Jones et al. 2021: Reptile backbone divisions and mobility

From the Jones et al. 2021 In brief:
“Jones et al. disprove the long-held idea that mammal backbone evolution involved a transition from reptile-like lateral bending to sagittal bending.”

Unfortunately, the study was conducted without a proper phylogenetic context. Outgroups included three salamanders (taxa unrelated to reptiles in the large reptile tree (LRT, 1811+ taxa).

Diadectes (Fig. 1) was cherry-picked as a ‘stem amniote‘ and ‘the ancestral condition for amniotes‘, but in the LRT Diadectes is a deeply nested lepidosauromorph amniote (= reptile) derived from Milleretta and not far from limnoscelids, pareiasaurs, procolophonids and turtles. This is a traditional mistake still taught at the university level due to taxon exclusion. At least two of the co-authors are from Harvard. So, kids, don’t go there. Harvard is not up-to-date!

In their results section
Jones et al. report, “there is less phylogenetic signal than expected under a Brownian motion model of evolution and that vertebral shape varies substantially within clades.”

That’s because they did not employ enough pertinent taxa. More taxa = more understanding of reptile phylogeny, just like a larger mirror collects more light and increases resolution in telescopes.

Figure 2. Diadectes (Diasparactus) zenos to scale with other Diadectes specimens.

Figure 1. Diadectes (Diasparactus) zenos to scale with other Diadectes specimens.

The whole concept of a transition from lateral undulation
to dorso-ventral undulation in the synapsid ancestors of mammals has been known for several decades. It was even included in Peters 1991, and not as an original hypothesis.

From the Jones et al. abstract:
“We show that the synapsid adaptive landscape is different from both extant reptiles and mammals, casting doubt on the reptilian model for early synapsid axial function, or indeed for the ancestral condition of amniotes more broadly. Further, the synapsid-mammal transition is characterized by not only increasing sagittal bending in the posterior column but also high stiffness and increasing axial twisting in the anterior column. Therefore, we refute the simplistic lateral-to-sagittal hypothesis and instead suggest the  synapsid-mammal locomotor transition involved a more complex suite of functional changes linked to increasing regionalization of the backbone.”

This was well-known decades ago.

Given this hypothesis, where do Jones et al. draw the transition zone?
Jones et al. indicate that dinocephalian synapsids walked like lizards by matching tracks (Fig. 1) and citing Smith 1993. They should have looked at dinocephalians more closely to see if Smith 1993 was correct. Turns out Smith 1993 was either incorrect or inaccurate.

Figure 2. Gaits illustrated by Jones et al. 2021. Compare the 'dinocephalian' to figure 3. It does not match.

Figure 2. Gaits illustrated by Jones et al. 2021. Compare the ‘dinocephalian’ to figure 3. It does not match.

This purported dinocephalian trackway
(Fig. 2, Smith 1993, Jones et al. 2021) does not match the hypothetical trackmaker (Fig. 2). The Smith 1993 trackway is so narrow the thumb prints overlapping the midline, something sprawling dinocephalians were unable to replicate (Fig. 3). That should have been checked, not just used ‘as is’ by Jones et al. 2021.

In similar fashion, too often workers use prior cladograms without checking for veracity. Science should be all about testing, checking, verifying. Too often it is about borrowing, trusting, accepting.

Figure 3. Dinocephalian in ventral view showing a widely splayed trackmaker.

Figure 3. Dinocephalian in ventral view showing a widely splayed trackmaker. Compare to figure 2 and 4.

Perhaps a better trackmaker
can be found for the Smith 1993 track in a larger relative to Hipposaurus (Fig. 4), a basal therapsid with the required 1) narrow pectoral girdle, 2) long slender limbs and 3) extremities that match the narrow-gauge tracks in size and configuration.

Figure 3. Image from Smith 1993, reprinted in Jones et al. 2021 falsified using their own data, then compared to a lithe large Hipposaurus with narrow toros and long limbs enabling a parasaggital gait matching the manus and pes.

Figure 4. Image from Smith 1993, reprinted in Jones et al. 2021 falsified using their own data, then compared to a lithe, large Hipposaurus with narrow toros and long limbs enabling a parasaggital gait matching the manus and pes.

Jones et al. 2021 discuss cervical, dorsal and lumbar regionalization,
without reporting that regionalization begins with Gephyrostegus (Fig. 5), a basalmost amniote (= reptile) in the LRT. This amphibian-like reptile was not mentioned by Jones et al. 2021. Smaller Diplovertebron (Fig. 5), a basal archosauromorph reptile, inherited and emphasized this regionalization.

Figure 1. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. None of these are congeneric.

Figure 5. Diplovertebron, Gephyrostegus bohemicus and Gephyrostegus watsoni. None of these are congeneric.

 

 

Regionalization of the vertebral column
diminishes in some lepidosauriformes (Fig. 6) reaching a minimum in snakes. Regionalizaton increases in some owenettids, macrocnemids (including fenestrasaurs and pterosaurs) and iguanids.

Figure 6. Saurosternon, the first taxon in the lepidosauromorph lineage with sternae. Note the lack of differences between cervical, dorsal and there are no lumbar vertebrae.

Among archosauromorphs
regionalization did not diminish as much, as shown by Ophiacodon (Fig. 7) a basal synapsid in the lineage of therapsids.

Figure 1. Varanosaurus, Ophiacodon, Cutleria and Ictidorhinus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology. This new reconstruction of Ophiacodon is based on the Field Museum (Chicago) specimen. Click to enlarge.

Figure 7. Varanosaurus, Ophiacodon, Cutleria and Ictidorhinus. These are taxa at the base of the Therapsida. Ophiacodon did not cross into the Therapsida, but developed a larger size with a primitive morphology. This new reconstruction of Ophiacodon is based on the Field Museum (Chicago) specimen. Click to enlarge.

By contrast, 
a basal archosauromorph diapsid, Archaeovenator (Fig. 8) reduces regionalization to a minimum. Lepidosauromorph turtles minimize lateral undulations when they evolve a carapace. So regionalization comes and goes.

Figure 2. Archaeovenator, a sister to Orovenator, is a protodiapsid.

Figure 8. Archaeovenator, a sister to Orovenator, is a protodiapsid.

 

 

One good reason for a lack of ribs in the lumbar region
was to make room for larger amniote eggs in the Earliest Carboniferous that even today greatly distends the abdomen of gravid lizards (Fig. 9).

Figure 4. Extant lizards, A. gravid, B. in the process of laying eggs, C. with egg clutch.

Figure 9. Extant lizards, A. gravid, B. in the process of laying eggs, C. with egg clutch.

Giving credit where credit is due
Jones et al. measured and graphed vertebral dimension across a wide swath of taxa, many closely related to one another. Expanding the taxon list to a wider gamut might have helped them see beyond the synapsids, a clade already well-studied for this factor.


References
Jones KE, Dickson BV, Angielczyk and Pierce SE 2021. Adaptive landscapes challenge the ‘‘lateral-to-sagittal’’ paradigm for mammalian vertebral evolution. Current Biology https://doi.org/10.1016/j.cub.2021.02.009
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.
Smith RM 1993. Sedimentology and ichnology of floodplain paleosurfaces in the Beaufort Group (Late Permian), Karoo sequence, South Africa. Palaios 8, 339–357.

A tadpole-like fish with a tetrapod-like backbone: Tarrasius problematicus

Updated March 1, 2020
with a DGS tracing of an in situ skull. 

Completely by convergence
this fish, Tarrasius problematicus (Traquair 1881; Viséan, Early Carboniferous 340mya; 10cm) developed morphologically distinct and diverse vertebrae (Fig. 1), like those of tetrapods, according to Sallan 2012.

Figure 1. From Sallan 2012, reconstruction of Tarrasius with colors added. Note the five vertebral regions.

Figure 1. From Sallan 2012, reconstruction of Tarrasius with colors added. Note the five vertebral regions. See Figure 1 updated below.

Figure 2. Diagram of Tarrasius reconstructions from Sallan 2012, colorized here with the addition of the DGS tracing at lower right.

Figure 1 updated. Diagram of Tarrasius reconstructions from Sallan 2012, colorized here with the addition of the DGS tracing at lower right.

From the Sallan abstract:
“Here, I show that, Tarrasius problematicus, a marine ray-finned fish from the Mississippian (Early Carboniferous; 359–318 Ma) of Scotland, is the first non-tetrapod known to possess tetrapod-like axial regionalization. Tarrasius exhibits five vertebral regions, including a seven-vertebrae ‘cervical’ series and a reinforced ‘sacrum’ over the pelvic area. Most vertebrae possess processes for intervertebral contact similar to tetrapod zygapophyses. The fully aquatic Tarrasius evolved these morphologies alongside other traits convergent with early tetrapods, including a naked trunk, and a single median continuous fin. Regional modifications in Tarrasius probably facilitated pelagic swimming, rather than a terrestrial lifestyle or walking gait, presenting an alternative scenario for the evolution of such traits in tetrapods. Axial regionalization in Tarrasius could indicate tetrapod-like Hox expression patterns, possibly representing the primitive state for jawed vertebrates. Alternately, it could signal a weaker relationship, or even a complete disconnect, between Hox expression domains and vertebrate axial plans.”

Sallan reports,
“Tarrasius problematic us (Traquair 1881) is a fossil ray-finned fish (Actinopterygii) found in the Mississippian marine sediments of Scotland, phylogenetically branching off either the actinopterygian or actinopteran stem.”

By contrast
in the large reptile tree (LRT, 1655+ taxa) Tarrasius (Figs. 1, 2) nests with  Pholidophorus (Fig. 3), far from the origin of tetrapods, but deep within the lineage.

Figure 1. From Sallan 2012 the NHM-P18062 skull of Tarrasius, the tadpole mimic. DGS colors added and used to create the reconstruction shown here. Note the complete lack of utility offered by the Sallan 2012 tracing.

Figure 2 updated. From Sallan 2012 the NHM-P18062 skull of Tarrasius, the tadpole mimic. DGS colors added and used to create the reconstruction shown here. Note the complete lack of utility offered by the Sallan 2012 tracing.

Odd that a taxon like this has five distinct vertebral types.
It doesn’t even have a pelvis and hind fins,

Tarranius problematicus (Traquair 1881; Sallan 2012; Viséan, Early Carboniferous, 340mya; 10cm) was considered similar to the bichir, Polypterus, but phylogenetically close to Eusthenopteron and Phanerosteon. Here it nests at the base of the Pholidophorus clade.

Figure x. Subset of the LRT focusing on basal vertebrates (= fish).

Figure x updated. Subset of the LRT focusing on basal vertebrates (= fish).

References
Sallan LC 2012. Tetrapod-like axial regionalization in an early ray-finned fish. Proceedings of the Royal Society B 279:3264–3271.
Traquair RH 1881. Report on the fossil fishes selected by the Geological Survey of Scotland in Eskdale and Liddesdale. I. Ganoidei. Trans. R. Soc. Edin. 30,
14–71.

https://en.wikipedia.org/wiki/Tarrasiiformes

doi:10.1098/rspb.2012.0784

Thelodus and Squatina: two overlooked human ancestors

Updated December 14, 2020
because so many more chondrichthyes and chimaera have been added to the LRT.

Figure y. Basal Gnathostomata with the addition of Rhinochimaera.
Figure 5. Shark skull evolution according to the LRT. Compare to figure 1.
Figure 5. Shark skull evolution according to the LRT. Compare to figure 1.

You might think these two flat bottom dwelling fish
were outliers, weird-ohs and anomolies. Not so! They were key players!

The taxonomic addition of Thelodus and Squatina
shifts sharks, ratfish and sturgeons to a node prior to the placoderm, Entelognathus, in the large reptile tree (LRT, 1470 taxa). The extant Squatina (angelshark) remains remarkably similar to its sister in the LRT, the Early Silurian Thelodus (Fig. 1) despite the 435 million year difference in appearance.

The basal taxon of any cladogram
is potentially the trickiest node, the one fraught with the most possible error. The choice (and it is a choice made by the clade maker) is vitally important. The software assumes this is indeed THE ancestral taxon. So it better be a valid ancestral taxon.

thelodus-squatina588
Figure 1. Squatina and Thelodus are sister taxa in the LRT, nesting as outgroup taxa to the Placodermi and bony fish. Not a specialized morphology, but the one from which we tetrapods and sharks share a last common ancestor.

Thelodus parvidens (Agassiz 1839; Early Silurian; 5–15cm in length) is the basalmost taxon in the LRT because it just barely shows skull bones. This jawless toothless(?) bottom feeder gave rise to sharks, like Squatina, and placoderms, like Entelognathus, which gave rise to bony fish and tetrapods, like humans.

FIgure 2. Squatina skull in two lateral views, with open and closed jaws. Even at this early stage some bones found in higher vertebrates can be identified here. Teeth appear here for the first time.
FIgure 2. Squatina skull in two lateral views, with open and closed jaws. Even at this early stage some bones found in higher vertebrates can be identified here. Teeth appear here for the first time.

Squatina oculata (Bonaparte 1840) is the extant smoothback angelshark, a bottom feeder sister to Thelodus and the basalmost tested shark. The gill arches are transformed to jaws with teeth. The general morphology is little changed from the Early Silurian and informs the genesis of many vertebrate traits.

Figure 3. Subset of the LRT focusing on basalmost taxa. Here jawless Thelodus is the new outgroup taxon, bringing with it the sharks, ratfish and sturgeons.
Figure 3. Subset of the LRT focusing on basalmost taxa. Here jawless Thelodus is the new outgroup taxon, bringing with it the sharks, ratfish and sturgeons. Squatina is similar, with toothy jaws transformed from anterior gill arches.

Ferrón and Botella 2017 wrote:
Thelodonts are an enigmatic group of Paleozoic jawless vertebrates that have been well studied from taxonomical, biostratigraphic and paleogeographic points of view, although our knowledge of their ecology and mode of life is still scant. Their bodies were covered by micrometric scales whose morphology, histology and the developmental process are extremely similar to those of extant sharks.”

“Currently, there are 147 described thelodont species, belonging to 54 different genera and grouped in six orders (Sandiviiformes, Loganelliiformes, Shieliiformes, Phlebolepidiformes, Thelodontiformes and Furcacaudiformes). Only 29 of these species are known from articulated specimens, which provide the information about the general aspect and some anatomical features of thelodonts. The remaining 118 species are described only on the basis of associations of (or a few) disarticulated scales.”

By the way
this further separates sharks from ‘spiny shark’ acanthodians, which nest with Cheirolepis and Brachyacanthus in the LRT.

Figure 4. Closeup of Thelodus' face labeled.
Figure 4. Closeup of Thelodus’ face labeled.

And once again,
we have living specimens little changed from deep time ancestors from our own family tree. These new taxa document the origin of jaws and teeth, bottom feeding, a flattened overall morphology, broad fins in contact with the substrate more useful for propulsion than the tail and the origin of the internal and external skeleton.

The hypothesis of relationships that nests Thelodus with Squatina
appears to be novel. A Google search failed to find any similar prior citation. If anyone can alert me to an earlier reference, then we’ll consider this independent discovery a confirmation of that earlier hypothesis.


References
Agassiz L 1839. Fishes of the Old Red Sandstone. In Merchison’s Silurian System. Künstliche Steikerne von Konchylien und Fische. Neues Jahrbuch Mineralogie.
Bonaparte CL 1840. Iconografia della fauna italica per le quattro classi degli animali vertebrati. Rome.
Ferrón HG and Botella H 2017. Squamation and ecology of thelodonts. . 2017; 12(2): e0172781.
Zhu M, Yu X-B, Ahlberg PE, Choo B and 8 others 2013. A Silurian placoderm with osteichthyan-like marginal jaw bones. Nature. 502:188–193.

wiki/Entelognathus
wiki/Theodus
wiki/Squatina
wiki/Smoothback Angelshark

https://sharksrays.org

http://www.southernfriedscience.com/no-bones-about-it/

http://oldredsandstone.com/new_page_6.htm

Thoracic transverse processes – here and there

In reptiles sometimes the dorsal (thoracic) vertebrae develop elongate transverse processes (Fig. 1). The phylogenetic pattern of these appearances is today’s topic, inspired by Hirasawa 2013.

Note (Fig. 1) that the turtle-like enaliosaur, Sinosaurophargis has elongate thoracic transverse processes. Turtles and near-turtles, like Odontochelys, don’t. So why did Hirasawa et al. (2013) add Sinosaurosphargis to their turtle family tree? Were they influenced by the convergent carapace?

The large reptile tree found the two clades (turtles and saurosphargids) were not related. Turtles nested with the new lepidosauromorphs, while saurosphargids nested with the new archosauromorphs.

from Hirasawa et al. 2013, pink arrow points to elongate transverse processes on Sinosaurosphargis. These are not present on Odontochelys and turtles.

Figure 1. from Hirasawa et al. 2013, pink arrow points to elongate transverse processes on Sinosaurosphargis. These are not present on Odontochelys and turtles. We’ll look at where in the tree such processes do appear  by convergence. 

The appearance of thoracic transverse processes within the Reptilia
You’ll recall that reptiles are essentially diphyletic. We’ll start with one of these clades, then look at the other in the large reptile tree.

The pattern of appearance within the new Lepidosauromorpha
The first appearance of transverse processes in the new Lepidosauromorpha is at the Kuehneosauridae, the gliding reptiles of the Permian to Cretaceous.

The only other clade is the Fenestrasauria (including the Pterosauria) of the Triassic to Cretaceous.

So, no turtles or near-turtles have elongate transverse processes.

The pattern of appearance within the new Archosauromorpha
The entire Synapsida develop elongate transverse processes in the new Archosauromorpha.

The next appearance includes the turtle-like basal enaliosaurs, Sinosaurosphargis (Fig. 1) + Largocephalosaurus.

Eusaurosphargis alone among thalattosaurs develops elongate transverse processes. It also has a wide, flattened torso, but gracile ribs.

Placodonts have elongate transverse processes. So do plesiosaurs, but not nothosaurs or pachypleurosaurs.

The pararchosauriforms from Doswellia to Tropidosuchus all have elongate transverse processes.  (Does Lagerpeton follow this pattern?)

Basal euarchosauriforms up to and including rauisuchids do not have elongate transverse processes. Derived rauisuchia from Yarasuchus and Ticinosuchus through all crocs and dinos (including birds and poposaurs) do have elongate dorsal transverse processes.

Pattern?
Wide flat taxa tend to have elongate transverse processes, whether they are trying to increase their width to glide or to flatten out on the ground or underwater. Even so, many flattened taxa do not have elongate transverse processes.

The stiffening of the torso (less undulating) appears to be the second reason, seen in synapsids, fenestrasaurs, pararchosauriforms and derived rauisuchians.

References
Hirasawa T, Nagashima H and Kuratani S 2013. The endoskeletal origin of the turtle carapace. Nature Communications 4:2107. online here.