared to the basal lepidosauromorph. Note the fusion of the some centrales into distal tarsal 4.

Lepidosauromorpha step two: Nyctiphruretus to Huehuecuetzpalli
Huehuecuetzpalli (Fig. 5) is a more arboreal late-survivor in the Early Cretaceous from an Early Triassic radiation of tritosaur lepidosaurs. All distal tarsals are reduced. One and two are absent. Five is fused to metatarsal five creating a twisted ‘hook’. The last centrale is fused to the astragalus and the hind limb is strongly rotated relative to the toes.  The calcaneum is smaller and able to detach from the fibula.

Are you starting to see that bones have a history of homology? Most tarsal fusion is not at all apparent unless comparisons are made in a phylogenetic context. Of course, this affects scoring in analysis.

Figure 1. Lepidosauriform tarsals. The centrale is larger than the distal tarsal 2 in Late Permian Nyctphuretus. Huehuecuetzpalli is Early Cretaceous, so like fenestrasaurs, its ankle also evolved since the Early Triassic split to lose smaller tarsals.

Figure 5. Lepidosauriform tarsals. The centrale is larger than the distal tarsal 2 in Late Permian Nyctphuretus. Huehuecuetzpalli is Early Cretaceous, so like fenestrasaurs, its ankle also evolved since the Early Triassic split to lose smaller tarsals.

Lepidosauromorpha step three: Macrocnemus to Peteinosaurus
Compared to taxa above (Fig. 5), Middle Triassic Macrocnemus (Fig. 6) loses distal tarsal 2 and retains the medial centrale.

Increasingly bipedal Langobardisaurus (Fig. 6) loses distal tarsal 1. The centrale is larger. The other tarsals are smaller.

Increasingly bipedal and sometimes flapping Cosesaurus (Fig. 6) loses distal tarsal 2. The centrale mimics distal tarsal 2. Distal tarsal 4 is not much larger than the centrale. The tibia migrates back in line with the axis of the pes.

Completely bipedal and flapping Peteinosaurus (Fig. 6) has a simple hinge ankle joint with alll four tarsal elements relatively larger and more similar in size.

Figure 3. Tritosaur lepidosaur tarsals. Note how the centrale moves distally to replace distal tarsal 1 and 2.

Figure 6. Tritosaur lepidosaur tarsals from Peters 2000. Note how the centrale moves distally to replace or fuse with distal tarsal 1 and 2. Or is the centrale really distal tarsal 2?

Archosauromorpha step two: Petrolacosaurus to Protorosaurus
Compared to the basal archosauromorph diapsid, Petrolacosaurus (Figs. 3, 7) In Protorosaurus (Fig. 7) distal tarsal 5 fuses to metatarsal 5, as in the lepidosaur tritosaur, Huehuecuetzpalli (Fig. 5) by convergence. The astragalus moves to a more central position as the medial centrale articulates directly with the tibia in a short-lived experiment that does not continue with taxa more directly in the archosauriform lineage (Fig. 8).

Figure 2. Petrolacosaurus and Protorosaurus pedes to establish homologies.

Figure 7. Petrolacosaurus and Protorosaurus pedes to establish homologies.

Archosauromorpha step three: Protorosaurus to Archosauriforms
Compared to Protorosaurus (Fig. 8), the tarsals are little changed in the basal archosauriform, Proterosuchus, with the note that, as mentioned above, the centrale does not contact the tibia. The distal tarsals are sub-equal in size. The calcaneum is laterally extended, backing up the similarly extended mt5.

In Euparkeria (Fig. 8) the tarsus is similar with symmetrical and block-like proximal tarsals. The calcaneum does not back up mt 5. Pedal digit 3 longer than 4 signaling a less sprawling, more upright, form of locomotion with a simple hinge ankle joint.

In increasingly bipedal PVL 4597 (Fig. 8), basalmost archosaur, distal tarsals 1 and 2 are absent, 3 and 4 are fused. The calcaneum has a posterior process, the ‘heel’. Mt 1 is longer creating a more symmetrical pes with a simpler hinge ankle joint.

Figure 8. Archosauriform pedes compared to Protorosaurus.

Figure 8. Archosauriform pedes compared to Protorosaurus.

Archosauromorpha step four: PVL 4597 to higher Archosauria
Compared to PVL 4597 (Fig. 9), the tarsus of the basal dinosaur Herrerasaurus (Fig. 9) is further reduced, and so is the calcaneum. The astragalus develops an anterior ascending process that also seen in Crocodylus (Fig. 9), which no longer has a simple-hinge ankle joint. Here fused distal tarsal 4/5 is thicker and the astragalus contacts mt 1, slightly rotating the tibia medially for a more sprawling configuration. Distinct from other archosaurs, mt 1 is the most robust in the set and mt 5 becomes a robust vestige in Crocodylus.

Figure 7. Archosaur tarsals compared.

Figure 9. Archosaur tarsals compared.

Blanco, Ezcurra and Bona 2020 report, 

  1. “the astragalus developed ancestrally from two ossification centres in stem archosaurs 
  2. the supposed tibiale of bird embryos represents a centrale.
  3. The tibiale never develops in diapsids.”

In counterpoint,

  1. Figure 1 indicates the two ossification centers go back to the basal tetrapod, Greererpeton. Protorosaurus was an oddball with the centrale contacting the tibia.
  2. Figure 1 indicates it is inappropriate to call the proximal tarsal in any reptile the ‘tibiale’ as the astragalus is present prior to basalmost taxa.
  3. See above.

Blanco et al. report,
“The proximal tarsus of archosaurs is ancestrally composed of a medial astragalus that articulates proximally with the tibia and fibula and a lateral calcaneum that articulates proximally with the fibula.” The authors do not identify the owner of such a tarsus, but let us presume it is that oddball Protorosaurus (Figs. 7, 8).

Blanco et al. reach back to captorhinids
to suggest an outgroup to archosaurs. Unfortunately, captorhinds are basal lepidosauromorphs in the LRT. So the authors are looking where they should not be looking for progenitors.

The Blanco et al. membership list of Archosaurs is over extended.
Blanco et al. employ the invalid clades ‘Pseudosuchia‘ and ‘Avemetarsalia‘, which includes the lepidosaur pterosaurs. They also employ the lepidosaur, Macrocnemus (Fig. 6) as an archosauriform outgroup. Their Archosauromorpha include the archosauriforms, Proterosuchus and Erythrosuchus and the archosaurs Caiman, Lewisuchus and Rhea. They are not aware that the old definition of Archosauromorpha now includes synapsids when given the taxon list of the LRT.

Figure 8. Basal pterosaur and basal dinosaur pedes (feet) compared. While convergent in many respects, certain traits separate these two unrelated clades.

Figure 10. Basal pterosaur and basal dinosaur pedes (feet) compared. While convergent in many respects, certain traits separate these two unrelated clades.

Pterosaurs are traditionally considered archosaurs, 
but that was shown to be invalid twenty years ago. Perhaps it would help if a basal archosaur dinosaur and a basal lepidosaur pterosaur were shown side-by-side (Fig. 10). We’ve already seen many instances of convergence in the tarsal evolution of archosauromorphs and lepidosauromorphs. This is just one more instance of the same. It is time for paleontologists to stop dragging their tails and get up to speed in this arena.


References
Blanco MVF, Ezcurra MD and Bona P 2020. New embryological and palaeontological evidence sheds light on the evolution of the archosauromorph ankle. Nature Scientific Reports (2020)10:5150. https://doi.org/10.1038/s41598-020-62033-8
Peabody FE 1951. The origin of the astragalus of reptiles. Evolution 5(4):339–344.

Tetrapod evolution without Ichthyostega and Acanthostega

Two recent papers,
(Clack 2009, Long et al. 2018, Figs. 1, 2), included traditional cladograms of tetrapod evolution ranging from taxa with fins to taxa with legs. Both included Ichthyostega and Acanthostega, taxa traditionally considered essential to any discussion of taxa documenting the transition from fins to legs.

Figure 1. Modified from Clack 2009 showing the taxa in the transition from fins to feet.

Figure 1. Modified from Clack 2009 showing the taxa in the transition from fins to feet.

The two studies do not have the same taxon list.
In Clack 2009 (Fig. 1) Panderichthys is a penultimate most basal taxon. In Long et al. 2018  (Fig. 2) Panderichthys is nearly a penultimate most derived taxon.

From Long et al. 2018, their cladogram of taxa in the transition from fins to feet.

Figure 2. From Long et al. 2018, their cladogram of taxa in the transition from fins to feet.

By contrast
the large reptile tree (LRT, 1586 taxa; subset Fig. 3), which employs many more pertinent taxa, nests Ichthyostega and Acanthostega distinctly off the main line leading from jawless Silurian fish to amniotes (= reptiles) and relegates them to the sidelines where they give rise to no other taxa. Apparently these two terminal (= dead end) taxa were evolving secondarily to a more aquatic niche or role. They both have no known descendants in the LRT. The LRT represents a new hypothesis of interrelationships from 2017 requiring confirmation or refutation with a similar taxon list.

Today I’ll summarize the subset topology recovered by the LRT
by graphically listing the included taxa that were transitional between jawless fish in the Silurian and basalmost reptiles in the Early Carboniferous. The list includes many taxa that have been traditionally omitted from prior more focused studies, like Clack 2009 and Long et al. 2018. The LRT minimizes taxon exclusion by testing all 1586 included taxa against one another, minimizing traditional biases and omissions.

Figure 2. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus. This tree does not agree with previous fish tree topologies.

Figure 3. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus for no reason during this post.  This tree does not agree with previous fish tree topologies. See figures 1 and 2.

And here they are:
(Figs. 4–6) from Silurian jawless fish like Thelodus to Early Carboniferous Silvanerpeton.

Figure 3. Basal vertebrates in the lineage of reptiles, part 1.

Figure 4. Basal vertebrates in the lineage of reptiles, part 1.

Figure 2. Basal vertebrates in the lineage of reptiles, part 2.

Figure 5. Basal vertebrates in the lineage of reptiles, part 2.

Towards the end,
of figure two fingers and toes first appear in a phylogenetic sense, not a chronological sense. Greererpeton is Early Carboniferous (320 mya) while Ichthyostega and Acanthostega are Latest Devonian (360 mya). To most paleontologists those 40 million years make all the difference permitting omission of Greererpeton and similar taxa To the LRT, Greererpeton is a late survivor from an earlier, perhaps Middle Devonian, radiation.

Figure 3. Basal vertebrates in the lineage of reptiles, part 3.

Figure 6. Basal vertebrates in the lineage of reptiles, part 3.

In this final group,
(Fig. 6) we find Tulerpeton, another taxon from the Latest Devonian (360 mya). It is very nearly a reptile, just two nodes apart from Silvanerpeton, the last common ancestor of all living reptiles. So Silvanerpeton laid amniotic eggs despite its otherwise amphibian-like appearance, and this increases the probability that the more primitive Greererpeton was a late survivor of an earlier Mid Devonian radiation.

Figure 1. From the Beginning - The Story of Human Evolution was published by Little Brown in 1991 and is now available as a FREE online PDF from DavidPetersStudio.com

Figure 7. From the Beginning – The Story of Human Evolution was published by Little Brown in 1991 and is now available as a FREE online PDF from DavidPetersStudio.com

I wish I knew back then
what I know now when I designed, wrote and illustrated “From the Beginning—The Story of Human Evolution” (Wm. Morrow 1991; Fig. 7). But then, it would have been a much bigger book.


References
Clack JA 2009. The fish-tetrapod transition: new fossils and interpretations. Evolution: Education and Outreach 2(2):213–223.
Long JA, Clement AM and Choo B 2018. Early Vertebrate Evolution. New insights into the origin and radiation of the mid-Palaezoic Gondwann stem tetrapods. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 1–17.

Reptilomorpha to scale

Taxa closer to Reptilia
(e.g. Silvanerpeton) than to Lissamphibia (e.g. Rana) are considered Reptilomorpha  by definition (Säve-Söderbergh 1934). Contra tradition and paradigm (see below), in the large reptile tree (LRT, 1440 taxa) most of the following taxa (Fig. 1) are basal (non-reptle) taxa that fulfill this definition. Some outgroup taxa are also shown along with Silvanerpeton, the last common ancestor of all reptiles (= amniotes) in the LRT.

Figure 1. Members of the Reptilomorpha and their proximal outgroups illustrated to scale and in their phylogenetic order from top (primitive) to bottom (derived).

Figure 1. Members of the Reptilomorpha. starting with Caerorhachis and their proximal outgroups illustrated to scale and in their phylogenetic order from top (primitive) to bottom (derived). Not all reptilomrophs have long limbs and large feet, but these traits are generally not found in non-reptilomorphs. Frogs are important convergent exceptions.

The clade Reptilomorpha
includes all members of the clade Reptilia (= Amniota), but we’re going to focus on the stem amniotes today, basically from Caerorhachis to Gephyrostegus.

According to Wikipedia
“As the exact phylogenetic position of Lissamphibia within Tetrapoda remains uncertain, it also remains controversial which fossil tetrapods are more closely related to amniotes than to lissamphibians, and thus, which ones of them were reptiliomorphs in any meaning of the word. These include the diadectomorphsseymouriamorphs, most or all “lepospondyls”, gephyrostegids, and possibly the embolomeres and chroniosuchians. In addition, several “anthracosaur” genera of uncertain taxonomic placement would also probably qualify as reptiliomorphs, including SolenodonsaurusEldeceeonSilvanerpeton, and Casineria.”

Most of these taxa
nest within the clade Reptilia in the LRT. Taxon exclusion has been the traditional cause of this problem, something the LRT was designed to take care of with high confidence because all candidates are tested.

Origin of amniotes
Wikipedia reports, “Exactly where the border between reptile-like amphibians (non-amniote reptiliomorphs) and amniotes lies will probably never be known, as the reproductive structures involved fossilize poorly, but various small, advanced reptiliomorphs have been suggested as the first true amniotes, including SolenodonsaurusCasineria and Westlothiana.”

In the LRT, these three taxa nest well within the Reptilia.
Exactly where the last common ancestor of all living reptiles has been known for several years. Phylogenetic analysis makes this easy. Whichever taxon is the last common ancestor all living mammals, birds, lizards and crocodilians marks the border. Here in the LRT, that taxon is Silvanerpeton (Fig. 1) from the Viséan (Early Carboniferous) with an even earlier genesis because several reptile ingroup taxa are coeval in the Viséan.

Figure 1. Subset of the LRT focusing on basal tetrapods and showing those taxa with lobefins (fins) and those with fingers and toes (feet). Inbetween we have no data.

Figure 1. Subset of the LRT focusing on basal tetrapods and showing those taxa with lobefins (fins) and those with fingers and toes (feet). Inbetween we have no data. By this cladogram and by definition, Microsauria is a clade within Reptilomorpha. 

Not all reptilomrophs have long limbs and large feet,
but these traits are generally not found in non-reptilomorph basal tetrapods. By convergence, frogs (genus: Rana) are important exceptions. Needless to say, longer, stronger limbs are ideal for terrestrial excursions, despite the fact that some reptiles, like snakes and skinks, get along very well without them.

Earlier we talked about the lack of posterior dorsal ribs in the earliest reptiles. This provided additional space for gravid females to grow amniotic eggs prior to laying them. A deep pelvis permitted the expulsion of larger eggs. A deeper pelvis is found in Eusauropleura (Fig. 1) and more derived taxa. Platyrhinops (Fig. 1), in this regard a reptile-mimic, also had a deep pelvis and long legs by convergence.

The earliest fishapods,  
like Panderichthys and Tiktaalik had a wide flat body with dorsal ribs that were much wider than deep — less chance for tipping while walking. This wide, flat morphology is retained up to Utegenia, when the dorsal ribs start curving to enclose a deeper, narrower torso (by convergence with other taxa, like Ichthyostega. At the same time the orbits moved laterally.

Not all reptilomorphs are small,
but the earliest reptilomorphs were no larger than than the juveniles of their ancestor, Greererpeton (Fig. 1), distinct from the giant ‘amphibians’ we looked at earlier.


References
Carroll RL 1991. The origin of reptiles. Pp. 331–53 in Schultze H-P and Trueb L editors. Origins of the higher groups of tetrapods — controversy and consensus. Ithaca: Cornell University Press.
Gauthier J, Kluge AG and Rowe T 1988. The early evolution of the Amniota. In The Phylogeny and Classification of the Tetrapods: Volume 1: Amphibians, Reptiles, Birds. Edited by MJ Benton. Clarendon Press, Oxford, pp. 103–155.
Ruta M, Coates MI and Quicke DLJ 2003. Early tetrapod relationships revisited. Biological Reviews 78 (2): 251–345.
Säve-Söderbergh G 1934. Some points of view concerning the evolution of the vertebrates and the classification of this group. Arkiv för Zoologi. 26A: 1–20.

Eusauropleura: now identified as a late-surviving basalmost reptile

The newest addition
to the large reptile tree (LRT, 1341 taxa) is Eusauropleura digitata (originally Sauropleura, Cope 1868; Romer 1930; Carroll 1970; Late Carboniferous, 310 mya; AMNH 6865; Figs. 1, 2) nests as a late-surviving basalmost reptile in the LRT.

The genesis for this genus
in the earliest Carboniferous is based on the more derived Silvanerpeton from the Viséan (335 mya). A dense layer of belly scales (not shown en masse), a larger manual digit 5, and a taller ilium, among other traits, distinguish this specimen from Gephyrostegus. A larger manus, ischium and giant caudal transverse processes (ribs) relative to the torso are unique traits among close relatives. Note the lack of ribs in the lumbar area, where large amniote eggs develop before they are laid. The eggs were relatively large based on the greater depth of the ischium.

Figure 1. Eusauropleura in situ and slightly reconstructed. Manus reconstruction with PILs enlarged.

Figure 1. Eusauropleura in situ and slightly reconstructed. Manus reconstruction with PILs enlarged.

Basal to Eusauropleura
are taxa close to the Reptilomorpha – Lepospondyli split. These include Eucritta and Utegenia (Fig. 2) all derived from the Late Devonian reptilomorph, Tulerpeton. This affirms the primitive state of basalmost reptiles, derived from Devonian tulerpetids. Further affirmation comes from the observation that the central vertebral elements of Eusauropleura “are very thin-walled, forming little more than a husk around the large notochord,” according to Carroll 1970.

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles.

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles. Note, none of these specimens preserves ossified carpals.

First considered a microsaur
(Cope 1868), then a gephyrostegid (Romer 1930, 1950; Carroll 1970), Eusauropleura was identified as more primitive than Gephyrostegus (Carroll 1970), but still terrestrial, not aquatic and close to the ancestry of reptiles, but not itself a reptile.

So what is a reptile?
As determined here in 2011, there is no list of traditional reptile skeletal traits that upholds the reptile status of Gephyrostegus. There is a new list. Irregardless of skeletal traits, only the nesting of Gephyrostegus as the last common ancestor of all reptiles in the LRT tells us it was laying eggs with an amnion, the ONLY trait needed to determine its reptile status. Silvanerpeton, from the earlier Viséan, was likely also a reptile because phylogenetic descendants of late-surviving Gephyrostegus are also found in coeval Viséan strata. Reptiles are that old. Given that the last common ancestor of Silvanerpeton and Gephyrostegus must also be a late-surviving member of that basalmost reptile radiation, whether the amnion was fully developed or not, something we may never know given the fragility of an amniotic membrane over 300 million years in stone. Earlier workers did not enter Eusauropleura, Silvanerpeton and Gephyrostegus into a wide gamut phylogenetic analysis and so did not recover a last common ancestor status for these amphibian-like reptiles.

Another specimen attributed to Eusauropleura
AMNH 6860 (Moodie 1909, Carroll 1970), is a bit more jumbled, more incomplete and more difficult to reconstruct. A complete ilium with an elongate posterior process is easy to see in this specimen. Such a process provides attachment points for more than one sacral rib, a traditional reptile trait, but this is difficult to determine in the scattered remains of the fossil. And is this really Eusauropleura?

Yet another specimen attributed to Eusauropleura
PU 16815 is an isolated pectoral girdle, bones lacking in the other specimens and therefore not readily comparable.

Scales
According to Carroll 1970, “Scales, both dorsal and ventral, are conspicuous in these specimens [Gephyrostegus and Eusauropleura]. The body was protected by heavy, oblong scales, overlapping to form a chevron pattern, between the pelvic and pectoral girdles. Were they not associated with the skeleton, they would be difficult to distinguish from those of [more primtive] embolomeres. Laterally the scales assume a more oval outline, become thinner, smaller and less extensively overlapping. The dorsal scales are small, thin and round. Where worn, all the scales exhibit a pattern of fine ridges, running parallel with the margins. These form a pattern of concentric ridges in the dorsal scales, similar to that of [more primitive] discosauriscids. Except for the heavier ossification of the dorsal scales, those of Eusauropleura are generally similar to those of Gephyrostegus.”

References
Carroll RL 1970. The ancestry of reptiles. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 257 (814):267–308. DOI: 10.1098/rstb.1970.0026
Cope ED 1868. Synopsis of the Extinct Batrachia of North America. Proceedings of the Academy of Natural Sciences of Philadelphia 1868:208-221.
Romer AS 1930. The Pennsylvanian tetrapods of Linton, Ohio. Bulletin of the American Museum of Natural History. 59 (2):144–147.
Romer AS 1950. The nature and relationships of the Paleozoic microsaurs: American Journal of Science 248:628-654.

wiki/Eusauropleura

Phylogenetic miniaturization preceding the origin of Reptilia

We looked at
Ossinodus and Acanthostega a few days ago. Today the relatives of those two, from Osteolepis to Gephyrostegus are shown to scale (Fig. 1). Look how small the first reptiles were. Certainly the transition to land was aided by having less weight to lug around without the support of water.

Figure 1. Taxa preceding reptiles in the LRT.  Look how small the first reptiles were. Certainly the transition to land was aided by having less weight to lug around without the support of water. 

Figure 1. Taxa preceding reptiles in the LRT. Look how small the first reptiles were. Certainly the transition to land was aided by having less weight to lug around without the support of water. 

Ossinodus still hasn’t gotten enough press
related to its placement in the origin of four legs with toes from fins. Tiktaalik (with lobefins) is its proximal outgroup. Or to the fact that Ossinodus is our first sabertooth! We need to find a complete manus and pes for Ossinodus to see if it had five toes ore more. Presently we don’t know.

Pederpes has five toes. The manus is not well enough known. The narrow skull suggested that Pederpes breathed by inhaling with a muscular action like most modern tetrapods, rather than by pumping air into the lungs with a throat pouch the way many modern amphibians do. The problem with this is Pederpes is basal to both lizards and frogs, which still breathe by buccal (throat pouch) pumping.

Ichthyostega had more than five toes, Which toes are homologous with our five are indicated here (Fig. 2). The extra digits appear between 1 and 2. Does anyone understand why this is so?

Figure 2. Ichthyostega pes with homologous digits numbered. The extra digits appear here between 1 and 2, perhaps due to a return to a more aquatic lifestyle (perhaps more swimming and less bottom walking).

Figure 2. Ichthyostega pes with homologous digits numbered. The extra digits appear here between 1 and 2, perhaps due to a return to a more aquatic lifestyle (perhaps more swimming and less bottom walking).

Arikanerpeton is a basal seymouriamorph in the large reptile tree (LRT). Utegenia is a basal lepidospondyl. Both are close but not very close to origin of reptiles. Perhaps the more direct route, at present, is through Eucritta. That taxon has small hands, but large asymmetric feet with long toes, like reptiles. The long toes of Eucritta (Fig. 3) are not at the ends of long legs, but really short legs, an odd combination.

Figure 3. Eucritta has long toes, but short legs. There's a story there that is presently hard to understand.

Figure 3. Eucritta has long toes, but short legs. There’s a story there that is presently hard to understand. Not sure how deep the pelvis was. Could go either way with present data. 

One wonders if
bullet-shaped Eucritta, coming after longer-legged Tulerpeton, was also secondarily aquatic, like Ichthyostega and Acanthostega.

References
Clack JA 1998. A new Early Carboniferous tetrapod with a mélange of crown group characters. Nature 394: 66-69.
Clack JA 2007. Eucritta melanolimnetes from the Early Carboniferous of Scotland, a stem tetrapod showing a mosaic of characteristics. Transactions of The Royal Society of Edinburgh 92:75-95.
Warren A and Turner S 2004. The first stem tetrapod from the Lower Carboniferous of Gondwana. Palaeontology 47(1):151-184.
Warren A 2007. New data on Ossinodus pueri, a stem tetrapod from the Early Carboniferous of Australia. Journal of Vertebrate Paleontology 27(4):850-862.

wiki/Ossinodus
wiki/Eucritta

Quick note: progress behind the scenes

Apologies
for not getting to the latest comments. I have not opened a week’s worth of snail-mail and bills, so you’re not alone.

Some changes to the LRT
happened while reexamining the data on which the matrix scores are input.

  1. Tulerpeton now nests between Ichthyostega and Eucritta.
  2. Bystrowiella now nests with Solenodonsaurus.

That’s really not a lot of news
for the amount of work that went into getting those. All the related taxa had little changes to toes, teeth, etc. …all toward a greater understanding of what’s going on here. It all started with attempting a lateral view of the skull of Bystrowiella (Fig. 1; (Witzmann and Schoch 2017; Middle Triassic), and see where it led…

Figure 1. Bystrowiella skull in lateral view. Note the large tooth roots on the premaxilla. we don't know how long those buck teeth would have been.

Figure 1. Bystrowiella skull in lateral view. Note the large tooth roots on the premaxilla. we don’t know how long those buck teeth would have been.

References
Witzmann F and Schoch RR 2017. Skull and postcranium of the bystrowianid Bystrowiella schumanni from the Middle Triassic of Germany, and the position of chroniosuchians within Tetrapoda. Journal of Systematic Palaeontology 29 pp.

The conquest of the land: 9 or 10x and counting…

Traditional paleontology 
has given us a picture of a more or less simple ladder of stem tetrapod evolution that had its key moment when an Ichthyostega-like taxon first crawled out on dry land. Then, according to the widely accepted paradigm, certain lineages returned to the water while others ventured forth onto higher and drier environs.

By contrast,
The large reptile tree (LRT, 1033 taxa) documents a bushier conquest of land, occurring in at least seven Devonian waves until the beachhead was secured by our reptile ancestors.

Dr. Jennifer Clack and her team have shown us that fish/amphibians can have limbs (Acanthostega and Ichthyostega) and not be interested in leaving the water. That comes later and later and, well, seven times all together.

Figure 6. Colosteus relatives according to the LRT scaled to a common skull length. Their sizes actually vary quite a bit, as noted by the different scale bars. Only Pholidogaster and Colosteus are taxa in common with traditional colosteid lists.

Figure 1. Colosteus relatives according to the LRT scaled to a common skull length. Their sizes actually vary quite a bit, as noted by the different scale bars. Only Pholidogaster and Colosteus are taxa in common with traditional colosteid lists.

The first wave:
simple small fins to simple small limbs
Arising from lobe-fin fish with one nostril migrating to the inside of the mouth, like Osteolepis, the much larger collosteid, Pholidogaster, had small limbs with toes. The smaller, but equally scaly and eel-like Colosteus, reduced those limbs to vestiges, showing they were not that important for getting around underwater in that wriggly clade. Neither shows signs of ever leaving the water and phylogenetically neither led to the crawling land tetrapods. However, like the living peppered moray eel (Gymnothorax pictus, Graham, Purkis and Harris 2009in search of crabs, these taxa might have made the first landfall without limbs. See terrestrial moray eel video here

Figure 1. Greererpeton reduced to a blueprint of body parts. Here there may be one extra phalanx on pedal digit 5 and one missing on pedal digit 2 compared to sister taxa. So an alternate is shown with that repair. The skulls at left are juveniles.

Figure 2. Greererpeton reduced to a blueprint of body parts. Here there may be one extra phalanx on pedal digit 5 and one missing on pedal digit 2 compared to sister taxa. So an alternate is shown with that repair. The skulls at left are juveniles.

The second wave:
fins to limbs on long flattened bottom feeders
Fully limbed Greererpeton and Trimerorhachis were derived from finny flat taxa like Panderichthys and Tiktaalik. Both Greererpeton and Trimerorhachis were likewise flat- and long-bodied aquatic forms that seem unlikely to have been able to support themselves without the natural buoyancy of water. Their descendants in the LRT likewise look like they were more comfortable lounging underwater like living hellbenders (genus Cryptobranchus. According to Wikipedia: “The hellbender has working lungs, but gill slits are often retained, although only immature specimens have true gills; the hellbender absorbs oxygen from the water through capillaries of its side frills.”  Only rarely do hellbenders leave the water, perhaps to climb on low pond rocks. If the Greererpeton clade was similar, this would have been the second meager and impermanent conquest of the land. And they would not have gone too far from the pond.

Figure 3. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

Figure 3. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

The third wave:
the Pederpes/Eryops clade experimented with overlapping ribs.
Arising from shorter Ossinodus and Acanthostega, a clade that included Pederpes, Ventastega, Baphetes and Eryops arose. This clade looks quite capable of conquering the land for the third time. Their overlapping ribs helped support their short backbone, for the first time lifting their belies off the substrate when doing so, matching Middle Devonian tracks. Some clade members, like Crassigyrinus (with its vestigial limbs) and Saharastega (with its flattened skull) appear to have opted for a return to a watery environment. And who could blame them? In any case, their big lumbering bodies were not well adapted to clambering over dry obstacles, like rocks and plants, that made terrestrial locomotion more difficult. And the biggest best food was still in the water. No doubt limbs helped many of them find new ponds and swamps when they felt the urge to do so, like living crocs. And they probably left the water AFTER some of the smaller and more able taxa listed below.

Figure 6. Proterogyrinus had a substantial neck.

Figure 4. Proterogyrinus had a substantial neck.

The fourth wave:
a longer neck and a smaller head gave us Proterogyrinus.
Ariising from fully aquatic fish/amphibians with overlapping ribs, like Ichthyostega, basal reptilomorphs, like low-slung, lumbering Proterogyrinus took the first steps toward more of a land-living life. The nostrils shifted forward, but were still tiny, at first. Bur the ribs were slender without any overlap. Perhaps this signaled improvements in lung power. Larger nostrils appeared in more devoted air breathers, like Eoherpeton and Anthracosaurus. All these taxa were still rather large and lumbering and so were probably more at home in the water.

Figure 4. Eucritta in situ and reconstructed. Note the large pes in green.

Figure 5. Eucritta in situ and reconstructed. Note the large pes in green.

The fifth wave:
goes small, gets longer legs and gives us Seymouria.
Eucritta is the first of the small amphibians with longer limbs relative to trunk length. This clade also arises from Ichthyostega-like ancestors. One descendant clade begins with a several long-bodied, short-legged salamander-like taxa. Discosauriscus is one of these. It begins life in water, but grows up to prefer dry land. Seymouria is the culmination of this clade. 

Figure 2. Utegenia nests as a sister to Diplovertebron.

Figure 6. Utegenia nests as a sister to Diplovertebron.

The sixth wave:
gives us salamanders and frogs.
Still tied to the water for reproduction and early growth with gills, this clade arises from the seymouriamorph/lepospondyl Utegenia, a short-legged, flat-bodied aquatic taxon. That plesiomorphic taxon gives rise to legless Acherontiscus and kin including modern caecilians. Reptile-mimic microsaurs, like Tuditanus arise from this clade. So do modern salamanders, like Andrias and long-legged, short bodied frogs, like Rana. Their marriage to or divorce from water varies across a wide spectrum in living taxa.

Figure 5. Various stem amniotes (reptiles) that precede Tulerpeton in the LRT. So these taxa likely radiated in the Late Devonian. And taxa like Acanthostega and Ichthyostega are late-survivors of earlier radiations documented by the earlier trackways.

Figure 7. Various stem amniotes (reptiles) that precede Tulerpeton in the LRT. So these taxa likely radiated in the Late Devonian. And taxa like Acanthostega and Ichthyostega are late-survivors of earlier radiations documented by the earlier trackways.

The seventh wave:
gives us the amniotic egg and the reptiles that laid them.
No one should have ever said you have to look like a typical reptile to lay an amnion-covered egg. And if they did, they were not guided by a large gamut phylogenetic analysis. This clade become fully divorced from needing water for reproduction, but basal members still liked the high humidity and wet substrate of the swamp. Arising from basalmost seymouriamorphs like Ariekanerpeton, stem reptiles included Silvanerpeton. These were small agile taxa with relatively long legs that would have had their genesis in the Late Devonian. Their first appearance in the fossil record was much later. The development of the amnion-enclosed embryo may have taken millions of years. The first phylogenetic reptiles appear in the form of amphibian-like Gephyrostegus and Tulerpeton in the Late Devonian, which still had six fingers and scales, but these lacked layers typically found in more fish-like taxa.

So the conquest of the land
by stem and basal tetrapods appears to have occurred seven times, according to the LRT, from distinct clades that were more or less ready to do so and in different ways. And, of course, odd extant fish, like the Peppered moray eel (wave 8) and the mudskipper, (wave 9) and maybe even snakes from stem sea snakes (wave 10) continue this tradition. What will THEY eventually evolve into, given enough time?

References
Clack JA 2006. The emergence of early tetrapods. Palaeogeography Palaeoclimatology Palaeoecology. 232: 167–189.
Clack JA 2009. The fin to limb transition: new data, interpretations, and hypotheses from paleontology and developmental biology. Annual Review of Earth and Planetary Sciences. 37: 163–179.
Coates MI 2014. The Devonian tetrapod Acanthostega gunnari Jarvik: Postcranial anatomy, basal tetrapod interrelationships and patterns of skeletal evolution. Earth and Environmental Science Transactions of the Royal Society of Edinburgh.
Coates MI and Clack JA 1990. Polydactly in the earliest known tetrapod limbs. Nature 347: 66-69.
Graham NAJ, Purkins SJ and Harris A 2009. Diurnal, land-based predation on shore crabs by moray eels in the Chagos Archipelago. Coral Reefs 28(2): 387–397. Online here.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.

wiki/Acanthostega

The Diplovertebron issue resolved…almost

Mystery solved!

Figure 1. Diplovertebron from Watson 1926. He drew this freehand. In DGS the traits are different enough to nest this specimen elsewhere on the LRT. Beware freehand!

Figure 1. Diplovertebron from Watson 1926. He drew this freehand. In DGS the traits are different enough to nest this specimen elsewhere on the LRT. Beware freehand!

Earlier I provided images from Watson 1926 describing a specimen of Diplovertebron (Fig. 1). It took the prodding of a reader (Dr. David M) and a reexamination of several journals to realize that Watson had drawn in freehand the same specimen others (refs. below) had referred to as Gephyrostegus watsoni or as small specimen of G. bohemicus. Since this specimen is not congeneric with Gephyrostegus in the LRT, perhaps the name should revert back to Diplovertebron. Unless the holotype (another specimens comprised of fewer bones) is not congeneric. Then it needs a new name.

Figure 1. Gephyrostegus watsoni (Westphalian, 310 mya) in situ and reconstructed. The egg shapes are near the hips as if recently laid.

Figure 2. The same specimen of Diplovertebron traced and reconstructed using DGS.

Diplovertebron punctatum (Fritsch 1879, Waton 1926; DMSW B.65, UMZC T.1222a; Moscovian, Westphalian, Late Carboniferous, 300 mya) aka:  Gephyrostegus watsoni Brough and Brough 1967) and  Gephyrostegus bohemicus (Carroll 1970; Klembara et al. 2014) after several name changes perhaps this specimen should revert back to its original name as it nests a few nodes away from Gephyrostegus.

This amphiibian-like reptile was derived from a sister to Eldeceeon, close to the base of the Archosauromorpa and Amniota (= Reptiliai). Diplovertebron was basal to the larger Solenodonsaurus and the smaller BrouffiaCasineria and WestlothianaDiplovertebron was a contemporary of Gephyrostegus bohemicus, Upper Carboniferous (~310 mya), so it, too, was a late survivor.

Overall smaller and distinct from Eldeceeon, the skull of Diplovertebron had a shorter rostrum, larger orbit and greater quadrate lean. The dorsal vertebrae formed a hump and had elongate spines. The hind limbs were much longer than the forelimbs. The tail is incomplete, but appears to have been short and deep.

Seven sphere shapes were preserved alongside this specimen. They may be the most primitive amniote eggs known.

Watson 1926 attempted a freehand reconstruction (see below) that was so different from this specimen that for a time it nested as a separate taxon, now deleted.

Figure 1. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. None of these are congeneric.

Figure 3. Watson’s Diplovertebron, the present Diplovertebron (former ©. watsoni) and Gephyrostegus bohemicus. Not sure where Fr. Orig. 128 came from, but that specimen is the same as Watson’s DMSW B.65 specimen at upper right drawn using DGS methods.

The large reptile tree
along with several pages here (PterosaurHeresies) and at ReptileEvoluton.com have been updated.

References
Brough MC and Brough J 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): 147–165.
Carroll RL 1970. The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf
Fritsch A 1879. Fauna der Gaskohle und der Kalksteine der Permformation “B¨ ohmens. Band 1, Heft 1. Selbstverlag, Prague: 1–92.
Klembara J, Clack J, Milner AR and Ruta M 2014. Cranial anatomy, ontogeny, and relationships of the Late Carboniferous tetrapod Gephyrostegus bohemicus Jaekel, 1902. Journal of Vertebrate Paleontology 34:774–792.
Watson DMS 1926. VI. Croonian lecture. The evolution and origin of the Amphibia. Proceedings of the Zoological Society, London 214:189–257.

wiki/Gephyrostegus
wiki/Diplovertebron

A word about competing phylogenetic hypotheses…

…from Coates et al. 2002:
re: basal tetrapods: “Debates about phylogenetic hypotheses concerning these basal nodes are often intense, and conflicts arise over differing taxon and character sets, scores, and coding methods (see Coates et al. 2000; Laurin et al.2000).

And that comes eight yeas before
the advent of ReptileEvolution.com and this blog. So, readers, don’t trust one or another analysis (even this one) before giving them a test on your own or waiting for all the fallout to… fall out. At present, they are competing analyses.

At present
there are broad swathes of agreement in many published trees. The disagreements will ultimately iron themselves out. That some workers object to seeing new solutions to problems they feel they have solved already is just part of the process.

References
Coates MI, Ruta M and Milner AR 2000. Early tetrapod evolution. Trends Ecol. Evol. 15: 327–328.
Coates MI and Ruta M 2001 2002. Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Laurin, M., Girondot, M., and de Ricqlès, A. 2000. Early tetrapod evolution. Trends Ecol. Evol. 15: 118–123.

Microsaurs in the Viséan and Middle Devonian footprints

Figure 1. Which came first? The tracks or the trackmakers? In this case the tracks came first, strong indications that the variety of Devonian trackmakers we have found were all commonplace in the Late Devonian. The variety of basal reptiles and microsaurs found in the Visean must also reflect a wide radiation of derived taxa, pointing to an earlier origin.

Figure 1. Which came first? The tracks or the trackmakers? In this case the tracks came first, strong indications that the variety of Devonian trackmakers we have found were all commonplace in the Late Devonian. The variety of basal reptiles and microsaurs found in the Visean must also reflect a wide radiation of derived taxa, pointing to an earlier origin.

The earliest known microsaur,
Kirktonecta milnerae (Clack 2011, UMZC 2002, Viséan, 330 mya), is not the basalmost microsaur, nor is it a basalmost lepospondyl, the parent clade. In the large reptile tree, Kirktonecta nests with Tuditanus, phylogenetically nesting much more recently than the Utegenia(Lepospondyl) /Silvanerpeton (stem-reptile) split.  That means what we have as taxa in the Visréan represents these taxa when they were commonplace, long after their origination and radiation.

On a related note,
the earliest known tetrapod trackways, the early Middle Devonian Zachelmie trackways, precede all known Devonian trackmakers in the Late Devonian. That means we no longer have to wait for the Late Devonian taxa to begin to evolve the earliest reptiles, but we can still use their morphologies. Now we can begin to evolve reptiles earlier, likely during the Tournasian, the first part of Romer’s Gap, a time for which there are (strangely) few to no fossils during the first 15 million years of the Carboniferous. This time succeeded a major extinction event, the Hangenberg event, in which most marine and freshwater groups became extinct or reduced, including the Ichthyostegalia. Evidently the places where these rare survivors were radiating are currently unknown in the fossil record. These survivors include basal temnospondyls and lepospondyls that also include basal microsaurs.

Fortunately,
the Ichthostegalia had already given rise to a wide range of stem-amphibians and stem-reptiles that ultimately produced all the post-Devonian tetrapods. Those Zachelmie trackways dated 10-18 million years earlier, give more time for reptilomorphs and reptiles to have their genesis and radiation. Post-extinction events traditionally produce new clades. So it appears to be with the genesis of the Reptilia (= Amniota).

The Early Devonian
is where we find Meemannia eos, an early ray-finned fish that was originally classified an early lobe-finned fish. So it didn’t take long after the origin of such fish to develop fingers and toes and move onto land.

This just in:
Recent work by Sallan and Galimberti 2015 showed that only small fish survived the Devonian / Carboniferous extinction event. Read more here. And a paper on Late Devonian catastrophes, impacts and glaciation here.

References
Clack JA 2011. A new microsaur from the early Carboniferous (Viséan) of East Kirkton, Scotland, showing soft tissue evidence. Special Papers in Palaeontology. 86:1–11.

Sallan L and Galimberti AK 2015. Body-size reduction in vertebrates following the end-Devonian mass extinction. Science, 2015; 350 (6262): 812 DOI: 10.1126/science.aac7373