Chronology and phylogeny of basal tetrapods

Bottom Line:
The place to make future basal tetrapod discoveries is in Late Devonian/Earliest Carboniferous strata (Fig. 1, light blue). That’s where an undiscovered radiation appears to have taken place based on the widespread dispersal of basal tetrapods in the Visean (Early Carboniferous, light green).

Figure 1. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered.

Figure 1. Subset of the LRT focusing on basal tetrapods, colorized according to chronology. Note the wide dispersal of Early Carboniferous taxa, suggesting a Late Devonian radiation as yet largely undiscovered.

Sometimes what you don’t see right away
is the important story. We should see lots of Devonian tetrapods, but currently we do not.

Earlier we considered the possibility that Acanthostega and Ichthyostega were secondarily a little more aquatic, based on ancestral taxa that were a little more terrestrial. That hypothesis is based on the current cladogram (subset in Fig. 1).

Tiktaalik was discovered by searching in the desired strata. So this process does work. Maybe we’ll find more basal tetrapods in slightly higher strata.

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

More on those fascinating Middle Devonian tetrapod tracks

Updated Dec 13, 2017. 

Surprisingly,
Middle Devonian tetrapod tracks (Fig. 1; Niedźwiedzki et al. 2010)  precede fossil taxa that could have made those tracks by tens of millions of years.

Wide-gauge 385 million year old tracks from Valentia
could only have been made by a tetrapod with laterally extended limbs found in 360 million year old strata, 25 million years later.

Figure 1. From Niedźwiedzki et al. 2010 showing the Valentia track (above), the Zalchemia track (below) and possible trackmakers (middle). Pink lines link corresponding forelimb and hind limb in the Zalchemia track.

Figure 1. From Niedźwiedzki et al. 2010 showing the Valentia track (above), the Zalchemia track (below) and possible trackmakers (middle). Pink lines link corresponding forelimb and hind limb in the Zalchemia track. Note the wide gauge of the Valentia track versus the narrow gauge of the earlier Zalchemie track.

Narrow-gauge older tracks from Zalchemie
(387 million years ago) also had a shorter stride on a longer torso, matching tetrapods without long lateral limbs, but with short stubs or limbs, like Tiktaalik appearing 12 million years later.

Figure 2. Chronology of Devonian stem tetrapod taxa and trackways. Frame one shows traditional tree without tracks. Frame two extends ghost lineages to consider the tracks as evidence of undiscovered fossils. Fossils represent rare discoveries typically long after major radiations to millions of individuals, increasing the odds of their being found.

Figure 2. Chronology of Devonian stem tetrapod taxa and trackways. Frame one shows traditional tree without tracks. Frame two extends ghost lineages to consider the tracks as evidence of undiscovered fossils. Fossils represent rare discoveries typically long after major radiations to millions of individuals, increasing the odds of their being found.

The problem is
the wider tracks come from an era in which Tiktaalik-like taxa are known as fossils, some 25 million years too soon based on fossil taxa like Ichthyostega, (Fig. 3).

Figure 3. Best Devonian Valentia track with various overlays.

Figure 3. Best Devonian Valentia track with various overlays.

The solution is
fossils of all sorts can be discovered close to the genesis of a clade, but are more likely to be discovered close to the maximum radiation (in terms of numbers of individuals), increasing the odds for preservation and discovery. Applying logic here, the skeletons must be appearing near the maximum radiation while the ichnites must be appearing near the genesis of the clade. But wait, there’s more:

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 5. Various stem amniotes (reptiles) that succeeded 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 taxa listed above
(Fig. 5) all succeed the Latest Devonian Tulerpeton in the large reptile tree (LRT, 1027 taxa). Their first appearance in the fossil record occurs much later.

And for all you future paleontologists:
there’s a great paper waiting for the next person or team to find these pre-Tulerpeton taxa in Late Devonian strata. Based on the stress to living things that occurred during the Latest Devonian extinction event, perhaps these taxa radiated quickly and widely.

References
Niedźwiedzki G, Szrek P, Narkiewicz K, Narkiewicz M and Ahlberg PE 2010. Tetrapod trackways from the early Middle Devonian period of Poland Nature 463, 43-48. doi:10.1038/nature08623

Rough chronology of basal tetrapods and basal reptiles

Today we’ll look at WHEN
we find fossils of basal tetrapods and basal reptiles. According to the large reptile tree (959 taxa, LRT, subset shown in Fig. 1), oftentimes we find late survivors of earlier radiations in higher strata. The origin of Reptilia (amphibian-like amniotes) extends back to the Devonian and Early Carboniferous now, not the Late Carboniferous as Wikipedia reports and as the Tree of Life project reports.

Figure 1. Color coded chronology of basal tetrapods and reptiles.We're lucky to know these few taxa out of a time span of several tens of millions of years.

Figure 1. Color coded chronology of basal tetrapods and reptiles.We’re lucky to know these few taxa out of a time span of several tens of millions of years. Click to enlarge.

The Late Devonian 390–360 mya
Here we find late survivors of an earlier radiation: Cheirolepis, a basal member of the Actinopterygii (ray-fin fish) together with Eusthenopteron and other members of the Sarcopterygii (lobe-fin fish). Coeval are basal tetrapods, like Acanthostega and basal reptiles, like Tulerpeton. These last two launch the radiations we find in the next period. The presence of Tulerpeton in the Late Devonian tells us that basal Seymouriamorpha and Reptilomorpha are waiting to be found in Devonian strata. We’ve already found basal Whatcheeriidae in the Late Devonian taxa Ichthyostega and Ventastega.

Early Carboniferous 360–322 mya
Here we find the first radiations of basal reptilomorphs, basal reptiles, basal temnospondyls,  basal lepospondyls and microsaurs, lacking only basal seymouriamorphs unless Eucritta is counted among them. It nests outside that clade in the LRT.

Late Carboniferous 322–300 mya
Here we find more temnospondyls, lepospondyls and phylogenetically miniaturized archosauromorphs, likely avoiding the larger predators and/or finding new niches. Note the first prodiapsids, like Erpetonyx and Archaeovenator, appear in this period, indicating that predecessor taxa like Protorothyris and Vaughnictis had an older, Late Carboniferous, origin. Not shown are the large basal lepidosauromorphs, Limnoscelis and Eocasea and the small archosauromorphs, Petrolacosaurus and Spinoaequalis.

Early Permian 300–280 mya
Here we find the first fossil Seymouriamorpha and the last of the lepospondyls other than those that give rise to extant amphibians, like Rana, the frog. Here are further radiations of basal Lepidosauromorpha, basal Archosauromorpha (including small prodiapsids), along with the first radiations of large synapsids.

Late Permian 280–252 mya
Here we find the next radiation of large and small synapsids, the last seymouriamorphs, and derived taxa not shown in the present LRT subset.

Early/Mid Triassic 252 mya–235 mya
Among the remaining basal taxa few have their origins here other than therapsids close to mammals. Afterwards, the last few basal taxa  listed here, principally among the Synapsida, occur later in the Late Triassic, the Jurassic and into the Recent. Other taxa are listed at the LRT.

What you should glean from this graphic
Taxa are found in only the few strata where fossilization occurred. So fossils are incredibly rare and somewhat randomly discovered. The origin of a taxa must often be inferred from phylogenetic bracketing. And that’s okay. This chart acts like a BINGO card, nesting known taxa while leaving spaces for taxa we all hope will someday fill out our card.