History of reptile Interrelationship hypotheses: Meckert’s PhD thesis

There is a long history
of workers creating hypotheses of reptile interrelationships going back to the mid 18th century (Carl von Linneaus 1758). That history, up until 1995 (Laurin and Resiz 1995 and Meckert 1995), was summarized by Dirk Meckert in his PhD thesis, which otherwise  concentrated on all available specimens of Barasaurus. You can download that thesis here online and read that short but fascinating history for yourself.

Some interesting notes arise from Meckert’s short history:

  1. Some studies united pareiasaurs and turtles. Others did not.
  2. Other studies united pareiasaurs, diadectids and procolophonids (which happened here just yesterday). Meckert wrote: “The Procolophoniformes contain Procolophonia and Testudinomorpha as sister-groups. Testudines are the sister-group of Pareiasauria within the Testudinomorpha.”
  3. Mesosaurs are commonly considered of uncertain affinities. But not here.
  4. Many prior studies had the synapsids branch off first. That is incorrect as shown here.
  5. No prior studies recognized the original dichotomy of lepidosauromorphs and archosauromorphs.
  6. No prior studies recognized Gephyrostegus bohemicus as a sister to the basalmost amniote.
  7. Diadectomorpha have been nested in and out of the Amniota. They’re in here.

No studies prior to reptileevolution.com
have included as many as 571 individual species as taxa, not counting the therapsid tree (with 52 additional taxa) and pterosaur tree (with 228 additional taxa) for a total of 851 taxa.

Other studies more recent than 1995
(not included in Meckert’s history) include

  1. http://www.palaeos.org/Reptilia and http://palaeos.com/vertebrates/amniota/reptiles.html
  2. http://whozoo.org/herps/herpphylogeny.html
  3. https://en.wikipedia.org/wiki/Amniote as determined by Benton, M.J. (2004). Vertebrate Paleontology. Blackwell Publishers. xii–452.
  4. University of Maryland (John Merck)
  5. online pdf, Amniote Origins and Nonavian Reptiles
  6. YouTube video by Walter Jahn
  7. Tree of Life
  8. Hedges 2012
  9. Gauthier, Kluge and Rowe 1988 online
  10. Hill 2005
  11. Mikko’s phylogeny archive
  12. ReptileEvolution.com
  13. Let me know if I missed any. I’ll add them here.

A while back
we looked at the differences between astronomy and paleontology. As noted earlier, time is never of the essence in paleontology — and that extends to idea acceptance. So many hypotheses of reptile interrelationships are still floating around out there. A definitive and all encompassing demonstration, like the large reptile tree, will probably just float forever with the other several dozen hypotheses out there, hashed, rehashed and rehashed again without end.

This is one of the frustrations of paleontology. And many think it is largely ego driven.

On that note
In astronomy the data, be it observation or spectral analysis, is immediate and widespread. You just have to look up with the right tool in the right direction. Or study the shared data (photos, etc.) Everyone can confirm the observation.

In paleontology the data comes out piecemeal, in low resolution, or imprecise tracings, not from every angle of view. Some key parts are lost and others are hidden beneath other bones or matrix. Sometimes you have to assemble dozens or hundreds of specimens for a proper study. No one is interested in confirming observations or analyses perhaps for years if ever. They’re all too busy with their own projects. Checking the characters and scores of an analysis can take weeks, months or years (as long as it took to build originally), and to do so requires the same amount of globe-hopping to see all the specimens in all the museums. No one is going to do that. They’d rather be making their own discoveries… and adding their taxa to established trees created by hungry PhD candidates, like Dirk Meckert in 1995, done at the nadir or advent of their experience.

The paleo-mantra remains: you must see the specimen!
And even that is no guarantee.

And if you want to break a paradigm or two,
like Ostrom did in the 1960s, you might have to wait for widespread (but never universal) acceptance. Paleontologists like their paradigms. They don’t like to give them up.

References
Benton MJ 2004. Vertebrate Paleontology. Blackwell Publishers. xii–452.
Carroll RL 1988. 
Vertebrate Paleontology and Evolution, WH Freeman & Co.
Laurin M and Reisz R 1995. 
A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society, 113: 165–223.
Linnaeus C 1758. 
Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Meckert D 1995.
 The procolophonid Barasaurus and the phylogeny of early amniotes. PhD thesis McGill University. Online Barasaurus dissertation

 

 

 

Convergent Evolution Among Reptiles – part 2

Yesterday we looked at part 1 of convergence among reptiles. Today we’ll add a few other examples from the large reptile tree.

Silesaurus and dinosaurs: Both developed a bipedal configuration without a calcaneal tuber. All other poposaurs have a tuber. In addition, ornithischian dinos shared a predentary with Silesaurus.

Legless lizards: snakes, legless skinks, legless geckos. It is widely known that these squamate taxa all lose their limbs independently. The origin of snakes  was last covered here with Tetrapodophis.

Finbacks: Spinosaurs, sphenacodonts, Lotosaurus, Arizonasaurus.These diverse taxa all have elongate dorsal spines creating a tall back fin.

Procolophonids and pareiasaurs: These unrelated taxa, Hypsognathus and Anthodon, both developed spiky laterally expanded cheek bones (quadratojugals).

Tetraceratops and basal therapsids: Both of these taxa had enlarged canines and many paleontologists consider Tetraceratops a basal therapsid. According to the large reptile tree, Tetraceratops is related to Tseajaia, which also has smaller canine teeth.

Trioceros and Triceratops: Both of these unrelated taxa developed three facial horns. Trioceros is a chameleon. Triceratops is a dinosaur.

Four clades of pterodactyloid-grade pterosaurs and two other demi-pterodactyloid pterosaurs: Various basal pterosaurs developed pterodactyloid-grade traits. Two dorygnathid taxa evolved into azhdarchids and ctenochasmatids. Two scaphognathid taxa evolved into cycnorhamphids + ornithocheirids and germanodactylids + pterodactyl ids. Wukongopterids developed some, but not all pterodactyloid-grade traits (they retain a long pedal digit 5 and long tail). Anurognathids do likewise (but they retain separate nares and a long pedal digit 5).

Azhdarchid pterosaurs and eopteranodontid pterosaurs: Both of these unrelated clades developed medium to large stork-like forms.

Longisquama and Lemur: Both of these unrelated taxa (Fig. 1) had a small skull with large orbits on a short skull, a long dorsal series, an attenuated tail held aloft, shorter forelimbs and very long leaping hind limbs.

Figure 1. Longisquama (Triassic fenestrasaur) compared to a modern Lemur. Similar body shapes might imply similar locomotory patterns.

Figure 1. Longisquama (Triassic fenestrasaur) compared to a modern Lemur. Similar body shapes might imply similar locomotory patterns. Click to enlarge.

Cartorhynchus, ichthyosaurs, plesiosaurs, certain mosasaurs and sea turtles: Cartorhynchus had large, ichthyosaur-like/plesiosaur-like fore flippers, and small hind flippers, like living sea turtles, but is related to basal pachypleurosaurs, all of which had distinct fingers.

Multiple tooth rows: captorhinids and Azendohsaurus + Trilophosaurus + Rhynchosauria: Developed independently. these taxa had multiple tooth rows developed on the maxilla and palatine.

Hyper-elongate necks: tanystropheids (Fig. 2) and sauropods. Both of these independent clades are famous for their long necks.

Figure 1. Click to enlarge. Four large Tanystropheus specimens in situ and reconstructed. The man silhouette is 6 feet (1.8m) tall.

Figure 1. Click to enlarge. Four large Tanystropheus specimens in situ and reconstructed. The man silhouette is 6 feet (1.8m) tall.

Revisions to the large reptile tree

Figure 1. Subset of a revised large reptile tree. Taxa that have moved are highlighted.

Figure 1. Subset of a revised large reptile tree. Taxa that have moved are highlighted.

A short note today.
Neil Brocklehurst was kind enough to run the large reptile tree through some testing that revealed weaknesses at several nodes. These weaknesses were caused by inappropriate scores. For the most part these have now been repaired.

The tree was and is still fully resolved (except at one node where a skull-only taxon nests with a skull-less taxon). Bootstrap scores are higher. Thank you, Neil.

Many scores were corrected with new data. Some were corrected with better judgement. I reinflated a crushed skull here. Several officially unnamed or misnamed taxa moved slightly. Saltoposuchus moved the most from the base of the proto-dinos to the middle of the basal crocs where it more closely resembles its new sister taxa. That taxon always seemed to be an oddball before.

Many of these mobile taxa are known from drawings only, some decades old. I removed multiples specimens of Mesenosaurus, leaving the holotype in place. When taxa resemble each other too much (fewer than 3-4 steps in difference) that leads to loss of resolution. The base of the synapsids close to Mesenosaurus was also rescored.  You can see the complete tree here. Other than these few changes, the basic tree topology remains largely unchanged.

It’s always refreshing to find errors and make repairs. And, thank you again, Neil.

 

Nesting turtles with pterosaurs redux 2011-2015

For those who don’t read the ‘Letters to the Editor’,
a recent comment on sister taxa inspired me to revisit the old experiment that nested pterosaurs and turtles together as a result of taxon exclusion, which you can review here.

By default nestings
can be interesting and silly. The point behind nesting pterosaurs with turtles back then was to examine the folly behind nesting pterosaurs with archosaurs — only possible due to a similar taxon exclusion that’s been going on for at least fifteen years now, following the publication of a phylogenetic analysis that nested pterosaurs with fenestrasaurs (Peters 2000) and has been ignored ever since.

Back in the day (July 2011) with 360 taxa,
when all other taxa were removed from the lepidosauromorph side of the large reptile treeProganochelys, the turtle, nested with MPUM6009, the pterosaur at the base of the Sauropterygia. That’s bizarre, but interesting and hopefully enlightening by analogy to the achosaur-link question.

Today,with 508 taxa,
and deleting all other lepidosauromorphs, the pterosaur now nests between Cathayornis and Struthio, the ostrich, + Gallus, the chicken. The turtle now nests with the frogs, between Doleserpeton and Gerobatrachus + Rana.

Hmm. Let’s fix that.
Let’s delete the amphibians and add the basal lizard Huehuecuetzpalli and guess what happens?

The three lepidosauromorphs:
the turtle, the lizard and the pterosaur, all nest together again in their own clade at the base of the Sauropterygia… in other words, nowhere near dinos, pre-dinos, parasuchians, Lagerpeton or Marasuchus. Delete Huehuecuetzpalli and Proganochelys nests with the turtle-like placodont, Henodus, as you might imagine, while the basal pterosaur bounces back to the birds. So one taxon in-between the turtle and pterosaur were needed this time to glue them together in a single clade and to trump the attraction of other candidate sisters.

Bottom line:
by including more and more taxa the large reptile tree provides more and more nesting sites, and thus the large reptile tree minimizes unwanted ‘by default’ nestings. Up to now other workers have been relying an tradition and paradigm for their taxon lists, and many of those traditions have been tested (and falsified) at reptileevolution.com. When workers base their smaller, more focused studies on a larger umbrella study, they will have greater success and greater confidence that their cladogram is a good one = with no ‘by default’ nestings.

News at the base of the Amniota, part 6: Cladogram of basal lepidosauromorpha

Yesterday we looked at primitive archosauromorpha at the base of the Amniota. Today we’ll look at basal lepidosauromorpha.

Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom. Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom.

Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom.

Figure 1. A new reconstruction of Gephyrostegus bohemicus. This species lived 30 million years after the origin of the Amniota in the Visean, 340 mya. Note the lack of posterior dorsal ribs. This trait shared by all basalmost amniotes, may provide additional space for massive eggs in gravid females, but is also shared with males, if there were males back then.

Figure 2. A new reconstruction of Gephyrostegus bohemicus. This species lived 30 million years after the origin of the Amniota in the Visean, 340 mya. Note the lack of posterior dorsal ribs. This trait shared by all basalmost amniotes, may provide additional space for massive eggs in gravid females, but is also shared with males, if there were males back then.

As mentioned earlier, the Amniota is divided at its base into Lepidosauromorpha (taxa closer to lepidosaurs) and Archosauromorpha. Gephyrotegus bohemicus (Fig. 2) is the last common ancestor and Silvanerpeton is the outgroup anamniote.

Figure 3. Urumqia liudaowanensis (Zhang et al. 1984) ~20 cm snout-vent length, Lower Permian.

Figure 3. Urumqia liudaowanensis (Zhang et al. 1984) ~20 cm snout-vent length, Lower Permian. Formerly considered a sister to Utegenia, an anamniote, it now nests as the basalmost of all lepidosauromorpha.

Urumqia liudaowanensis (Zhang et al. 1984) ~20 cm snout-vent length, Upper Permian, was originally considered a discosaurid seymouriamorph close to UtegeniaHere (Fig. 1) it nests at the base of the lepidosauromorph reptiles despite its late appearance in the fossil record. Note the gastalia are much wider than the dorsal ribs, likely to retain large eggs in gravid females. Distinct from G. bohemicus, Urumqia had shorter limbs, longer posterior dorsal ribs and a robust tail with elongate caudals. The palate included a larger suborbital fenestra, not homologous to later taxa with this trait. The cheek included a small lateral temporal fenestra. The carpals and tarsals are poorly ossified.

Figure 2. Bruktererpeton, a gephyrostegid and a basal lepidosauromorph amniote.

Figure 4. Bruktererpeton, a gephyrostegid and a basal lepidosauromorph amniote.

Bruktererpeton fiebigi — (Boy and Bandel, 1973; Fig. 3) is an older (Namurian/ Bashkirian, 320 Ma) sister to Gephyrostegus bohemicus (Ruta, Jefferey and Coates, 2003; Klembara et al., 2014). The pectoral girdle and limbs are more gracile. The scapula is taller. The intercentra are smaller. Other traditional amniote traits, if present, are not preserved.

Figure 3. Thuringothyris. A basal lepidosauromorph.

Figure 5. Thuringothyris. A basal lepidosauromorph.

Thuringothyris  mahlendorffae — (Boy and Martens, 1991; type: MNG 7729; (Müller et al., 2006) referred MNG 10183; Artinskian, Early Permian, 280 Ma) is half the size of Bruktererpeton and documents all traditional amniote traits. Note the derived shape of the humerus and the reduced intercentra.

Traditional amniote traits include:

  1. loss/fusion of the intertemporal
  2. absence of the otic notch
  3. loss/reduction of palatal fangs
  4. appearance/expansion of the transverse flange of the pterygoid
  5. loss of labyrinthine infolding of the marginal teeth
  6. reduction of the intercentra
  7. addition of a second sacral vertebra
  8. narrowing and elongation of the humeral shaft
  9. appearance of the astragalus from fused tarsal elements.
Figure 4. Cephalerpeton. A basal lepidosauromorph.

Figure 6. Cephalerpeton. A basal lepidosauromorph.

Cephalerpeton — (Gregory 1948) representing a new sister clade to the Captorhinomorpha, Cephalerpeton had an elongate humuerus with a narrow shaft. The much larger and later Reiszorhinus is a sister.

Figure 5. Two specimens of Concordia, a basal lepidosauromorph.

Figure 7. Two specimens of Concordia, a basal lepidosauromorph.

Concordia — (Müller & Reisz 2005, Stephanian, Late Pennsylvanian, Carboniferous, 4 cm skull length) was considered the oldest known captorhinid, but here (Fig. 1) it nests with Cephalerpeton as a sister to captorhinids.

Figure 6. Romeria texana, a basal capitorhinomorph, lepidosauromorph, amniote.

Figure 8. Romeria texana, a basal capitorhinomorph, lepidosauromorph, amniote.

Romeria texana —(Price1937) Artinskian, Early Permian, ~280 mya, ~25 mm skull length, was the basalmost captorhinid. Here (Fig, 1) the skull is wider and flatter.

Figure 6. Saurorictus, a basal lepidosauromorph in the lineage of Milleretta, compared to sister taxa.

Figure 9. Saurorictus, a basal lepidosauromorph in the lineage of Milleretta, compared to sister taxa.

Saurorictus — (Modesto and Smith 2001, SAM PK-8666, skull length ~2.2 cm, estimated total length 15 cm, Late Permian), derived from a sister to Thuringothyris, Concordia and Cephalerpeton, Saurorictus is the taxon basal to all other lepidosauromorpha including diadectomorpha, chelonia and lepidosauria. (sorry, cut off from bottom of cladogram, Fig. 1). It was considered the most complete captorhinid from the Late Permian.

Size comparisons

Figure 1. Basal amniotes to scale. Click to enlarge.

Figure 10. Basal amniotes to scale. Click to enlarge.

Here, Fig. 10, there is a size reduction in ‘second generation’ basal amniotes/basal lepidosauromorpha. You’ll note that several former anamniotes now nest within the amniota. They were judged anamniotes by the skeletal traits, not by their phylogenetic nesting, which has not been adequately tested until now.

References
Boy JA and Bandel K 1973. Bruktererpeton fiebigi n.gen.n.sp. (Amphibia: Gephyrostegida). Der erste Tetrapode aus dem Rheinisch-Westfälischen Karbon (Namur B; W-Deutschland). Palaeontographica 145: 39–77.
Boy JA and Martens T 1991. Ein neues captorhinomorphes Reptil aus dem thüringischen Rotliegend (Unter-Perm; Ost-Deutschland). Palaeontologische Zeitschrift 65 (3-4): 363–389.
Gregory JT 1948. The structure of Cephalerpeton and affinities of the Microsauria. American Journal of Science 246:550–568
Modesto SP and Smith RMH 2001. A new Late Permian captorhinid reptile: a first record from the South African Karoo. Journal of Vertebrate Paleontology 21(3): 405–409.
Müller J, Berman DS, Henrici AC, Martens T and Suminda S 2006. The basal reptile Thuringothyris mahlendorffae (Amniota:Eureptilia) from the Lower Permian of Germany. Journal of Paleontology 80:726-739.
Müller J and Reisz RR 2005. An early captorhinid reptile (Amniota: Eureptilia) from the Upper Carboniferous of Hamilton, Kansas. Journal of Vertebrate Paleontology. 25(3): 561-568.
Price LI 1937. Two new cotylosaurs from the Permian of Texas. Proceedings of the New England Zoölogical Club 16:97-102.
Zhang F, Li Y, and Wan X. 1984. A new occurrence of Permian seymouriamorphs in Xinjiang, China. Vertebrate Palasiatica22(4):294-306.

News at the base of the Amniota, part 5: Cladogram of basal archosauromorpha

Earlier here and elsewhere we looked at the origin of the Amniota. Today we’ll take a look at the cladogram (Fig. 1) and some of the taxa no one expected to see on this side of the anamniote/amniote transition series.

Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom. Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom.

Figure 1. Cladogram of basal amniotes, a subset of the large reptile tree. Dots represent phylogenetic size reductions. Bootstrap scores are shown. Archosauromorpha in gray. Lepidosauromorpha in black at the bottom.

As before, the Amniota is divided at its base into the new Lepidosauromorpha (taxa closer to lepidosaurs) and the new Archosauromorpha (closer to archosaurs).

Figure 1. A new reconstruction of Gephyrostegus bohemicus. This species lived 30 million years after the origin of the Amniota in the Visean, 340 mya. Note the lack of posterior dorsal ribs. This trait shared by all basalmost amniotes, may provide additional space for massive eggs in gravid females, but is also shared with males, if there were males back then.

Figure 2. A new reconstruction of Gephyrostegus bohemicus. This species lived 30 million years after the origin of the Amniota in the Visean, 340 mya. Note the lack of posterior dorsal ribs. This trait shared by all basalmost amniotes, may provide additional space for massive eggs in gravid females, but is also shared with males, if there were males back then.

Gephyrotegus bohemicus (Fig. 1, Westphalian, 310 mya) is the last common ancestor of all amniotes and Silvanerpeton (Viséan, 340 mya) is the outgroup anamniote (or very possible also an amniote).

Utegenia nests as the common ancestor of frogs, salamanders, caecelians and microsaurs.

Figure 3. Utegenia nests as the common ancestor of frogs, salamanders, caecelians and microsaurs but the only known specimens are from the Earliest Permian.

Note the placement of the seymouriamorph, Utegenia (Fig. 3), at the base of the Lepospondyli, which includes extant amphibians and microsaurs… and just outside the base of the Amniota.

Basal Archosauromorpha

Figure 3. Two specimens attributed to Eldeceeon that nest together.

Figure 4. Two specimens attributed to Eldeceeon that nest together. The lack of posterior dorsal ribs was first noticed in the holotype.

Eldeceeon rolfei  – (Smithson 1994, ~27 cm in total length, Early Carboniferous (Viséan) ~335 mya), is from the same formation that yielded Silvanerpeton and Westlothiana. Eldeceeon is known from two dissimilar specimens that nest together. They have a smaller skull and slightly shorter limbs with smaller girdles while retaining a deep ventral pelvis.

Gephyrostegus-watsoni588

Figure 5. Gephyrostegus watsoni (Westphalian, 310 mya) reconstructed. Embryo is hypothetical. Note the lack of posterior dorsal ribs.

Gephyrostegus watsoni – (Brough and Brough 1967) was originally named Diplovertebron punctatum (Watson 1926, Fig. 5), but reassigned to Gephyostegus bohemicus by Carroll (1970) despite the size difference. Carroll thought G. watsoni was a juvenile. Klembara et al. (2014) agreed. The high arched neural spines, small intercentra, and the extreme lean of the posterior skull mark this small basal amniote/gephyrostegid distinct from all others. Egg shapes were found nearby along with insects. The embryo shown is hypothetical.

Figure 4. Solenodonsaurus reconstructed.

Figure 6. Solenodonsaurus reconstructed. The largest of the basal amniotes, likely aquatic. Note the intertemporal is still present. That doesn’t matter. It still nests with amniotes.

Solenodonsaurus janenschi – (Broili 1924) Early Permian ~13 cm skull length was considered the sister to all other amniotes by all prior workers, but here Solenodonsaurus nests as a basal archosauromorph, basal to chroniosuchids.

Figure 5. Three chorniosuchids to scale.

Figure 7. Three chorniosuchids to scale.

Chroniosuchids – (Tverdokhlebova 1972) Early Permian ~7 cm skull length, were considered aberrant pre-reptiles by all prior workers, but here they nest within the Archosauromorpha. Note the convergent appearance of an antorbital fenestra.

Figure 7. Casineria reconstructed.

Figure 8. Casineria reconstructed.

Casineria kiddi – (Paton, Smithson & Clack 1999) Viséan, Carboniferous, ~335 mya). Tiny Casineria lies at the end of a phylogenetic series of decreasing size beginning with Proterogyrinus.

Figure 8. Westlothiana reconstructed.

Figure 9. Westlothiana reconstructed. The gray area is hypothetical as if gravid.

Westlothiana – (Smithson & Rolfe 1990) lived ~338 mya, earlier than any other known reptile. This reconstruction has longer anterior dorsal ribs and shorter posterior dorsal ribs than originally reconstructed. A longer torso is a different solution to egg containment.

brouffia588overall

Figure 10. Broffia reconstructed. The smallest of all basal amniotes, this could be a juvenile or just a small adult.

Brouffia orientalis – (Carroll and Baird 1972) Westphalian, Late Carboniferous, (CGH IIIB 21 c. 587) and counterpart (MP451), specimen 1 of Brough and Brough (1967) was considered very small Gephyrostegus with two sacrals and an intertemporal. Carroll (1970) considered it not congeneric. Carroll and Baird (1972) considered it a primtiive reptile with a single sacral and no intertemporal. The missing skull of the sister taxon Casineria (Fig. 8) probably looks like this one.

We’ll look at basal Lepidosauromorpha tomorrow.

References
Broili F von 1924. Ein Cotylosaurier aus der oberkarbonischen Gaskohle von Nürschan in Böhmen. Sitzungsberichte der Mathematisch-Naturwissenschaftlichen Abteilung der Bayerischen Akademie der Wissenschaften zu München 1924: 3-11.
Brough MC and Brough J 1967. Studies on early tetrapods. III. The genus Gephyrostegus. Philosophical Transactions of the Royal Society B252: 147-165.
Brough MC and Brough J 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 252 (776): 147–1
Carroll RL 1970. The Ancestry of Reptiles. Philosophical Transactions of the Royal Society London B 257:267–308. online pdf
Carroll RL 1970. The ancestry of reptiles. Philosophical Transactions of the Royal Society B257: 267-308.
Clack JA and Klembara J 2009. An articulated specimen of Chroniosaurus dongusensis and the morphology and relationships of the chroniosuchids. Special Papers in Palaeontology, 81: 15–42.
Danto M, Witzmann F and Müller J 2012. Redescription and phylogenetic relationships of Solenodonsaurus janenschi Broili, 1924, from the Late Carboniferous of Nyrany, Czech Republic. Fossil Record 15 (2) 2012, 45–59.
Klembara J, Clack J, and Cernansky A 2010. The anatomy of palate of Chroniosaurus dongusensis (Chroniosuchia, Chroniosuchidae) from the Upper Permian of Russia. Palaeontology 53: 1147-1153.
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.
Laurin M and Reisz 1999. A new study of Solenodonsaurus janenschi, and a reconsideration of amniote origins and stegocephalian evolution. Canadian Journal of Earth Sciences 36:1239-1255.
Paton RL Smithson TR and Clack JA 1999. An amniote-like skeleton from the Early Carboniferous of Scotland. Nature 398: 508-513.
Schoch RR, Voig S and Buchwitz M 2010. A chroniosuchid from the Triassic of Kyrgyzstan and analysis of chroniosuchian relationships. Zoological Journal of the Linnean Society 160: 515–530. doi:10.1111/j.1096-3642.2009.00613.x
Smithson TR 1994. Eldeceeon rolfei, a new reptiliomorph from the Viséan of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (3-4): 377–382.
Smithson TR & Rolfe WDI 1990. Westlothiana gen. nov. :naming the earliest known reptile. Scottish Journal of Geology no 26, pp 137–138.
Tverdochlebova GI 1972. A new Batrachosaur Genus from the Upper Permian of the South Urals, Paleontol. Zh., 1972: 95–103.

News at the base of the Amniota, part 4: Keratinized epidermal scales

Earlier here, here and here we looked at various aspects of life for basal amniotes in the Viséan to the Westphalian (340-310 mya). Today we’ll look at another trait common to basal amniotes.

Figure 1. Amniote scales from Didelphis (opossum, background) and Iguana.

Figure 1. Amniote scales from Didelphis (opossum, background) and Iguana. They probably had their origin in the Viséan as basal amniotes spent less and less time in water and needed a form of waterproofing to avoid desiccation.

Phylogenetic Miniaturization and the Genesis of Keratinized Scales
Keratinized scales (Fig. 1) more or less insulate many living amniotes and all living reptiles from evaporative water loss. Anamniotes (frogs, etc.) don’t have a waterproof skin, but may have scattered osteoderms. While many mammals replace scales with fur and birds replace scales with feathers, keratinized scales are retained on the tail of the opossum and the feet of all birds. Scalation also protects against ant and termite bites.

Figure 1. A new reconstruction of Gephyrostegus bohemicus. This species lived 30 million years after the origin of the Amniota in the Visean, 340 mya. Note the lack of posterior dorsal ribs. This trait shared by all basalmost amniotes, may provide additional space for massive eggs in gravid females, but is also shared with males, if there were males back then.

Figure 2. A new reconstruction of Gephyrostegus bohemicus, this most primitive amniote preserves dorsal dermal scales. Ossified ventral scales are more common and sometimes transformed into gastralia rods.

Scale origins
Basal gnathostomes (basal fish) had ossified skin first and an ossified skeleton later. In teleosts and tetrapods the integumentary (skin) skeleton has undergone widespread reduction and/or modification (Vickaryous and Sire 2009). Sarcopterygians (stem tetrapods) had cosmoid scales, characterized by an intrinsic, interconnected canal system with numerous flask-shaped cavities and superficial pores. In Ichthyostega and other basal tetrapods, dentine, enameled, guanine and pore-canal systems were lost, leaving bone (osteoderms) as the remaining dermal element wherever present. Osteoderms are structurally quite variable and are found in a wide variety of tetrapods, including amphibians. Often they have been lost and independently regained. Temnospondyl amphibians, like Greererpeton, have a combination of thin and overlapping scales, granular pellets, and/or robust plates.

Amniote scale origins
While scales or their impressions are rarely preserved, Carroll (1969) reported some indication of dorsal epidermal scales in Gephyrostegus bohemicus (Fig.2). In Cephalerpeton Carroll and Baird (1972) reported that skin impressions had a slightly pebbly texture, but without evidence for discrete scales. Brough and Brough (1967) found ventral scales in tiny Brouffia (their specimen no. 1) and provided a similar description for the developing gastralia of Gephyrostegus. Not sure why the first and most substantial scales first appeared ventrally, rather than dorsally, where the sun shines, except that the ventral surface is in contact with the substrate.

Figure 2. Reptile hatchling.

Figure 2. Reptile hatchling about actual size. A larger surface-to-volume ratio increases the danger from desiccation unless ‘waterproofed’ with scales.

The importance of scales
In basalmost amniotes forays onto land likely increased in duration. Dermal protection from desiccation is more important in smaller amniotes due to their larger surface-to-mass ratio (Hedges and Thomas, 2001). This is especially applicable to hatchlings and juveniles, some of which may have been less than 2 cm in snout/vent length because the adults were so small. Basal amniote juveniles would have rivaled certain microsaur juveniles as the smallest tetrapods of their day, and perhaps they competed in similar niches. Scales may have given the advantage to reptiles.

With regard to microsaurs
Carroll and Baird (1968) reported, “Extremely delicate dorsal scales of elongate-oval shape are present between (and occasionally overlapping) the ribs of the better-ossified United States National Museum specimen [of the microsaur Tuditanus]. If allowance is made for their insubstantial nature, the scales of Tuditanus are essentially similar to those of ther Carboniferous microsaurs (Carroll, 1966). There is no evidence of rodlike ventral scales such as occur in other lepospondyls (Baird, 1965), or wheat-shaped gastralia like those of primitive reptiles.”

Going one step further in the middle Jurassic, tiny basal mammals traded scales for insulating hair and thicker fur (assumed by phylogenetic bracketing). On the other hand, the first Jurassic dinosaurs to preserve protofeathers, like slender Sinosauropteryx (Ji and Ji, 1996), were a meter long, so not tiny. Basal volant birds became progressively smaller.

References
Baird D 1965. Paleozoic lepospondyl amphibians.  American Zoologist 5: 287-294.
Brough MC and J Brough 1967. The Genus Gephyrostegus. Philosophical Transactions of the Royal Society London, Series B, Biological Sciences 252:47–165.
Carroll RL 1966. Microsaurs from the Westphalian B of Joggins, Nova Scotia. Proceedings of the Linnean Society of London 177: 63-97.
Carroll RL 1969. Problems of the origin of reptiles. Biological Reviews 44:393–431.
Carroll RL and Baird D 1968. The Carboniferous amphibian Tuditanus (Eosauravus) and the distinction between microsaurs and reptiles. American Museum novitates 2337: 1-50.
Carroll RL. and D Baird 1972. Carboniferous stem-reptiles of the family Romeriidae. Bulletin of the Museum of Comparative Zoology 143:321–363.
Hedges SB and R Thomas 2001. At the lower size limit in amniote vertebrates: A new diminutive lizard from the West Indies. Caribbean Journal of Science 37:168–173.
Ji Q and S Ji 1996. On the discovery of the earliest bird fossil in China (Sinosauropteryx gen. nov.) and the origin of birds. Chinese Geology 10(233): 30–33.
Vickaryous MK and Sire J-Y 2009. The integumentary skeleton of tetrapods: origin evolution, and development. Journal of Anatomy 214:441-464. online here.