Ichthyostega and Acanthostega: secondarily more aquatic

More heresy here
as the large reptile tree (LRT, 1036 taxa) flips the traditional order of fins-to-feet upside down. Traditionally the late Devonian Ichthyostega and Acanthostega, bridge the gap between lobe-fin sarcopterygians, like Osteolepis.

In the LRT
Acanthostega, ‘the fish with limbs’, nests at a more derived node than its precursor, the more fully limbed, Ossinodus (Fig. 1). Evidently neotony, the retention of juvenile traits into adulthood, was the driving force behind the derived appearance of Acanthostega, with its smaller size, stunted limbs, smaller skull, longer more flexible torso and longer fin tail.

Figure 1. Ossinodus is the more primitive taxon in the LRT compared to the smaller Acanthostega, the tadpole of the two.

Figure 1. Ossinodus is the more primitive taxon in the LRT compared to the smaller Acanthostega, essentially the neotenous ‘tadpole’ of the two.

Likewise
Ichthyostega is more derived than both fully-limbed Ossinodus and Pederpes, which had five toes. As in Acanthostega, the return to water added digits to the pes of Ichthyostega. In both taxa the interosseus space between the tibia and fibula filled in to produce a less flexible crus.

Figure 2. Ossinodus, Pederpes were more primitive than the more aquatic Icthyostega.

Figure 2. Long-limbed Ossinodus and Pederpes were more primitive than the more aquatic Icthyostega.

So, Acanthostega and Ichthyostega were not STEM tetrapods.
Instead, they were both firmly nested within the clade Tetrapoda. Ossinodus lies at the base of the Tetrapoda. The proximal outgroups are similarly flattened Panderichthys and Tiktaalik. The extra digits displayed by Acanthostega and Ichthyostega may or may not tell us what happened in the transition from fins to feet. We need to find a derived Tiktaalik with fingers and toes.

Figure 3. Tiktaalik specimens compared to Ossinodus.

Figure 3. Tiktaalik specimens compared to Ossinodus.

In cases like these
it’s good to remember that ontogeny recapitulates phylogeny. Today and generally young amphibians are more fish-like (with gills and fins) than older amphibians.

It’s also good to remember
that the return to the water happened many times in the evolution of tetrapods. There’s nothing that strange about it. Also the first Devonian footprints precede the Late Devonian by tens of millions of years.

Figure 4. From the NY Times, the traditional view of tetrapod origins.  Red comment was added by me.

Figure 4. From the NY Times, the traditional view of tetrapod origins. 

Phylogenetic analysis teaches us things
you can’t see just by looking at the bones of an individual specimen. A cladogram is a powerful tool. The LRT is the basis for many of the heretical claims made here. You don’t have to trust these results. Anyone can duplicate this experiment to find out for themselves. Taxon exclusion is still the number one problem that is largely solved by the LRT.

You might remember
earlier the cylindrical and very fish-like Colosteus and Pholidogaster convergently produced limbs independently of flattened Ossinodus, here the most primitive taxon with limbs that are retained by every living tetrapod. By contrast, the Colosteus/Pholidogaster experiment did not survive into the Permian.

References
Ahlberg PE, Clack JA and Blom H 2005. The axial skeleton of the Devonian trtrapod Ichthyostega. Nature 437(1): 137-140.
Clack JA 2002.
 Gaining Ground: The origin and evolution of tetrapods. Indiana University Press.
Clack JA 2002. An early tetrapod from ‘Romer’s Gap’. Nature. 418 (6893): 72–76. doi:10.1038/nature00824
Clack JA 2006. The emergence of early tetrapods. Palaeogeography Palaeoclimatology Palaeoecology. 232: 167–189.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.
Jarvik E 1996. The Devonian tetrapod Ichthyostega. Fossils and Strata. 40:1-213.
Säve-Söderbergh G 1932. Preliminary notes on Devonian stegocephalians from East Greenland. Meddelelser øm Grönland 94: 1-211.
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/Ichthyostega
wiki/Acanthostega
wiki/Ossinodus
wiki/Pederpes

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Tulerpeton restoration

A reconstruction
puts the in situ bones back into their in vivo places.

A restoration
imagines the bones and soft tissues that are missing from the data. Adding scaled elements from a sister taxon is usually the best way to handle a restoration as we await further data from the field.

Figure 1. Tulerpeton restored based on the bauplan of Silvanerpeton and to the same scale.

Figure 1. Tulerpeton restored based on the bauplan of Silvanerpeton and to the same scale.

We looked at
Tulerpeton, the Upper Devonian taxon known chiefly from its limbs, earlier. I reconstructed the limbs several ways, but did not attempt a restoration. Here (Fig. 1) that oversight is remedied based on the bauplan of Viséan sister, Silvanerpeton. 

Among the overlapping elements,
in Tulerpeton the pectoral girdle and forelimbs are larger. An extra digit is present laterally.

References
Clack JA 1994. Silvanerpeton miripedes, a new anthracosauroid from the Visean of East Kirkton, West Lothian, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 84 (for 1993), 369–76.
Coates MI and Ruta M 2001
 2002. Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Lebedev OA 1984. The first find of a Devonian tetrapod in USSR. Doklady Akad. Navk. SSSR. 278: 1407–1413.
Lebedev OA and Clack JA 1993. Upper Devonian tetrapods from Andreyeva, Tula Region, Russia. Paleontology36: 721-734.
Lebedev OA and Coates MI 1995. postcranial skeleton of the Devonian tetrapod Tulerpeton curtum Lebedev. Zoological Journal of the Linnean Society. 114 (3): 307–348.
Mondéjar-Fernandez J, Clément G and Sanchez S 2014. New insights into the scales of the Devonian tetrapods Tulerpeton curtum Lebedeve, 1984. Journal of Vertebrate Paleontology 34:1454-1459.

wiki/Silvanerpeton
wiki/Tulerpeton

Tulerpeton nests between Ichthyostega and Eucritta

Updated Dec 13, 2017 re-nesting Tulerpeton between Ichthyostega and Eucritta. 

Yesterday we looked at the nesting of Tulerpeton (Lebedev 1984; Latest Devonian; PIN 2921/7) as a basal tetrapod, which is the traditional nesting.

I thank
Dr. Michael Coates for sending a pdf of his 1995 study of Tulerpeton. From the improved data I was able to make new reconstructions of the manus and pes. The differences shift the nesting of Tulerpeton to the last common ancestor of Eucrtta and Seymouriamorpha.

Figure 1. Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2.

Figure 1. Tulerpeton parts from Lebedev and Coates 1995 here colorized and newly reconstructed. Manus and pes enlarged in figure 2. Note the in situ placement of the pedal phalanges. The clavicle is shown as originally published and withe the ventral view reduced in width to compare its unchanged length to the original lateral view image.

In the new reconstruction
only the manus retained 6 digits, with the lateral sixth digit a vestige. The pes has a new reconstruction with only 5 digits, very much in the pattern of Gephyrostegus and Eucritta. Both have five phalanges on digit 5. In the new reconstructions all of the PILs (Peters 2000) line up in sets.

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

Figure 2. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here with PILs added. Note the broken mt5 and the reinterpretation of the squarish elements as phalanges, not distal carpals. The tibiale is rotated 90º to cap the tibia.

Lebedev and Coates report:
“A cladistic analysis indicates that Tulerpeton is a reptilomoprh stem-group amniote and the earliest known crown-group tetrapod. The divergence of reptilomorphs from batrachomorphs (frogs and kin) occurred before the Devonian Carboniferous boundary. Polydactyly persisted after the evolutionary divergence of the principal lineages of living tetrapods. Tulerpeton was primarily air-breathing.”

Autapomorphies
Manual digit 6 is present as a novelty. Perhaps it is a new digit after damage. More primitive taxa do not have this digit. An anocheithrum (small bone atop the cleithrum) is present. Metatarsal 1 in Tulerpeton is the largest in the set. The posterior ilium rises. The femur has a large, sharp, fourth (posterior) trochanter.

Scales
on Tulerpeton are also found similar in size and number are also found in related taxa.

Taxon exclusion
and digital graphic segregation AND reconstruction AND comparative anatomy all contributed to the new data scores. As usual, I have not seen the specimen, but I did add it to a large gamut data matrix, the likes of which are not typically employed.

Figure 1. Tulerpeton restored based on the bauplan of Silvanerpeton and to the same scale.

Figure 1. Tulerpeton restored based on the bauplan of Silvanerpeton and to the same scale.

References
Coates MI and Ruta M 2001 (2002). Fins to limbs: What the fossils say. Evolution & Development 4(5): 390–401.
Lebedev OA 1984. The first find of a Devonian tetrapod in USSR. Doklady Akad. Navk. SSSR. 278: 1407–1413.
Lebedev OA and Clack JA 1993. Upper Devonian tetrapods from Andreyeva, Tula Region, Russia. Paleontology36: 721-734.
Lebedev OA and Coates MI 1995. postcranial skeleton of the Devonian tetrapod Tulerpeton curtum Lebedev. Zoological Journal of the Linnean Society. 114 (3): 307–348.
Peters D 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41

wiki/Tulerpeton

Devonian fish heads

New data on February 27, 2017
focuses on Kenichthys (Zhu and Ahlberg 2004), a sarcopterygian fish in which the posterior naris has migrated to the jaw line, on its way to the inside of the mouth (Fig. A).

Figure A. From Zhu and Ahlberg 2004 demonstrating the migration of the posterior naris in Youngolepis to the rim of the jaw in Kenichthys, and to the inside of the mouth in Eusthenopteron.

Figure A. From Zhu and Ahlberg 2004 demonstrating the migration of the posterior naris in Youngolepis to the rim of the jaw in Kenichthys, and to the inside of the mouth in Eusthenopteron.

In a quest for understanding
the origins of everything reptilian, today we’ll take a look at the skulls of three Devonian fish with skull bones homologous with those of reptiles.

FIgure 1. Cheirolepis skull. Most bones have readily identified homologs with tetrapods, but the medal skull bones and the pineal opening do some shifting. Yes, that's the pineal appearing in the inter frontal.

FIgure 1. Cheirolepis skull. Most bones have readily identified homologs with tetrapods, but the medal skull bones and the pineal opening do some shifting. Yes, that’s the pineal appearing in the inter frontal. DIs the supratemporal split to give rise to temporals? And did the parietals split to give rise to post parietals? Or did the tabulars and post parietals  migrate?

Cheirolepis trailli (Agassiz 1835; Middle Devonian, 390 mya; 30-55?cm in length; Fig. 1) is considered one of the earliest actinopterygian (ray-finned) fish with ‘standard’ dermal skull bones. Those bones were homologous with those of coeval and later sarcopterygian fish, like Eusthenopteron (Fig. 2) and Osteolepis (above right, Fig. 3)and tetrapods, like Ichthyostega. That series of small rostral bones will fuse to become the nasal in tetrapods. The maxilla will become much shallower, the rostrum will lengthen.

In Cheirolepis (now the basal taxon in the large reptile tree, LRT, 955 taxa) the orbit is far forward. The jaws opened up to right angles to form a large gape. The pineal opening pierced the inter-frontal (originally the singular frontal). Note the great depth of the maxilla on the cheek. That doesn’t last in tetrapods. Bony gill covers were present. Those disappear, too. The pectoral fins were lobed and muscular, but the hind fins were not. The pelvis and dorsal fins were large and broad-based. The tail was heterocercal, like a shark’s tail. The body was deep and broad anteriorly, narrower posteriorly. The lumbar  region becomes wider and less streamlined in basal tetrapods.

The homologies of the tabular and postparietal
are questionable here, so two solutions are shown (Fig. 1). The one on the left is likely correct based on the random appearance of post parietals in Osteolepis (Fig. 3) from the random splitting of elongate parietals.

The homologies of the frontals and parietals
are reidentified here for Cheirolepis (Fig. 1) and Eusthenopteron (Fig. 2) based on tetrapod skull bones and the pattern in Osteolepis skull roofing bones (Fig. 3). In their original identity, the pineal opening pierced the parietals. Here the pineal opening migrates from the inter frontal to the frontals, heading toward the parietals in basal tetrapods. The pineal basically follows the lateral eyes as they also migrate posteriorly.

Figure 2. Eusthenopteron skull showing some changes from the Cheirolepis skull.

Figure 2. Eusthenopteron skull showing some changes from the Cheirolepis skull. Here the post parietals have not split rom the parietals. Pink bone on verbal column is the future sacral, the posterior most vertebra with tiny transverse processes (ribs).

Eusthneopteron foordi (Whiteaves 1881; Late Devonian, 385 mya; 1.8m in length) was one of the first fish genera known to share a long list of traits with basal tetrapods.

Distinct from Cheirolepis
Eusthenopteron had choanae (palatal openings for the passage of air, internal nares). The posterior maxilla was not so deep. The orbits were smaller and set further posteriorly. The mandible bones were more like those of tetrapods. Limb bones appear within the pectoral and pelvic fins, but no distinct wrist, ankle, metapodial or digit bones are yet present. Fin rays remain. The jaws were rimmed with tiny teeth. The palate had several large fangs. The tail was not so heterocercal, but stretched out more or less in line with the vertebral column.

Not as visible in these figures…
While all fish have anterior and posterior external naris for odor-laden water to enter and exit, in Eusthenopteron and Osteolepis the posterior naris has migrated to the orbit to become the tear duct. Now, that’s a clue that these fish were spending time poking their eyes above the water and perhaps not gulping air like a lungfish, but breathing and smelling through its new choanae (internal nares).

Figure 3. Osteolepis cranial shield bones from Graham-Smith 1978 and reidentified here (in white)

Figure 3. Osteolepis cranial shield bones from Graham-Smith 1978 and reidentified here (in white). These are due to individual variation.

Variation in the skull shield of Osteolepis (Fig. 4)
is shown above (Fig. 3). Bones originally labeled intertemporals are here considered supratemporals. Original supratemporals are here considered tabulars. Note the random splitting of the parietals, originally considered anterior parietals (APa) and parietals (Pa). Here those bones are parietals and post parietals based on tetrapod homologies. In one Osteolepis specimen (Fig. 3 lower right) extra bones (supernumeraries – sa) appear, but do not appear in related taxa.

Figure 2. Ostelepis has a large bone basal to the pelvic fin. IMHO it is too far back to be a possible ischium, contra Panchen.

Figure 4. Ostelepis, more or less actual size. The heterocercal tail is retained here.

References
Agassiz JLR 1835. On the fossil fishes of Scotland. Report of the British Association for the Advancement of Science, British Association for the Advancement of Science, Edinburgh.
Graham-Smith W 1978. On the Lateral Lines and Dermal Bones in the Parietal Region of Some Crossopterygian and Dipnoan Fishes. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 282 (986):41-105.
Schultze H-P 1984. Juvenile specimens of Eusthenopteron foordi Whiteaves, 1881 (Osteolepiform rhipidistian, Pisces) from the Late Devonian of Miguasha, Quebec, Canada. Journal of Vertebrate Paleontology 4: 1-16.
Whiteaves JF 1881. On some remarkable fossil fishes from the Devonian rocks of Scaumenac Bay, in the Province of Quebec. Annals and Magazine of Natural History. 8: 159–162.
Zhu M and Ahlberg P 2004. The origin of the internal nostril of tetrapods. Nature 432:94-97.

Cheirolepis fossil images
wiki/Cheirolepis
wiki/Eusthenopteron

Acherontiscus at the base of the caecilian clade

Acherontiscus caledoniae (Carroll 1969; Namurian, Carboniferous; 1967/12/1 Royal Scottish Museum; Fig. 1) is a tiny slender aquatic amphibian with vestigial limbs and a large pectoral girdle.

FIgure 1. Acherontiscus, a basal adelogyrinid, close to the origin of caecilians, derived from a sister to Microbrachis.

FIgure 1. Acherontiscus, a basal adelogyrinid, close to the origin of caecilians, derived from a sister to Microbrachis.

Carroll wrote: Acherontiscus combiines cranial characteristics typical of lepospondyls with a vertebral structure resembling that of embolomeres” (like Proterogyrinus). “This form cannot be placed in any recognized amphibian orders but presumably represents an isolated lineage which originated prior to the establishment of the definitive characteristics which differentiate all known lepospondyls and labyrinthodonts.” 

As a lepospondyl
“This genus provides the first conclusive evidence of the presence of multiple central element in the trunk region.”

Here,
in the large reptile tree (LRT) Acherontiscus nests between the microsaur, Microbrachis, and Adelogyrinus + Adelospondylus. Carroll recognized “The pattern of the skull roof of Acherontiscus resembles most closely that of the microsaur Microbrachis” a taxon presently known only from later Late Carboniferous strata (305 mya).

Diagnosis:
“Small stegocephalian amphibia with both pleurocentra and intercenta well-developed cylinders. Skull with lateral line canals, orbits far forward, no otic notch, teeth without labyrinthine infolding of enamel. Demoral pectoral gidle well developed. Long trunk region.”

References
Carroll RL 1969. A new family of Carboniferous amphibians. Palaeontology 12(4):53–548.

Odd Gerrothorax nests with Greererpeton

Figure 1. Gerrothorax, lacks a supratemporal rim and has laterally extended ribs, similar to those in Greererpeton.

Figure 1. Gerrothorax, lacks a supratemporal rim and has laterally extended ribs, similar to those in Greererpeton. There is some variation and perhaps some confusion regarding the identity of the bones between the orbits. Here an alternate is shown that is similar to Greererpeton.

Gerrothorax pulcherrimus (Nilsson 1934, Jenkins et al. 2008; Late Triassic) was and is considered a plagiosaurine temnospondyl. Here it nests with Greererpeton with which it shares a lack of a supratemporal-tabular rim, the straight lateral ribs and other traits. The ‘interfrontal’ is here identified as fused frontals. The separated ‘frontals’ are here identified as prefrontals.

Jenkins et al. showed the skull raised like a toilet seat cover during feeding, rather than opening the mandible on this flattened bottom feeder.

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. Images are from the PhD thesis of Step;hen Godfrey 1986.

Greererpeton burkemorani (Romer 1969, Smithson 1982, Godfrey 1989; Early Carboniferous, 320 mya; 1.5 m in length). Godfrey thought it nested closer to Proterogyrinus than to Ichthyostega. Here Greererpeton nests as an offshoot of the temnospondyls along with Gerrothorax (Fig. 1).

The skull was flattened with orbits on top of the skull. The lacrimal does not contact the naris. The torso included some 41 presacral vertebrae. The ribs were robust and extended laterally. The pectoral girdle was robust. The limbs were small.

No complete pedes are known for Greererpeton, according to Godfrey 1989, but that may have changed since then. So I wonder if digit two had 3 phalanges, as in sister taxa, not 2. And I wonder if digit five had 3 phalanges, as in sister taxa, not 4 (Fig. 2) as illustrated in Godfrey 1989. I have not seen manus or pes data for Gerrothorax. If you have it, please send it.

Juveniles are known with larger orbits not so dorsally oriented, but otherwise similar (Fig. 2), but note the squamosal divides the postorbital and supratemporal in the adult, making one wonder if the difference is indeed ontogenetic.

Figure 6. Subset of the large reptile tree focusing on basal tetrapods, updated with Gerrothorax.Figure 6. Subset of the large reptile tree focusing on basal tetrapods, updated with Gerrothorax.

Figure 6. Subset of the large reptile tree focusing on basal tetrapods, updated with Gerrothorax.

References
Godfrey SJ 1986. The skeletal anatomy of Greererpeton burkemorani Romer 1969, an Upper Mississippian temnospondyl amphibian. PhD thesis, McGill University, Montreal.
Godfrey SJ 1989. The postcranial skeletal anatomy of the Carboniferous tetrapod Greererpeton burkemorani Romer, 1969. Philosophical Transactions of The Royal Society B Biological Sciences 323(1213):75-133.
Jenkins FA Jr, Shubin NH, Gates SM and Warren A 2008. Gerrothorax pulcherrimus from the Upper Triassic Fleming Fjord Formation of East Greenland and a reassessment of head lifting in temnospondyl feeding. Journal of Vertebrate Paleontology. 28 (4): 935–950.
Nilsson T 1934. Vorläufige mitteilung über einen Stegocephalenfund aus dem Rhät Schonens. Geologiska Föreningens I Stockholm Förehandlingar 56:428-442.
Romer AS 1969. A temnospondylus labyrinthodont from the Lower Carboniferousw. Kirtlandia 6:1-20.
Smithson TR 1982. The cranial morphology of Greererpeton burkemorani Romer (Amphibia: Temnospondyli). Zoological Journal of the Linnean Society 76(1):29-90.

wiki/Greererpeton
wiki/Gerrothorax

Did Acanthostega lose an intertemporal?

Or was it overlooked?
(See Fig. 1, blinks organge/green above the orbit.)

Figure 1. Acanthostega skull from Porro et al. 2015 showing lack and presence of the intertemporal.

Figure 1. Acanthostega skull from Porro et al. 2015 showing lack and presence of the intertemporal above the postorbital and between the postfrontal and supratemporal — or else it is fused to the parietal.

Ancestral osteolepiformes
like Osteolepis have an intertemporal. So do many (but not all) basal tetrapods. Porro et al 2015 did not indicate one (Fig. 1), but I have added a green one where I think one would be, dorsal to the postorbitals and lateral to the parietals. It blinks on an off in that animation. It might have been overlooked because the whole skull roof has shifted forward. Typically the intertemporal is located somewhat behind the orbit (Fig. 2), where the supratemporal is in figure 1.

Ventastega, a closely related taxon,
(Fig. 2, green added) indicates the presence of an intertemporal. Earlier an intertemporal was located in Ichthyostega where one was overlooked before.

Figure 2. Ventastega from Ahlberg et al. 2008, showing their interpretation of an intertemporal (green).

Figure 2. Ventastega from Ahlberg et al. 2008, showing their interpretation of an intertemporal (green) above a missing postorbital.

Side note
Most Acanthostega relatives do not have such an elevated mandible tip. This hook-jaw morphology is similar to the spawning phase of the male sockeye salmon, which does not have such a hooked jaw in its ocean phase.

References
Ahlberg PE, Clack JA, Luksevics E, Bom H and Zupins I 2008. Ventastega curonica and the origin of tetrapod morphology. Nature 453: 1199-1204.
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.
Jarvik E 1952. On the fish-like tail in the ichtyhyostegid stegocephalians. Meddelelser om Grønland 114: 1–90.
Porro LB, Rayfield EJ and Clack JA 2015. Descriptive Anatomy and Three-Dimensional Reconstruction of the Skull of the Early Tetrapod Acanthostega gunnari Jarvik, 1952. PLoS ONE 10(3): e0118882. doi:10.1371/journal.pone.0118882

wiki/Acanthostega