Air-breathing Polypterus: it’s in our direct ancestry

The Nile bichir
(genus: Polypterus; Fig. 1) is a small, long-bodied, extant fish at home in hot, swampy, oxygen-starved waters. It has lungs, but no trachea. It breathes through a spiracle. Juveniles have large, pink external gills for breathing underwater. For decades, the big question has been: “What is it?”

According to Wikipedia,
“Polypterus was discovered, described, and named in 1802 by Étienne Geoffroy Saint-Hilaire. It is a genus of 10 green to yellow-brown species. Naturalists were unsure whether to regard it as a fish or an amphibian. If it were a fish, what type was it: bony, cartilaginous, or lungfish? Some regarded Polypterus as a living fossil, part of the missing link between fishes and amphibians, helping to show how fish fins had evolved to become paired limbs.”

Figure 1. The Nile bichir (Polypterus), skull, skeleton and bones colorized for ease of comparison. Compare to the placoderm, Entelognathus, (Fig. 2) and the stem tetrapod Tinirau (Fig. 3).

Figure 1. The Nile bichir (Polypterus), skull, skeleton and bones colorized for ease of comparison. Compare to the placoderm, Entelognathus, (Fig. 2) and the stem tetrapod Tinirau (Fig. 3).

Surprisingly,
when added to the large reptile tree (LRT, 1447 taxa) Polypterus did not nest with the distinctly different basal actinopterygian, Cheirolepis (Fig. 4), but between the Silurian placoderm, Entelognathus (Fig. 2) and the Middle Devonian stem tetrapod/crossopterygian, Tinirau (Fig. 3). Thus, Polypterus is a very ancient fish, with a genesis predating all tested Devonian crossopterygians and actinopterygians.

Figure 2. The placoderm, Entelognathus, is widely considered the outgroup to the crossopterygians, the stem tetrapods. Compare the skull bones to those of Polypterus (Fig. 1) and Tinirau (Fig. 3).

Figure 2. The placoderm, Entelognathus, is widely considered the outgroup to the crossopterygians, the stem tetrapods. Compare the skull bones to those of extant Polypterus (Fig. 1) and Middle Devonian Tinirau (Fig. 3).

Romer (1946) wrote (quoted from Wikipedia),
“The weight of Huxley’s opinion is a heavy one, and even today many a text continues to cite Polypterus as a crossopterygian and it is so described in many a classroom, although students of fish evolution have realized the falsity of this position for many years…. Polypterus…is not a crossopterygian, but an actinopterygian, and hence can tell us nothing about crossopterygian anatomy and embryology.” If this were true, Polypterus would have nested with the actinopterygian, Cherolepis, in the LRT.

Hall (2001) reported, 
“Phylogenetic analyses using both morphological and molecular data affirm Polypterus as a living stem actinopterygian.” Remember, a ‘stem’ actinopterygian, by definition, is not an actinopterygian. It’s something else preceding that clade. Currently Polypterus cannot have its genes tested against any other placoderms or crossopterygians, but we can include this taxon in phylogenetic analysis.

So often
paleontologists keep looking, too often in frustration, where they think a taxon should nest (e.g pterosaurs as archosaurs, caseids as synapsids, whales as artiodactyls, etc.), instead of just letting the taxon nest itself in a wide gamut analysis, like the LRT. It’s really that easy and you can be confident of the results because all other candidates are tested at the same time.

Figure 3. The stem tetrapod, Tinirau. Compare to Polypterus (Fig. 1) and Entelogenathus (Fig. 2).

Figure 3. The stem tetrapod, Tinirau. Compare to Polypterus (Fig. 1) and Entelogenathus (Fig. 2).

So, where does that leave the basal actinopterygian, Cheirolepis?
In the LRT, Cheirolepis nests with the small crossopterygian, Gogonasus (Fig. 4) and these more circular cross-section taxa form a clade outside of the main line of flat stem tetrapods. That also solves several long-standing problems.

Now the late appearance of the basal fish, Cheirolepis
and other actinopterygians makes sense. They are derived from crossopterygians that have losing their lobe fins while retaining their fin rays.

Figure 4. The former most primitive ray-fin fish, Cheirolepis (Middle Devonian) nests with the crossopterygian, Gogonasus, in the LRT, distinct from Polypterus. Note the fleshy pectoral fins, the anterior advancement  and transformation of the pelvic fin and the loss of the anterior dorsal fin in Cheirolepis.

Figure 4. The former most primitive ray-fin fish, Cheirolepis (Middle Devonian) nests with the crossopterygian, Gogonasus, in the LRT, distinct from Polypterus. Note the fleshy pectoral fins, the anterior advancement  and transformation of the pelvic fin and the loss of the anterior dorsal fin in Cheirolepis.

Placoderms had fleshy fins.
The pectoral fins of Cheirolepis remained lobe-like, while the pelvic fins lost their lobes. That’s the progression, not the other way around.

If you see a fish with great distance
between the pectoral and pelvic fins (e.g. Polypterus, sharks, sturgeons, paddlefish, etc.) it is primitive. In Cheirolepis the distance is shortening. In many derived fish, the pelvic girdle is just beneath the gills. Very strange, when you think about it.

Not only can Polypterus breathe air,
it can lift its chest off the substrate with its robust forelimbs (Fig. 5). It can survive on land for several months at a time. Now those Middle Devonian trackmakers no longer seem so outlandish.

The three flathead fish, Entelognathus, Polypterus and Tinirau,
lead directly to Panderichthys, Tiktaalik and other flattened basal tetrapods.

That makes Polypterus,
like Didelphis, Caluromys and Lemur,  a living representative close to the direct lineage of tetrapods, mammals, primates and humans.

If you’re interested in Polypterus
the above YouTube videos will prove enlightening.


References
Geoffry Saint-Hillaire E 1802. Description d’un nouveau genre de poisson, de l’ordre des abdominaux. Bull. Sci. Soc. Philom., Paris, 3(61):97-98.
Hall BK 2001. John Samuel Budgett (1872-1904): In Pursuit of Polypterus, BioScience 51(5): 399–407.
Romer AS 1946. The early evolution of fishes, Quarterly Review of Biology 21: 33-69.

wiki/Polypterus
wiki/Tinirau
wiki/Cheirolepis
wiki/Entelognathus

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Where do sea horses come from?

A little off topic,
but I was curious to see how the odd morphology of the sea horse came to be, who its ancestors were and what transitional taxa went through on their evolutionary journey through deep time. Hope you find this interesting.

The relationship between sticklebacks and sea horses
has been known for many decades. Both are members of the clade Gasterosteiformes, which is in the clade of spiny finned fish, Acanthopterygii, which is in the clade of bony ray-finned fish, Actinopterygii.

A helpful guide
is Gregory 1933, available online as a PDF. Most of the images below come from that book.

No phylogenetic analysis was performed here,
so think of the following images as broad evolutionary brush strokes, not a narrow ladder of succession. Few details are offered because most are apparent at first glance. Precise last common ancestors remain unknown. These are rare derived representatives of deep time radiations.

Even so,
the early appearance of body armor in the stickleback, G. aculeatus; the diminution of the tail (except in the pipefish/fantails, Dunckerocampus and Solenostomus); the gradual loss of the fusiform shape; the elongation of the rostrum and the reduction of the mouth are all apparent in this series of illustrations.

Figure 1. Stickleback to sea horse evolution through pipefish. Sticklebacks have some of the body armor that overall encases and stiffens sea horses and sea dragons like Hippocampus and two species of Phyllopteryx. The ghost pipeish Solenostomus, is distinct from the more slender, elongate types of pipefish.

Figure 1. Stickleback to sea horse evolution through pipefish. Sticklebacks have some of the body armor that overall encases and stiffens sea horses and sea dragons like Hippocampus and two species of Phyllopteryx. The ghost pipeish Solenostomus, is distinct from the more slender, elongate types of pipefish.

The skulls of the taxa shown above
(Fig. 2) detail other changes, such as how far anteriorly the quadrate and palate bones shift on these fish with an ever longer rostrum and ever smaller mouth losing tiny teeth. The hyomandibular (hy) is the stapes in tetrapods. Not sure about the homology of the squamosal and the labeled preopercular, but the following is offered. Sometimes fish and tetrapods have different names for the same bones, as we learned earlier here.

Figure 2. Most of the bones of the tetrapod skull are also found in sea horses with some odd changes, like the placement of the quadrate well anterior to the orbit. Hippocampus illustration from Franz-Odendaal and Adriaens 2014. Not the parasphenoid passing midway through the orbit, while in tetrapods the orbit is typically raised above this anteriorly-directed splint-like bone arising from the basicranium. Everything below it is mouth cavity.

Figure 2. Most of the bones of the tetrapod skull are also found in sea horses with some odd changes, like the placement of the quadrate well anterior to the orbit. Hippocampus illustration from Franz-Odendaal and Adriaens 2014. Not the parasphenoid passing midway through the orbit, while in tetrapods the orbit is typically raised above this anteriorly-directed splint-like bone arising from the basicranium. Everything below it is mouth cavity.

So many bones
are displaced or lost in sea horses distinct from their basal vertebrate locations (e.g. Cheirolepis, Fig. 3) that an evolutionary series illustration (Fig. 2) proves helpful in understanding the lumping and splitting of clade members. Sarcopterygians, like Osteolepis (Fig. 3), split off early from other ray-finned fish, which is why they appear share more traits and proportions with Cheirolepis. Note the jaw hinge remains posterior to the orbit in these two.

Figure 2. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

Figure 3. Cheirolepis skull (left) with skull bones colorized as in Osteolepis (right) and Enteognathus, figure 1. Colors make bone identification much easier. Note the post opercular bone differences between Osteolepis and Cheirolepis indicating separate and convergent derivation, based on present data.

Understanding where we came from,
and where our cousins went in their evolutionary journeys are the twin missions of this blogpost in support of ReptileEvolution.com.


References
Franz-Odendaal TA and Adriaens D 2014. Comparative developmental osteology of the seahorse skeleton reveals heterochrony amongst Hippocampus sp. and progressive caudal fin loss. EvoDevo 2014, 5:45
Gregory WK 1933. Fish skulls: a study of the evolution of natural mechanisms. American Philosophical Society. ISBN-13: 978-1575242149 PDF

wiki/Seahorse
diverosa.com/Syngnathidae

Ever hear of Palaeothentes?

Currently there is no Wikipedia page for this taxon.
Even so, I found it to be far more important at filling gaps and shaking up paradigms than it seemed at first. The small, dull-looking taxa tend to be like that, as readers now know.

You could find this rat-sized Miocene taxon
(Fig. 1) in Carroll’s 1988 book, Vertebrate Paleontology, now well-worn and in pieces due to constant page flipping and scanning. Today’s research has revealed several more precise and more recent resources.

Figure 1. Not a marsupial, and not a shrew opossum, Palaeothentes nests in the LRT at the base of the Apatemys + Trogosus clade nest to the clade of living shrew opossums within Glires.

Figure 1. Not a marsupial, and not a shrew opossum, Palaeothentes nests in the LRT at the base of the Apatemys + Trogosus clade next to the clade of living shrew opossums within Glires.

According to Abello and Candella 2010,
Palaeothentes minutes (Ameghino 1887) is a paucituberculatan (details below) from the Santa Cruz Formation The results indicate that Palaeothentes would have been an agile cursorial dweller, with leaping ability, similar to the extant paucituberculatan Caenolestes fuliginosus and the didelphid Metachirus nudicaudatus.”

Okay, so now we have a problem.
In the large reptile tree (LRT, 1445 taxa) Caenolestes is not a marsupial. It nests with Rhyncholestes and more distantly Apatemys and more distantly, the extant tree shrew, Tupaia and the extant shrew, Scutisorex. As noted earlier, shrew opossums are placental shrews, not marsupial opossums in the LRT.

Wikipedia reports,
“Like several other marsupials, they do not have a pouch, and it appears that females do not carry the young constantly, possibly leaving them in the burrow.”
That’s describes most rodent/rabbit/tree shrew mothers and their young.

Wikipedia reports,
“Paucituberculata is an order of South American marsupials. Although currently represented only by the eight living species of shrew opossums, this order was formerly much more diverse, with more than 60 extinct species named from the fossil record, particularly from the late Oligocene to early Miocene epochs.”

Let’s solve that problem
by adding Palaeothentes to the LRT. Doing so recovers this taxon at the base of the Apatemys + Trogosus clade, next to the clade that includes Caenolestes, within Glires, far from Marupialia.

I suspect taxon exclusion
is the cause for the present lack of confirmation for traditional consensus. Many PhDs over several decades have followed tradition in nesting and testing shrew opossums with marsupials without testing them against apatemyids apparently. That’s why the LRT is here, to test taxa that have never been tested together before.

But wait! There’s a novel twist here~~~~~~!
Carroll 1988 reports, “Caenolestids have long been recognized as being very distinct from other South American marsupials, but they share with them a highly distinctive pattern of the spermatozoa, which become paired within the epididymis. Paired sperm are not known in any placental groups or among the Australian marsupials.” 

Sorry.
Physical traits have to trump genes and sperm. It just has to be that way because the LRT includes fossil taxa, which never preserve sperm. There have to be rules that all participants abide by. Interesting that the gene for paired spermatozoa is localized to one continent, just as genes separate other placentals into afrotheres and laurasiatheres. By the way, “The data show that paired spermatozoa exhibit a significant motility advantage over single spermatozoa in a viscous medium” according to Moore and Taggart 1995, who tested Monodelphis, a South American opossum.

Finally
we have a last common ancestor for arboreal Apatemys (Eocene, North America) and terrestrial Trogosus (Eocene, North America), two former enigma taxa with little to no relationship with other better known mammal clades. All members of Glires had their genesis sometime in the Jurassic, based on the presence of highly derived multituberculates (clade: Glires) in the Jurassic.

Wikipedia considers
apatemyids and trogosinae (Tillodontia) to be members of the Cimolesta, “an extinct order of non-placental eutherian mammals.” This bungling of the mammal family tree is due to taxon exclusion and the lack of a phenomic (trait-based) wide gamut cladogram that includes all the taxa present in the LRT. Paleontology needs to toss off a wide range of useless tradition with a reptile revolution led by someone out there confirming (or refuting) the widest gamut cladogram presently available: the LRT.


Palaeothentes lemoinei (Ameghino 1887, MPM-PV 3566; Miocene) was considered a prehistoric shrew opossoum (clade: Paucituberculata) but here nests next to shrew opossums, at the base of the Apatemys + Trogosus clade within Glires. The skull is 2x wider than tall, the canines are still large, the last premolar is large with a flat occlusal surface and the nasals split to form a zigzag suture with the frontals.


References
Abello MA and Candela AM 2010. Postcranial skeleton of the Miocene Marsupial Palaeothentes(Paucituberculata, Palaeothentidae): Paleobiology and Phylogeny. Journal of Vertebrate Paleontology 30(5):1515-1527.
Ameghino F 1887. Enumeracions sistematicad e las especies de mamiferos
fosiles coleccionados por Carlos Ameghino en los terranos eocenos de la Patagonia austral y depositados en el Museo La Plata. Boletin Museo de La Plata, 1:1-26.
Carroll RL 1988. Vertebrate Paleontology and Evolution. W. H. Freeman and Co. New York.
Forasiepi AMSánchez-Villagra MR, Schmelzle T,  Ladevèze S and Kay RF 2014. An exceptionally well-preserved skeleton of Palaeothentes from the Early Miocene of Patagonia, Argentina: new insights into the anatomy of extinct paucituberculatan marsupials. Swiss Journal of Palaeontology, 133(1):1-21.
Moore HD and Taggart DA 1995. Sperm pairing in the opossum increases the efficiency of sperm movement in a viscous environment. Biol. Reprod. 52(4):947-53.
Osgood WH 1921. A monographic study of the American marsupial, Caenolestes. Field Museum of Natural History, Zoological series 14:1–156.

wiki/Apatemyidae
wiki/Paucituberculata
https://en.wikipedia.org/wiki/Shrew_opossum
wiki/Vertebrate_Paleontology_and_Evolution

Paucituberculata -Trouessart 1898, Ameghino 1894


There was some news
about Palaeothentes recently (see below). Note, the experts consulted here consider this genus a marsupial.

New Bolivian Marsupials from the Middle Miocene

UV light vs. LCA (last common ancestor) approach to flapping flight in birds

Schwarz et al. 2019
employed ultraviolet (UV) light to report, “In contrast to previous studies, we show that most of the vertebral column of the Berlin Archaeopteryx possesses intraosseous pneumaticity, and that pneumatic structures also extend beyond the anterior thoracic vertebrae in other specimens of Archaeopteryx. With a minimum Pneumaticity Index (PI) of 0.39, Archaeopteryx had a much more lightweight skeleton than has been previously reported, comprising an air sac-driven respiratory system with the potential for a bird-like, high-performance metabolism.

“The neural spines of the 16th to 22nd presacral vertebrae in the Berlin Archaeopteryx are bridged by interspinal ossifications, and form a rigid notarium-like structure similar to the condition seen in modern birds. this reinforced vertebral column, combined with the extensive development of air sacs, suggests that Archaeopteryx was capable of flapping its wings for cursorial and/or aerial locomotion.”

Schwarz et al. did not perform a phylogenetic analysis
nor did they mention anything about the elongate locked down coracoid present on this specimen. In the large reptile tree (LRT, 1445 taxa, subset Fig. 1) the Berlin specimen of Archaeopteryx (MB.Av.101) nests at the base of all flapping birds, including the Enantiornithes, the first clade to split off. As in the amniotic egg issue, the last common ancestor is where you find the genesis of traits common to all descendant taxa. So, Schwarz et al. are correct: the Berlin specimen is indeed close to the origin of flapping flight.

Figure 3. Subset of the LRT focusing on basal birds and pre-bird theropods. Note many of the various Solnhofen birds nest apart from one another and the Daiting specimen nests outside the birds (Aves).

Figure 1. Subset of the LRT focusing on basal birds and pre-bird theropods. Note many of the various Solnhofen birds nest apart from one another and the Daiting specimen nests outside the birds (Aves). The Berlin Archaoepteryx is the last common ancestor of all flapping birds.

It’s not the reinforced vertebral column
that determines if a bird (or pterosaur) flaps or not. It’s the elongation of an immobile coracoid these two flapping clades share in common at the genesis of this behavior.

Distinctlively different
due to lacking a coracoid, bats employ the hyper-elongation of the clavicle to do the same thing by convergence.

Figure 1. Generic freehand Archaeopteryx (Berlin specimen) from Schwarz et al. 2019, retraced from Wellnhofer 2008 compared to bone-by-bone tracing for ReptileEvolution.com.

Figure 2. Generic freehand Archaeopteryx (Berlin specimen) from Schwarz et al. 2019, retraced from Wellnhofer 2008 compared to bone-by-bone tracing for ReptileEvolution.com. Wellnhofer’s drawing appears to be a generic Archaeopteryx. Tracings of all specimens show no two are alike.

One more thing…
if possible, don’t freehand your reconstructions (Fig. 2) and don’t redraw freehand reconstructions from Wellnhofer 2008 ~ especially if you’re going to go through all the trouble of extracting more precise data on a fossil than has been recovered before. Do your own more precise bone tracings and reconstructions!


References
Schwarz D, Kundrat M, Tischlinger H, Dyke G and Carney RM 2019. Ultraviolet light illuminates the avian nature of the Berlin Archaeopteryx skeleton. Nature.com
Wellnhofer P 2008. Archaeopteryx. Der Urvogel von Solnhofen. (Verlag Dr. Friedrich Pfeil, München), pp. 256.

When elbows and knees start bending in basal tetrapods

You might remember,
earlier we looked at the non-traditional origin of fingers and toes in the Tetrapoda in the basal dvinosaur/plagiosaur, Trypanognathus (Figs. 1, 7). 

Today
we’ll look at the origin of elbows and knees able to bend (distinct from lobefins and kin).

And then a quick peek
at bendable limbs large enough to sustain/lift the weight of the skull and body off the substrate, an ability chronicled in Middle Devonian tracks.

Backstory
Lobefin fish, like Eusthenopteron through Panderichthys, have one proximal limb bone, two more distally and many tiny bones further out, ending in lepidotrichia (fin filaments). In these taxa the radius is much longer than the ulna. The tibia is much longer than the fibula.

Figure 1. Basal tetrapods to scale. Yellow taxa are temnospondyls. Red taxa are reptilomorphs.

Figure 1. Basal tetrapods to scale. Yellow taxa are temnospondyls. Red taxa are reptilomorphs.

Dvinosaurs (basalmost tetrapods, and by definition, reptiles and humans; Fig. 2) like Trypanognathus (Figs. 1, 2), have the same limb bone arrangements (1 bones, 2 bones, then several bones), but the ulna length catches up to the radius and the fibula resembles the tibia. Notably this occurs on limbs too small to support weight. Only tiny fingers and toes are present. These replace the lepidotrichia. There is little indication that a strongly bendable elbow or knee is present.

Figure 2. Subset of the LRT focusing on the basal tetrapods including Trypanognathus.

Figure 2. Subset of the LRT focusing on the basal tetrapods. Trypanognathus forelimb and hind limb shown at right. Also see figure 7.

In colosteids,
like Colosteus (Fig. 3) and Pholidogaster, the limbs have modern proportions, with bendable elbows and knees, but they remain far too small to support the weight of the head and torso. Another traditionally considered colosteid, Greererpeton (Fig. 1) nests at the base of the next derived clade in the large reptile tree (LRT, 1444 taxa).

Figure 5. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Figure 3. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Thereafter
limbs get bigger, as documented in Ossinodus (Fig. 1), able to support weight lifted above the substrate. At this stage and with this innovation basal tetrapods split into three clades: Temnospondyli, Lepospondyli and Reptilomorpha in the LRT. Even so, lateral undulation of the backbone is the main driver for stride length.

Temnospondyli
Limbs are larger in temnospondyls (Fig. 1). Bodies are rounder. Tails are longer. The limbs would have been advanced more by lateral undulation than by extension and flexion.

Acanthostega represents a rare reversal,
among temnospondyls. Phylogenetically it is a smaller, apparently neotonous taxon with extra fingers and toes, a reversal to a longer radius than ulna, smaller limbs, but also a smaller, narrower body and a robust tail. Both girdles were quite large and both the humerus and femur had large processes not seen in more primitive dvinosaurs.

Figure 2. Dendrerpeton without raised orbits from Holmes et al. 1998.

Figure 4. Dendrerpeton without raised orbits from Holmes et al. 1998. This configuration is similar to that of basal lepospondyls and reptilomorphs including microsaurs.

Lepospondyli
Trimerorhachis (Fig. 5) is either a basal taxon retaining a wide torso and relatively small limbs or it is yet another reversal because its sister taxon in the LRT is Dendrerepton (Fig. 4) a small taxon with robust limbs and a small body. This clade gives rise to frogs, like Rana, which hyper-emphasizes the limbs and reduces the torso, along with Cacops, which shortens the torso and emphasizes the girdles.

Figure 1. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition.

Figure 5. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition. The limbs in dorsal view would have been advanced more by lateral undulation than by extension and flexion.

Reptilomorpha
This clade includes reptiles, like Silvanerpeton, their proximal ancestors and microsaurs, like Tuditanus, in the LRT. Basal taxa were increasingly terrestrial with robust limbs on smaller bodies. Much later several clades within both Reptilia and Microsauria (e.g. Diplocaulus) returned to a more aquatic niche, typically reducing the limbs. In some reptiles (e.g. Ichthyosaurus, Orcinus) limbs evolved back into fins/flippers and tails evolved fish-like flukes.

Figure 1. Subset of the LRT focusing on basal tetrapods and showing those taxa with lobefins (fins) and those with fingers and toes (feet). Inbetween we have no data.

Figure 6. Subset of the LRT from an earlier post focusing on basal tetrapods and showing those taxa with lobefins (fins) and those with fingers and toes (feet). Inbetween we have now have data in the form of Trypanognathus (Fig. 7) at the base of the Dvinosauria.

Figure 1. Trypanognathus in situ, colorized to bring out ribs and limbs.

Figure 7. Trypanognathus in situ, colorized to bring out ribs and limbs, too small to support the body. Also see figure 2.

Plagiosuchus: confluent orbital-temporal fenestra, like birds and mammals

Just another case of convergence here
reminding us not to define clades on traits, but only on two select taxa, their last common ancestor and all descendants.

Plagiosuchus pustuliferus (von Huene 1922; Middle Triassic, 240mya; SMNS 57921) nests with the smaller, Gerrothorax in the large reptile tree (LRT, 1444 taxa). Plagiosuchus has a narrower skull and upper temporal fenestrae confluent with the orbits. This is partly due to the loss of the prefrontals, postfrontals and postorbitals.

Both taxa belong to the first clade
to split off from the remainder of basal tetrapods at the transition from fins to fingers and toes. Both taxa retained gills and likely never left the water.

Figure 1. Plagiosuchus skull from Damiani et al. 2009, lateral view and colors added here.

Figure 1. Plagiosuchus skull from Damiani et al. 2009, lateral view and colors added here. Lost or fused are the postorbital, postrfrontal and prefrontal bones on this basal tetrapod not far from fish. Note the relatively short dentary and long post-dentary bones.

The wide skull and laterally oriented dorsal ribs
indicate Plagiosuchus was a full-time bottom dweller with little use of its limbs other than to paddle them around like fins, supporting itself like another sit-and-wait predator, the extant frogfish.


References
Damiani R, Schoch RR, Hellrung H, Wernburg R and Gastou S 2009. The plagiosaurid temnospondyl Plagiosuchus pustuliferus (Amphibia: Temnospondyli) from the
Middle Triassic of Germany: anatomy and functional morphology of the skull. Zoological Journal of the Linnean Society, 2009, 155, 348–373.
von Huene F 1922. Beiträge zur Kenntnis der Organisation einiger Stegocephalen der schwäbischen Trias. Acta Zoologica3: 395–459.

wiki/Plagiosuchus

Untangling the Sclerothorax chimaera

Tough one today
with many puzzle pieces.

The genus Sclerothorax
was first named by Huene 1932 based on two giant salamander-sized Early Triassic specimens. One was a torso and anterior tail lacking a skull and ventral pectoral girdle (HLD-V 608; Fig. 1). The other was a skull and ventral pectoral girdle (HLD-V 607; Fig. 1). Apparently there were no bones in common. Both are shown here at about one-quarter natural size.

Figure 1. Sclerothorax holotypes (two specimens) described by Huene 1932. Colors added. The NMK specimens (at right) are traced from new specimens added by Shoch et al. (2007, Fig. 3), but newly traced here.

Figure 1. Sclerothorax holotypes (two specimens) described by Huene 1932. Colors added. The NMK specimens (at right) are traced from new specimens added by Shoch et al. (2007, Fig. 3), but newly traced here.

The 608 specimen torso and tail are notable
for their exceedingly tall neural spines topped by spine tables, like those of Eryops, and overlapping ribs, like those of Eryops, Sclerocephalus, Mastodontsaurus (Fig. 2) and Petobatrachus.

The 607 skull is notable
for being shorter than the interclavicle like no other basal tetrapods.. Schoch et al. 2007 report, “At first sight, this (second) specimen seemed so different from the first find that Huene himself was struck. Yet his efforts in further preparing the second specimen revealed the morphology of the dorsal spines which he found similar to the first specimen, albeit affected by compaction and consequently distorted.”

Figure 2. Click to enlarge. The largest amphibians of all time include Mastodonsaurus, Prionosuchus, Koolasuchus, Siderops, Crassigyrinus and the extant Andrias, the giant Chinese salamander.

Unfortunately
those purported high dorsal spines on the 607 skull specimen are not visible with the present data, nor were they illustrated (Fig. 1). No doubt the torso with overlapping ribs also resembles that of the hippo-sized Mastodontsaurus (Fig. 3). To that point, Shoch et al. nested Sclerothorax with Mastodonsaurus in their phylogenetic analysis.

Figure 2. NMK-S118 and 117 specimens assigned to Sclerothorax. Colors added.

Figure 3. NMK-S118 and 117 specimens assigned to Sclerothorax. Colors added. Snout restored two ways. Trying to identify sutures in such a textured skull with lateral line canals is fraught with difficulties. Note the dorsal ribs on the 117 specimen. They do not appear to overlap and appear to be laterally oriented, creating a broad, flat torso, but we are seeing them in ventral aspect.

Contra earlier studies,
in the large reptile tree (LRT, 1443 taxa) the NMK S-118 posterior skull referred specimen nested with the similarly-sized Early Permian Trimerorhachis (Fig. 6), a flat-head, flat-torso taxon without overlapping dorsal ribs or high dorsal spines. Distinct from most basal tetrapods, the jugal does not contribute to the orbit rim. The HLD-V 608 torso and tail holotype specimen nest at the base of Peltobatrachus + Sclerocephlaus, and between the Ossinodus clade and the Eryops clade. So the two specimens are not congeneric in the LRT and that skull does not belong with that torso and tail (Fig. 4). (As always, I am willing to be convinced otherwise with better data.)

Figure 3. Images from Schoch et al. 2007 combining various specimens to create a Sclerothorax chimaera.

Figure 4. Images from Schoch et al. 2007 combining various specimens to create a Sclerothorax chimaera. Note the small size of the limbs relative to the torso.

When it used skull traits from Schoch et al. (2007)
(Fig. 5) the LRT lost resolution. I also discovered the lateral view of the reconstructed skull  did not match the dorsal view with regard to the placement of nares, orbits and certain sutures. A repair of the lateral view is presented here (Fig. 5).

Figure 4. Sclerothorax skull in 4 views from Schoch et al. 2007, colors added. A second skull is shown in lateral view to match the elements and proportions of the dorsal view.

Figure 5. Sclerothorax skull in 4 views from Schoch et al. 2007, colors added. A second skull is shown in lateral view to match the elements and proportions of the dorsal view.

The change in the skull sutures
presented here (Fig. 3) and the subsequent nesting of the 607 skull specimen with Trimerorhachis (Fig. 6) is supported by the preservation of long, only slightly curved and laterally oriented ribs in the NMK S-117 specimen (Fig. 3), like those of Trimerorhachis (Fig. 6).

Figure 1. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition.

Figure 6. Trimerorhachis. Like Sclerothorax and distinct from other basal tetrapods the jugal does not contact the orbit rim. 


Fossils typically come to rest

with their major axis parallel to the bedding plane. In this way taxa with a wide, flat, skull and torso will usually be preserved in dorsal aspect. By contrast, taxa preserved in lateral view are more likely to have a deeper than wide torso, as in the Sclerothorax holotype (Fig. 1).

Eryops also had tall neural spines,
overlapping ribs and a deep pelvis in common with Sclerothorax. Perhaps Sclerothorax had a large skull and strong limbs, like Eryops, suitable for terrestrial locomotion.

By contrast
the 607 skull retains lateral line canals, like those of Trimerorhachis, so we might expect shorter limbs and an aquatic environment for the 607 skull specimen.

Figure 7. Subset of the LRT focusing on basal tetrapods. The two Sclerothorax taxa are highlighted in separate clades.

Figure 7. Subset of the LRT focusing on basal tetrapods. The two Sclerothorax taxa are highlighted in separate clades.

Phylogenetically separating
the 607 skull from the holotype torso resolves the lateral line canal issue when the skull was joined to the torso as a chimaera.


References
Huene F v 1932. Ein neuartiger Stegocephalen−Fund aus dem oberessischen Buntsandstein. Palaönontologische Zeitschrift 14: 200–229.
Schoch RR, Fastnacht M, Fichter J and Keller T 2007. Anatomy and relationships of the Triassic temnospondyl Sclerothorax. Acta Palaeontologica Polonica 52 (1): 117–136.

wiki/Sclerothorax