Recalibrating clade origins, part 2

Marjanovic 2019 reports on
the origin of several clades based on fossils and molecules. Yesterday we looked at part 1, which focused on the abstract. Today: the origin of several more listed clades.

Gnathostomata (Chondrichthyes + Osteichichthyes)
Marjanovic cautiously proposes the mid-Florian (Early Ordovician, 475 mya) for the origin using traditional taxa and cladograms.

By contrast, the LRT splits off quasi-jawless sturgeons before the appearance of jawed sharks + other bony fish. It also splits off the jawed Loganellia + Rhincodon + Manta clade before the Polyodon + ratfish + sharks + skates clade and the Pachycormus + Hybodus clade before the dichotomy that resulted in the rest of the bony fish (the now polyphyletic ‘Osteichthyes‘)… so direct comparisons are not apples and apples here. Sturgeons first appear much later in the fossil record. Loganellia appears in the Early Silurian with an earlier genesis. So Marjanovic’s estimate may be a little early.

Osteichthyes (Actnopterygii + Sarcopterygii)
Marjanovic reports, “The oldest known uncontroversial crown-group osteichthyan is the oldest known dipnomorph, Youngolepis.” He suggests, “the minimum age for this calibration is the same as that for the next node,” the Silurian/Devonian boundary, 420 mya.

The LRT includes placoderms within one branch of the bony fish, so Entelognathus along with the stem-lungfish Guiyu, both in the Late Silurian are older than Marjanovic suggests with an earlier genesis. Sturgeons, which traditional workers consider a member of the Osteichyes, phylogenetically preceded Longanellia, which is known from Early Silurian strata. So, again we’re not comparing similar cladograms here. The LRT tests a wider gamut of taxa, which is an advantage in that it opens further possibilities than tradition dictates.

Dipnomorpha + Tetrapodomorpha (lungfish + lobe fin ancestors of tetrapods)
Marjanovic reports, “I suggest a hard minimum age of 420mya.” (See above).

The LRT includes Late Siluirian Guiyu within the stem-lungfish clade. so the split occurred earlier.

Tetrapoda (Amphibia + total group of Amniota)
Marjanovic reports, “the richer and better studied Famennian (end-Devonian) record, which has not so far yielded tetrapods close to the crown-group but has yielded more stemward tetrapods and other tetrapodomorphs (Marjanović and Laurin, 2019), should be used to place a soft maximum age around very roughly 365 Ma.”

Figure 3. Tersomius texensis, an amphibamid lepospondyl close to Dendrerpeton.

Figure 1. Tersomius texensis, an amphibamid lepospondyl close to Dendrerpeton.

In the LRT the last common ancestor of Amphibia + Amniota is Tersomius (Fig. 1), a late survivor in the Early Permian of an earlier genesis and radiation. The oldest taxa from this clade in the LRT are the basal amniotes / amphibian-like reptiles, Silvanerpeton and Eldeceeon from the Viséan (335 mya), with a long list of late surviving taxa between them and Tersomius, some eight nodes beyond the Late Devonian Acanthostega and Ichthyostega (365 mya). So the Tournaisian (355 mya) split suggested by Marjanovic seems about right.

Amniota (Theropsida + Sauropsida)
Marjanovic reports, “I refrain from recommending a maximum age other than that of the preceding Node, even though such an early age would imply very slow rates of morphological evolution in the earliest thero- and sauropsids.”

The LRT recovers a different basal dichotomy (Archosauromorpha + Lepidosauromorpha) and a different last common ancestor for all amniotes (Silvanerpeton) than Marjanovic is working with. Silvanerpeton is Viséan in age (~335 mya). In the LRT ‘Amniota’ is a junior synonym for Reptilia.

Crown group of Diapsida (Lepidosauromorpha + Archosauromorpha)
Marjanovic reports, “I cannot express confidence in a maximum age other than that of  Node 106, which I cannot distinguish from the maximum age of Node 105 as explained above. This leaves Node 107 without independent calibrations in the current taxon sample.”

The LRT finds two origins for reptiles with a diapsid skull architecture. So the tradtional clade ‘Diapsida’ is also a junior synonym for Reptilia and Marjanovic is using an outdated and under represented cladogram. Lepidosauromorph diapsids first appear with Paliguana in the earliest Triassic. Archosauromorph diapsids first appear with Erpetonyx and Petrolacosaurus in the Late Carboniferous with an earlier genesis. These taxa are not mentioned by Marjanovic.

Archosauria (Crocodile total group + Bird total group)
Marjanovic reports, “I accept the Permian-Triassic boundary (251.902 ± 0.024 Ma: ICS; rounded to 252) as the soft maximum age on the grounds that a major radiation of archosauromorphs at the beginning of the Triassic seems likely for ecological reasons.”

The LRT restricts membership within the Archosauria to just Crocodylomorpha + Dinosauria. So the maximum age for this dichotomy is younger and the last common ancestor is the PVL 4597 specimen (late Middle Triassic, 230mya) traditionally assigned to Gracilisuchus, but nesting apart from the holotype.

The LRT finds the Archosauriformes first appeared in the Late Permian (260mya), arising from a sister to Youngoides romeri (FMNH UC1528) thereafter splitting into clades arising from the larger Proterosuchus and the smaller Euparkeria.

Alligatoridae (Alligatorinae + Caimaninae)
Marjanovic reports, “Given this uncertainty, I have used a hard minimum age of 65 Ma for present purposes, but generally recommend against using this cladogenesis as a calibration for time trees.”

The LRT does not include pertinent taxa surrounding this split.

Figure 1. Megapodius is the extant bird nesting at the base of all neognathae (all living birds except ratites).

Figure 2. Megapodius is the extant bird nesting at the base of all neognathae (all living birds except ratites). And it looks like a basal bird. It also looks a bit like the Solnhofen bird, Jurapteryx. It is easy to imagine diverse forms arising from this bauplan and the LRT indicates that is exactly what happened.

Crown group of Neognathae (Gallanseres + Neoaves)
Marjanovic further defines this clade as, “The last common ancestor of Anas, Gallus and Meleagris on one side and Taeniopygia.” More commonly Marjanovic nests a duck, a chicken and a turkey on one side and a zebra finch on the other as the basal dichotomy of all living birds, sans ostriches, kiwis and kin. Marjanovic reports, “As the soft maximum age I tentatively suggest 115 Ma, an estimate of the mid-Aptian age of the (likewise terrestrial) Xiagou Fm of northwestern China, which has yielded a diversity of stem-birds but no particularly close relatives of the crown.”

Taxa listed by Marjanovic are all highly derived taxa in the LRT where the scrub fowl, Megapodius (Fig. 2) and the tinamou, Crypturus, are basal neognaths. These would have had their genesis in the Earllest Cretaceous given that Early Cretaceous clades that redevelop or retain teeth are more derived.

More tomorrow…

Marjanovic D 2019. Recalibrating the transcriptomic timetree of jawed vertebrates.
bioRxiv 2019.12.19.882829 (preprint)

The antorbital and lateral temporal fenestrae of the frog , Rana

Earlier we looked at the evolution of the frog, Rana. And it continues to be the most popular blog post of the past year.

Today, after adding Rana to the matrix of the large reptile tree (still not updated), I think it’s time we looked at the antorbital fenestra of Rana, and the lateral temporal fenestra as well (Fig. 1).

Figure 1. Rana, the bull frog, with naris in red, orbit in purple, antorbital fenestra in dark blue and lateral temporal fenestra in orange. The reduction of the the skull bones in Rana created these fenestrae.

Figure 1. Rana, the bull frog, with naris in red, orbit in purple, antorbital fenestra in dark blue and lateral temporal fenestra in orange. The reduction of the the skull bones in Rana created these fenestrae.

One usually thinks of additional skull fenestrae in the province of reptiles. As we saw earlier, the antorbital fenestra comes and goes in several reptiles. So does the lateral temporal fenestra. Amphibians (non-amniote tetrapods) typically do not have skull fenestrae. Neither to most basal reptiles.

Relative to the body, the skull of Rana is enormous. So are the hind limbs. Frogs leap, as everyone knows, and if the skull is going to be large it also has to be lightweight to enable longer leaps. So the skull bones are reduced to their bare minimum creating fenestrae.

Proximal outgroup taxa, including long-legged Triadobatrachus, likewise have reduced skull bones.

More distant outgroup taxa, including short-legged Gerobatrachaus and Doleserpeton and Utegenia have relatively smaller skulls and shorter hind limbs — and no skull fenestrae.



The Evolution of Frogs

While they are the most common of living amphibians, frogs are among the oddest amphibians of all time with their short torso and long hind legs. The skull has undergone great changes from the primitive state with many bones reduced (Fig. 1).

Frog skull evolution. Here, starting with the Permian Utegenia (with relatives back to the Visean) you can see stages in the evolution of the frog skull through Doleserpeton and Gerobatrachus to Rana, the bull frog.

Figure 1. Frog skull evolution. Here, starting with the Permian Utegenia (with relatives back to the Visean) you can see stages in the evolution of the frog skull through Doleserpeton and Gerobatrachus to Rana, the bull frog.

The wide flat skull of Utegenia (Earliest Permian with roots in the Visean, Laurin 1996) is our starting point. It represents a basic seymouriamorph skull with a full complement of skull bones, palatal fangs and sharp marginal teeth.

In Doleserpeton (Early Permian, Sigurdsen and Bolt 2010) the palate changes the most as the interpterygoid vacuity greatly expands. The palatal fangs are gone. The marginal teeth are tiny and continue behind the orbit. The vomer expands with a shagreen of tiny teeth. The intertemporal fuses to the postfrontal. The otic notch deepens. Note the palatines, now form part of the ventral orbit in lateral view.

In Gerobatrachus (Early Permian, Anderson et al. 2008) these trends continue as the skull widens, the orbits enlarge and the palatal bones become more gracile. Here the intertemporal bones may not have fused to the postfrontal, but the specimen has many cracks and is exposed ventrally.  Here the otic notch is not so deep and the voters are tiny.

In Rana (extant) the skull bones are reduced to struts (no more post parietal shelf) to accommodate the giant orbit and narial opening. The otic notch is angular here, not a smooth curve. It appears as if the intertemporal were still present, but sources available at present don’t delineate the cranial bones. Send a good reference if you have it and I’ll make changes as necessary.

The evolution of frogs from Utegenia, through Doleserpeton, Gerobatrachus, Triadobatrachus and Rana.

Figure 2. The evolution of frogs from Utegenia, through Doleserpeton, Gerobatrachus, Triadobatrachus and Rana. Click to enlarge. It wasn’t until Triadobatrachus in the Early Triassic that frogs developed their long hind limbs, and long ankle bones, but they still retained a short pelvis and long torso.

We skipped one
In Triadobatrachus (Fig. 2, Early Triassic, Piveteau 1936, Rage and Rocek 1989) the hind limbs and ankle bones begin to elongate and the vertebral count is reduced. Distinct from Gerobatrachus, the skull of Triadobatrachus was relatively smaller with a narrower skull roof and a larger orbit that extended into the cheek where the bones were reduced. The frontal and parietal were fused together, but note the parietals were not completely fused to each other! The intertemporal was fused to the parietal. A large postparietal shelf was absent. The lower jaw was toothless. The presacral vertebral count was reduced to 14. The tail was reduced to a nub of at least six vertebrae. The ilium included a larger anterior process, but note the thigh muscles did not extend to the anterior tip of the ilium as in reptiles and mammals. The femur, tibia and fibula were elongated as were two proximal ankle bones, the tibiale and fibulare. Unfortunately the hands and feet remain unknown. Despite the much longer hind legs, Triadobatrachus was considered an ineffective jumper.

Anderson JS et al. 2008.  A stem batrachian from the Early Permian of Texas
and the origin of frogs and salamanders. Nature 453:
Kuznetzov VV and Ivakhnenko MF 1981. Discosauriscids from the Upper Paleozoic in Southern Kazakhstan. Paleontological Journal 1981:101-108.
Laurin M 1996. A reappraisal of Utegenia, a Permo-Carboniferous seymouriamorph (Tetrapoda: Batrachosauria) from Kazakhstan. Journal of Vertebrate Paleontology 16(3):374-383.
Piveteau J. 1936. Une forme ancestrale des amphibiens anoures dans le Trias inférieur de Madagascar. Comptes Rendus hebdomadaires des séances de l’Académie des Sciences 202:1607–1608.
Rage J-C and Rocek Z 1989. Redescription of Triadobatrachus massinoti (Piveteau, 1936) an anuran amphibian from the early Triassic. Palaeontographica Abt. A 206(1-3):1-16. online pdf
Sigurdsen T and Bolt JR 2010. The Lower Permian amphibamid Doleserpeton (Temnospondyli: Dissorophoidea), the interrelationships of amphibamids, and the origin of modern amphibians. Journal of Vertebrate Paleontology 30(5):1360-1377.