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

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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

News at the base of the Amniota, part 8: The list of Viséan amniotes has grown.

Earlier (in seven prior blog posts) we looked at the new basalmost amniotes and how they evolved.

With the news
that the Amniota (and amniote eggs) extended back to the Viséan (326-345 mya) let’s take a look at those basalmost amniotes along with the genera that may have been more primitive, but survived more than 30 million years later, into the Westphalian (303-311 mya) and beyond to the Early and Late Permian. That’s a stretch of 80 million years for the most successful taxa. And what made them so successful? Or were they all just as successful, just not found yet in higher strata?

Figure 1. Basal amniotes to scale colorized according to the time strata in which their fossils were found. Visean, yellow; Namurian, pink; Westphalian, blue; Permain, tan.

Figure 1. Click to enlarge. Basal amniotes to scale colorized according to the time strata in which their fossils were found. Visean, yellow; Namurian, pink; Westphalian, blue; Permian, tan.

It’s well worth remembering
at this point that in similar fashion, basal primates, like lemurs, also co-exist today with derived primates, like apes and humans. So that happens. At least some of these basalmost amniotes (the Permian forms, like Utegenia, Fig. 1) developed successful traits so well matched to their own niche they survived for tens of millions of years thereafter.

Also remember
that fossilization is a rare event. Even more rare is the discovery of rare fossils. So we’re very lucky to have even single examples of these taxa. They were probably more widespread both across the globe and through time.

Two important points
1) We don’t find amniotes prior to the Viséan. So these Viséan amniotes  (Fig. 1) are likely the earliest representatives of their kind.

2) The Viséan is a short 15 to 35 million years after the very first tetrapods developed limbs from fins some 360 million years ago in the Late Devonian. So evolution was rapid during those first 15 million years. Not so rapid for the next 80 million years, at least for certain taxa.

That’s exciting to think about.