Hagfish and nematodes side-by-side in detail for the first time

Summary for those in a hurry
After this comparison, nematodes and hagfish need to be added to the base of the vertebrate/ echinoderm/ deuterostome family tree as outgroup taxa. In other words, hagfish are big nematodes with a notochord. And in turn, so are we.

Figure 1. The hagfish Myxine in vivo patrolling the sea floor. Note the nematode-like tentacles surrounding the mouth end at lower left.

Hagfish (clade: Myxini)
are very low on the vertebrate family tree. According to Wikipedia, “They are the only known living animals that have a skull but no vertebral column, although hagfish do have rudimentary vertebrae.”

With origins in the Cambrian or Ediacaran,
we know of only one fossil hagfish, Gilpichthy greenei (Bardack and Richardson 1977, FMNH PE18703, 5cm; Fig. 2) from the famous Mazon Creek Formation, Late Carboniferous, 307 mya.

Figure 2. Gilpichthys, a Pennsylvanian hagfish, enlarged and full scale.

Without vertebrae,
the Atlantic hagfish (genus Myxine, Linneaus 1758, 50cm, other genera up to 127cm) nest between Vertebrata and more basal taxa. (Not yet added to the LRT).

Outgroup taxa include
lancelets and nematodes (= round worms).

Yesterday
one of those insightful bells rung when I realized nematodes have eversible teeth made of keratin, as in hagfish. Something obvious had, once again, been overlooked. Peters 1991 listed nematodes as vertebrate ancestors based on overall morphology. Hagfish were not included then.

Now
let’s see what other details link nematodes to hagfish, a relationship overlooked by all prior authors, probably due to the great size difference (most nematodes are <2.5mm long), or perhaps due to taxon exclusion. According to Wikipedia, “Taxonomically, they [nematodes] are classified along with insects and other moulting animals in the clade Ecdysozoa,”

Figure 3. Nematodes and hagfish side-by-side, focusing on the eversible mouth parts and keratin teeth.

Classification
According to Wikipedia, “The classification of hagfish had been controversial. The issue was whether the hagfish was a degenerate type of vertebrate-fish that through evolution had lost its vertebrae (the original scheme) and was most closely related to lampreys, or whether hagfish represent a stage that precedes the evolution of the vertebral column (the alternative scheme) as is the case with lancelets. Recent DNA evidence has supported the original scheme.”

We have learned time and again, you can never trust DNA evidence, especially when taxon exclusion is in play. Instead, look at the traits of the taxa under study. And look at lots of taxa to make sure none of them share more traits.

Smithsonian Magazine listed 14 (edited to 7) fun facts about hagfish.

1. Hagfishes live in cold waters around the world, from shallow to 1700 m.
2. Hagfish can go months without food.
3. Hagfish can absorb nutrients straight through their skin.
4. Hagfish have two rows of tooth-like structures made of keratin they use to burrow deep into carcasses. They can also bite off chunks of food. While eating carrion or live prey, they tie their tails into knots to generate torque and increase the force of their bites.
5. No one is sure whether hagfish belong to their own group of animals, filling the gap between invertebrates and vertebrates, or if they are more closely related to vertebrates.
6. The only known fossil hagfish, [Gilphichthys, above] looks modern.
7. Hagfish produce slime. When harassed, glands lining their bodies secrete stringy proteins that, upon contact with seawater, expand into the transparent, sticky slime.

Figure 4. Illustration of a nematode with labels from corodon.com. This model has been based on the fresh-water nematode Ethmolaimus. Compare to the hagfish in figure 1.

How does the hagfish compare to an aquatic nematode?

1. Tail — The post-anal region forms a tail in both
   
2. Mucus — Moens et al. 2005 report, “Many aquatic nematodes secrete mucus while moving.” The authors did not mention hagfish, which are famous for mucus. Some nematodes also exude adhesive from post-anal, tail tip glands.
3. Sensory tentacles — The mouth is in the centre of the anterior tip and may be surrounded by 6 lip-like lobes in primitive marine forms, three on each side. Primitively the lips bear 16 sensory papillae or setae.
4. Burrowing into their prey — Both hagfish and nematodes attach their lips to larger prey, make incisions and pump out the prey’s contents with a muscular pharynx.
5. Swimming — In water nematodes swim by a graceful eel-like motion as they throw their stiff but elastic bodies into sinusoidal curves by contracting longitudinal muscles (the elasticity of the cuticle and hydrostatic skeleton more or less returns the body to its original straight shape). The notochord in the hagfish gives the same sort of elasticity to the famously wriggly body capable, as in nematodes, to form corkscrews and knots.
   
6. Niche — Nematodes represent 90% of all animals on the ocean floor, not counting hagfish. Both play important roles in dead vertebrate decomposition.
7. Embryo development — An alternative way to develop two openings from the blastopore during gastrulation, called amphistomy, appears to exist in some animals, such as nematodes.
8. Size –– some species of hagfish and nematode reach 1m in length, though most nematodes are <2.5mm
9. Eyes — A few aquatic nematodes possess what appear to be pigmented eye-spots, but most are blind. So are hagfish.
10. Reproduction — Usually male and female, sometimes hermaphroditic
11. Tough skin and subcutaneous sinus — largely separated from underlying tissue

Evolution from nematode to hagfish

1. Head — radially symmetrical evolves to bilaterally symmetrical
2. Mouth — three or six lips with teeth on inner edges reduced to two
3. Skin and skeleton — Hydroskeleton and cuticle evolve to notochord and ‘eelskin’
4. Nerve chord —Dorsal, ventral and lateral in nematodes, reduced to just dorsal in hagfish
5. Brain – circular nerve ring in nematodes, dorsal concentration in hagfish

Pikaia gracilens
(Walcott 1911, Middle Cambrian, Fig. Z) has been compared to lancelets and hagfish. Like hagfish, Pikaia retained twin tentacles, but also had cirri instead of rasping eversible teeth.

Figure z. Pikaia gracilens from Mallatt and Holland 2013 showing hagfish and lancelet affinities.

Added 24 hours later
as the question of mouth and anus origin from the original blastopore (Fig. zz) arises again in the comments section.

Figure zz. Blastopre evolution to produce an anus and mouth at the same time in a marine nematode. This is the transitional taxon from protostome nematodes to deuterostomes. This is how it happened. This is how it was ignored in many Western textbooks.

Malakov 1997 writes,
“The blastopore initially has a spherical Caenorhabtitis sp. (Ehrenstein & Schierenberg, shape, but then stretches to become an elongated 1980). oval-shape (Fig. 2). Subsequent development results Embryogenesis in enoplids appears to have several in the lateral edges ofthe blastopore approaching and u.nusual features. Firstly, variability occurs in the eventually connecting with the centre. Two openblastomere arrangement in the stages of early cleavings, one at the anterior end the other at tl1e posterior age. At the four-cells stage various configurations end of the embryo, are persistent remnants of the have been observed, viZ., tetrahedral, rhombic, Tblastopore. The anterior opening provides the beginshaped. These configurations have been variously ning of the definitive mouth, and the posterior one, encountered in the development of nematodes bethe definitive anus.”

See figure z (above). Hagflish and vertebrates arose form marine nematodes exhibiting this form of early cell division. This is how deuterostomes arose.

Malakov 1997 reports, “From these results it may be concluded that enoplids represent an early evolutionary branch, which seperated (sic) from the ancestral nematode stem prior to all other groups of nematodes.”

Figure 5. Medial section of Acipenser (sturgeon) larva with temporary teeth from Sewertzoff 1928. Note this specimen has marginal teeth and deeper teeth.

Getting back to baby sturgeon teeth…
Several months ago I cited Sewertzoff 1928 (Fig. 5) who found tiny teeth in the tiny lava of the large sturgeon, Acipenser. Those tiny teeth disappear during maturity, as you might recall. The question is: are those teeth homologs of keratinous hagfish + nematode teeth? Or homologs of enamel + dentine shark and bony fish teeth? McCollum and Sharpe  2001 in their review of the evolution of teeth reported, “The aim of this review is to see what this developmental information can reveal about evolution of the dentition.”

Unfortunately McCollum and Sharpe 2001 delivered the usual history of citations that indicate teeth started with sharks, overlooking sturgeon, nematode, lamprey and hagfish teeth. Phylogenetic bracketing indicates that baby sturgeon teeth are keratinous, not homologous with dentine + enamel shark teeth, which phylogenetically evolve later, first in sharks and later retained by bony fish. Let me know if this is incorrect.

Figure 6. Ventral view of the GLAHM V830 specimen of Thelodus. This appears to have fang-like teeth, but teeth are too soon. These are barbels = cirri.

Sturgeon barbels:
Are they homologs of hagfish + nematode barbels? Soft tissues, like barbels, are unlikely to fossilize, but one intervening bottom-dwelling taxon, Thelodus (Fig. 6), preserves barbels anterior to the ventral oral opening. Open water thelodonts do not preserve barbels. Catfish barbels appear to be a reversal because a long line of more primitive taxa do not have barbels. The same can be said of the catfish-mimic eel ancestor, the cave fish Kryptoglanis.

The relationship between hagfish and nematodes
should have been known for decades, but apparently this hypothesis of interrelationships has been overlooked, ignored or set to the side until now. If someone else recovered this hypothesis of interrelations previously, let me know so I can promote that citation.

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References
Bardack D and Richardson ES Jr 1977. New aganathous fishes from the Pennsylvanian of Illinois. Fieldiana Geology 33(26):489–510.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Malakov VV 1998. Embryological and histological peculiarities of the order Enoplida, a primitive group of nematodes. Russian Journal of Nematology 6(1):41–46.
Mallatt J and Holland ND 2013. Pikaia gracilens Walcott: stem chordate, or already specialized in the Cambrian? Journal of Experimental Zoology, Part B, Molecular and Developmental Evolution 320B: 247-271.
McCollum M and Sharpe PT 2001. Evolution and development of teeth. Journal of Anatomy 199:153–159.
Moens T et al. (6 co-authors) 2005. Do nematode mucus secretions affect bacterial growth? Aquatic Microbial Ecology 40:77–83.
Morris CS, Caron JB 2012. Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia. Biological Reviews 87: 480-512.
Nielsen C, Brunet T and Arendt D 2018. Evolution of the bilaterian mouth and anus. Nature Ecology & Evolution 2:1358–1376.
Nielsen C 2019. Blastopore fate: Amphistomy, Protostomy or Dueterostome. In eLS (eds) John Wiley & Sons Ltd.  DOI: 10.1002/9780470015902.a0027481
Peters D 1991. From the Beginning – The story of human evolution. Wm Morrow.
Sewertzoff AN 1928. The head skeleton and muscles of Acipenser ruthensus. Acta Zoologica 13:193–320.

wiki/Hagfish
wiki/Nematode
wiki/Pikaia
cronodon.com/BioTech/Nematode.html
pterosaurheresies.wordpress.com/2020/08/07/chordate-origins-progress-since-romer-1971/
Hagfish Day, occurs every year on the third Wednesday of October:
smithsonianmag.com/science-nature/14-fun-facts-about-hagfish-77165589/

Hagfish YouTube video 

Grand Canyon tetrapod tracks: odd, or misinterpreted?

Short summary for those in a hurry.
There are several reasons to think only the original interpretation (Fig. 1) is odd.

Rowland, Caputo and Jensen 2020 bring us their interpretation
of an odd 313mya trackway (Figs. 1–3) from the latest Early Carboniferous (Pennsylvanian) in Grand Canyon National Park (AZ, USA).

Figure 1. Animation of the interpretation of Rowland, Caputo and Jensen 2020 of the Grand Canyon Early Carboniferous trackmaker.

From the abstract
“We report the discovery of two very early, basal-amniote fossil trackways on the same bedding plane in eolian sandstone of the Pennsylvanian Manakacha Formation in Grand Canyon, Arizona. 

Only trackway 1 (Figs. 1–3) is under review here. And that may be more than one trackway.

“It displays a distinctive, sideways-drifting, footprint pattern not previously documented in a tetrapod trackway. We interpret this pattern to record the trackmaker employing a lateral-sequence gait while diagonally ascending a slope of about 20˚, thereby reducing the steepness of the ascent.”

Only one interpretation was provided by the authors. Here you’ll see one more.

“These trackways are the first tetrapod tracks reported from the Manakacha Formation and the oldest in the Grand Canyon region. The narrow width of both trackways indicates that both trackmakers had relatively small femoral abduction angles and correspondingly relatively erect postures.”

Is this the correct interpretation? Another (Fig. 2) is presented.

“They represent the earliest known occurrence of dunefield-dwelling amniotes―either basal reptiles or basal synapsids―thereby extending the known utilization of the desert biome by amniotes, as well as the presence of the Chelichnus ichnofacies, by at least eight million years, into the Atokan/Moscovian Age of the Pennsylvanian Epoch.”

“The depositional setting was a coastal-plain, eolian dunefield in which tidal or wadi flooding episodically interrupted eolian processes and buried the dunes in mud.”

Could these tracks be interpreted more parsimoniously?
What if the trackmaker was just an ordinary lissamphibian, like Celtedens (Figs. 2, 6) or a reptilomorph, like Amphibamus (Fig. 4)? Both have more of a matching manus and pes than any coeval amniote (details below). What if the trackmaker had a more sinuous spine, similar to that of coeval tetrapods and Celtedens or Amphibamus? What if the trackmaker had sprawling limbs, similar to other coeval tetrapods and Celtedens or Amphibamus? The trackmaker did not leave a tail drag mark, so what if the trackmaker had a short tail, like Celtedens or Amphibamus? What if there were multiple trackmakers? Is it possible that one or two trackmakers closely followed another one in a mating ritual or pursuit?

Figure 2. The Chelichnus-like tracks together with Celtedens, an amphibian trackmaker with a short tail, sinuous spine, splayed limbs and fewer digits than coeval amniotes. The unused tracks would have been created by a pursuing Celtedens-like trackmaker.

If so, 
here’s an animation based on an alternate taxon walking in a more typical fashion (Fig. 2) with less freehand invention. More splayed limbs and a sinuous spine are employed here matching coeval tetrapods in morphology and gait. In this scenario the unused tracks (Fig. 2) were created by a second and third Celtedens-like trackmaker pursuing in lock step with the first trackmaker.

Figure 3. Figure from Rowland, Caputo and Jensen 2020, with color overlays and PILs added at left.

From the first line of the Introduction
“Amniotes evolved early in the Pennsylvanian or late in the Mississippian Epoch.”

The authors were so sure the tracks were made by amniotes
they plugged the word “Amniotes” into the first line of the Introduction. In the large reptile tree (LRT, 1725+ taxa) we have several amniotes (= reptiles) from the EARLY Mississippian (Viséan). These were overlooked by Rowland, Caputo and Jensen 2020. The authors and those they cited were not up to date with the most recent phylogenic hypotheses of interrelationships.

Still on the subject of amniotes, the authors note,
“Because this trackway records the presence of relatively long digits with acuminate claws, we infer that the trackmaker was an amniote.” The Early Cretaceous lissamphibian, Celtedens (Figs. 2, 5) and the Late Carboniferous reptilomorph, Amphibamus (Fig. 4), also have long, slender digits with claws that taper to a point. The longest digits are medial on each manus and pes. Amniotes had more asymmetrical extremities with digit 4 typically the longest. The authors followed their initial bias and did not consider morphologically similar, but phylogenetically dissimilar trackmakers.

The keyword “Lissamphibian”
is not found in the Rowland, Caputo and Jensen text. Celtedens is a lissamphibian known only from two Early Cretaceous specimens. However, given the presence of related Gerobatrachus, Apteon and Doleserpeton specimens in the Early Permian, the radiation of Celtedens-like taxa was likely in the Carboniferous. The Late Carboniferous basal reptilomorph, Amphibamus, is likewise not mentioned and was not considered a potential trackmaker despite its appropriate match both morphologically and temporally.

The manus and pes of the Grand Canyon trackmaker 
were nearly equal in size. The pes in Carboniferous amniotes is typically larger than then manus. The authors agree, noting, “the manus prints of the Manakacha tracks are not conspicuously smaller than the pes prints, contrary to the typical pattern in Chelichnus.” Celtedens also has a pes that is larger than the manus, but the lissamphibians Apteon, Doleserpeton and Triassurus have subequal extremities. So does the reptilomorph, Amphibamus (Fig. 4).

Figure 4. Late Carboniferous Amphibamus is a potential trackmaker for the Grand Canyon latest Early Carboniferous tracks. with medial digits the longest, like the trackmaker.

The digits of the trackmaker
were relatively symmetrical with digits 2, 3 and 4 making impressions. The digits in Carboniferous amniotes are typically asymmetrical with 4 the longest and largest. The authors note, “Chelichnus tracks typically consist of only three or four digits of a pentadactyl trackmaker.” 

Taxonomic affinity of the trackmaker
The authors report, “Impressions of three digits are present in each track (Figs 4 and 6), however no plausible Pennsylvanian candidate trackmaker taxon was tridactyl. Thus, we interpret the prints to be shallow undertracks made by a pentadactyl animal whose lateral digits were not impressed deeply enough into the sediment to translate into the preserved bedding plane. Without impressions of all five digits on each foot we are unable to measure foot slenderness and other characters that are useful for distinguishing among the tracks of various basal amniote taxa.”

Figure 5. Microbrachis slightly revised with a new indented supratemporal here rotated to the lateral side of the skull above the squamosal and quadratojugal. Otherwise this image is from Carroll, who did not indent the supratemporal.

Yes, three digits is a little unsettling for a tetrapod trackmaker. 
The Middle Pennsylvanian microsaur, Microbrachis (Fig. 5), had a three digit manus and a five digit pes with #1 and #5 smaller than the medial digits, but these were mere vestiges, unable to support the animal on a terrestrial substrate.

Figure 6. Celtendens is the closest relative to Karaurus in the LRT.

Celtedens and Amphibamus had four fingers and five toes. 
So, they are not a perfect match for the Grand Canyon trackmaker, but they are close, at least one finger closer than any coeval amniote. Early Cretaceous Celtedens (Fig. 5) is too small to be the trackmaker. However, two hundred million years separate the two. On the other hand, Amphibamus (Fig. 4) is a better size match to the trackmaker and much closer temporally/stratigraphically.

The authors note, 
“a lateral-sequence gait is the most parsimonious footfall-sequence interpretation that is compatible with the pattern of tracks in this trackway. Tetrapods, in fact, routinely use a lateral-sequence gait when walking slowly; while one foot is off the ground, this gait provides a larger stable triangle than other footfall sequences. Moreover, a lateral-sequence gait facilitates undulations of the spine, which lengthen the step.” 

Actually, the diagram provided by Rowland, Caputo and Jensen minimizes undulations of the spine and the steps are not lengthened, but shortened. What they inadvertently describe is the more parsimonious and typical movement of the lissamphibian Celtedens presented here (Fig. 2).

“As indicated by expulsion rims adjacent to many of the tracks (Figs 4,5B,5C and 6), interpreted to occur on the downhill side, the trackmaker’s body was oriented straight up the slope.”

In figure 2 the Celtedens-like tetrapod also ascends the hill, but diagonally, taking big steps, not tiny lateral steps.

“Fossil trackways that record diagonal movement on the slope of a sand dune are common in the ichnology literature.”

“Francischini et al. documented an occurrence within the Permian eolian Coconino Sandstone of Arizona in which the angle of progression of an Ichniotherium trackway―inferred to have been made by the diadectid reptiliomorph Orobates ―differs markedly from the angle that the feet were pointing, similar to the case documented here in the Manakacha Formation. However, none of such previously documented cases of a tetrapod moving diagonally across the face of a sand dune record such a regular pattern of impressions of all four feet, as does Trackway 1 described here, and none have been interpreted to record a lateral-sequence gait.”

The possibility of two or three trackmakers in quick succession creating trackway 1 did not occur to the authors of this paper.

The possibility of an Amphibamus-like or Celtedens-like trackmaker did not occur to the authors of this paper. Instead they went straight for an imagined, headline-generating anachronistic atypical trackmaker, a taxon not present in coeval strata walking unlike any known taxon past or present.

Best to go with Occam’s razor and maximum parsimony.

Add taxa, especially when matching tracks to trackmakers, to make sure you don’t overlook more obvious matches.

Add pursuing trackmakers if there are too many tracks for one ordinary trackmaker.

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References
Rowland SM, Caputo MV and Jensen ZA 2020. Early adaptation to eolian sand dunes by basal amniotes is documented in two Pennsylvanian Grand Canyon trackways. PLoSONE 15(8): e0237636. https://doi.org/10.1371/journal.pone.0237636

The first four citations found in Rowland, Caputo and Jensen 2020:
Ahlberg PE and Milner AR 1994. The origin and early diversification of tetrapods. Nature 1994; 368: 507–514.
Clack JA 2002. Gaining Ground: the origin and evolution of tetrapods. Bloomington: Indiana University Press.
Benton MJ. 2005. Vertebrate Palaeontology. 3rd ed. Blackwell Science.
Ford DP and Benson RBJ 2020. The phylogeny of early amniotes and the affinities of Parareptilia and Varanopidae. Nature Ecology & Evolution 2020; 4: 57–65.

Sallen 2016 presents a fascinating flawed look at fish tails

Sallen 2016 reports,
“The symmetrical, flexible teleost fish ‘tail’ has been a prime example of recapitulation — evolutionary change(phylogeny) mirrored in development (ontogeny).”

Sallan’s cladogram (Fig. 1) lays out the traditional cladogram of fish. Note the position of the bichir (Polypterus), at a basal node and the sturgeon + paddlefish (Acipcenser + Polyodon) near the middle.

Figure 1. Cladogram from Sallan 2016 (above) and young fish tails (below).

Unfortunately,
taxon exclusion mars the cladogram of Sallan 2016 according to the the large reptile tree (LRT, 1704+ taxa; Figs. 2, 5). Due to tradition Sallan has chosen the wrong outgroup. Jawless sturgeons and shark-like paddlefish should be the outgroups here, not lungfish-like bichirs (Polypterus), which are highly derived taxa close to lungfish and tetrapods.

Figure 2. Same taxa as above, but rearranged to fit the LRT tree topology. Remember, sturgeons, paddlefish and sharks are basal taxa in the LRT. Esox is a catfish related to placoderms.

Salan reports,
“Paleozoic ray-finned fishes (Actinopterygii), relatives of teleosts, exhibited ancestral scale-coveredtails curved over their caudal fins. For over 150 years, this arrangement was thought to be retained in teleost larva and overgrown, mirroring an ancestral transformation series. New ontogenetic data for the 350-million-year-old teleost relative Aetheretmon overturns this long-held hypothesis.”

By contrast,
in the LRT Aetheretmon nests with Pteronsculus (Figs. 5–7)) far from the base of all bony fish, much closer to lobefin fish and tetrapods.

The Sallan point is still made:
Many fish tails do have two parts, especially when hatchlings.

Unfortunately, Sallan does not understand
the topology of the family tree of fish due to taxon exclusion. This is something the LRT minimizes by testing a wider gamut of taxa. As readers know, we see this same taxon exclusion problem all the time in paleontology.

Figure 3. Muskie (Esox) tail ontogeny from Sallan 2016 (middle row). Top row (to scale) and photo (below) added here. You might remember, Esox is a derived catfish without barbels.

Salan writes,
“These two tails appear at a shared developmental stage in Aetheretmon, (Fig. 4) teleosts and all living actinopterygians. Ontogeny does not recapitulate phylogeny; instead, differential outgrowth determines final morphology.”

That appears to be so, but it still needs a valid tree topology.

Figure 4. Fish tail ontogeny in extinct Aetheretmon and extant Monotrete. Note the upper and lower lobes. In the LRT these two fish are not closely related. Aetheretmon is basal to lobefins. Monotrete is a puffer fish.

Salan speculates:
“The double tail likely reflects the ancestral state for bony fishes.”

No, the ancestral state for bony fish is the heterocercal tail documented by sturgeons and whale sharks, and this goes back to armored osteostracans according to the LRT (Fig. 5).

Figure x. Subset of the LRT, focusing on fish for July 2020.

Salan speculates,
“Many tetrapods and non-teleost actinopterygians have undergone body elongation through tail outgrowth extension, by mechanisms likely shared with distal limbs.”

Not sure what those ‘mechanisms’ would be, but basal tetrapods and stem tetrapods in the LRT have relatively short, straight tails and elongated bodies with great distances between the fore and hind limbs. Look at Panderichthys.

Figure 6. Aetheretmon is known from the oldest complete growth series for vertebrates.

Figure 7. Pteronisculus, a Triassic sister to Early Carboniferous Aetheretmon in the LRT and it is easy to see why.

Sallan is ‘Pulling a Larry Martin’
by putting too much emphasis on one trait without testing all the traits on many more taxa. Only after a valid phylogenetic context is established can one begin to figure out if trait A came before trait B or not.

Sallan goes into great detail describing
the successive stages of growth in Aetheretmon, but this is problematic because the cladogram is invalid. “First things first” is a motto all paleontologists should ascribe to. First get the phylogeny correct. Fish workers are relying on an invalid family tree. The LRT is here to fix that.

Its worth remembering,
many fish on the other branch of bony fish (perch, anglers, etc., Fig. 5, orange right column) bring the pelvic fins beneath the pectoral fins, shortening the gut cavity and elongating the tail to extremes in some cases (oarfish). This is all distinct from the longer torso, shorter tail trend in the stem tetrapod branch of bony fishes (Fig. 5, yellow left column).

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References
Sallan 2016. Fish ‘tails’ result from outgrowth and reduction of two separate ancestral
tails. Current Biology 26, R1205–R1225.
White EI 1927. The fish fauna of the Cementstones of Foulden, Berwickshire. Transactions of the Royal Society Edinburgh 55:255–287.

https://www.the-scientist.com/the-nutshell/a-tale-of-two-tails-32394

Fish cladogram: Cambrian period to the present day

When one layers established time periods
over the fish portion of the large reptile tree (LRT, 1673+ taxa; Fig. 1) the surprising length of certain ghost lineages and the ability of several clades to survive several hundred million years becomes apparent.

Figure 1. Subset of the LRT focusing on basal vertebrates (= fish). Colors indicate time periods. This chart documents the lack of fossils for several clades and genera in the Silurian and Devonian.

The antiquity of Silurian members in the highly derived lungfish clade
(Guiyu and Psarolepis) helps one understand the coeval Silurian appearances of so-called primitive fish, like acanthdians and placoderms (Entelognathus). Traditional cladograms assumed early taxa must be more primitive, not realizing that phylogenetic analysis indicates a vast undiscovered radiation of taxa in the Silurian (Fig. 1). Most of these are still waiting to be discovered.

What do Silurian and Early Devonian fossil fish in the LRT have in common?
Many were flat bottom dwellers with small eyes.

By contrast, coeval spiny sharks had large eyes and were free-swimmers. Even so they lost their flexible fin rays, they lost large teeth, they kept a large mouth, and they had vestigial skeletons. Such traits are associated today with slow-moving deep sea fish.

So known Silurian fish were not open sea visual predators with great swimming skills. Their ecological absence must have a reason. I wonder if such taxa were gobbled up before they could drift to muddy or silty anoxic regions of the sea floor where they could wait undisturbed to be buried for fossilization? Even a few exceptions are lacking. Very puzzling…

According to Google:
“In North America geologic activity over the last 417 million years has removed or covered up most Silurian rocks. Well-preserved fossils from Silurian reefs can be found in the Great Lake States of Minnesota, Wisconsin, Michigan, and Illinois.” So Silurian exposures are comparatively rare.

How do left column fish differ from right column (Fig. 1) fish?
As a general rule (allowing for many exceptions) left column fish do not appear to be the fast, open water swimmers seen in the majority of primitive right column fish in the Silurian and Devonian. It is noteworthy that not one taxon in the right column has a Silurian through Permian representative. I will add them as they come to my attention. It is also noteworthy that the left column has very few living representatives. I count nine.

Traditional cladograms
put more emphasis on time and exclude extant taxa. That’s why traditional cladograms often nest spiny sharks and placoderms near the base of the basal vertebrates, prior to sharks and bony fish. And they attempt to add tube-feeding sturgeons somewhere in the middle of bony fish. In the LRT taxon exclusion is minimized and more natural evolutionary patterns are recovered based on phenomics (traits).

Some previously unrecognized relationships recovered by the LRT include:

1. The wide radiation of clades in the Silurian.
2. Devonian taxa take us rapidly to tetrapods, documented by Middle Devonian tracks
3. Note the proximity of Silurian lobefins to Viséan (Early Carboniferous) tetrapods, including reptiles.
4. Note the unbalanced fossil record with regard to the major dichotomy splitting bony fish
5. Proamia is known from the Devonian while a sister taxon, Amia, is known from extant taxa, separated by 360 million years. This is the closest we get to a right column fish fossil in the Silurian or Devonian.
6. The time span between tiny Silurian Loganiella and giant extant sisters Rhincodon + Manta is about 430 million years.
7. A similar time span splits Hemicyclaspis from living sturgeons.
8. A longer time span (~500 my) splits Branchiostoma from its Cambrian precursors.
9. When comparing the LRT to traditional cladograms, check to make sure they have similar outgroup taxa. Too often taxon exclusion is an unaddressed issue in those papers, which make them fitting subjects for the next few blogposts.

Cautionary note:
The choosing of fish taxa for the LRT has not been random, but was made on the basis of availability and possible importance. At present the fossil record is skewed toward left column fish prior to the Permian. As more taxa are discovered and added, the subjective second reason will hopefully pale to become less of a factor.

 

Eusauropleura: now identified as a late-surviving basalmost reptile

The newest addition
to the large reptile tree (LRT, 1341 taxa) is Eusauropleura digitata (originally Sauropleura, Cope 1868; Romer 1930; Carroll 1970; Late Carboniferous, 310 mya; AMNH 6865; Figs. 1, 2) nests as a late-surviving basalmost reptile in the LRT.

The genesis for this genus
in the earliest Carboniferous is based on the more derived Silvanerpeton from the Viséan (335 mya). A dense layer of belly scales (not shown en masse), a larger manual digit 5, and a taller ilium, among other traits, distinguish this specimen from Gephyrostegus. A larger manus, ischium and giant caudal transverse processes (ribs) relative to the torso are unique traits among close relatives. Note the lack of ribs in the lumbar area, where large amniote eggs develop before they are laid. The eggs were relatively large based on the greater depth of the ischium.

Figure 1. Eusauropleura in situ and slightly reconstructed. Manus reconstruction with PILs enlarged.

Basal to Eusauropleura
are taxa close to the Reptilomorpha – Lepospondyli split. These include Eucritta and Utegenia (Fig. 2) all derived from the Late Devonian reptilomorph, Tulerpeton. This affirms the primitive state of basalmost reptiles, derived from Devonian tulerpetids. Further affirmation comes from the observation that the central vertebral elements of Eusauropleura “are very thin-walled, forming little more than a husk around the large notochord,” according to Carroll 1970.

Figure 2. Eusauropleura to scale with ancestral and descendant taxa including Eucritta, Utegenia, Silvanerpeton and Gephyrostegus, the last common ancestor of all reptiles. Note, none of these specimens preserves ossified carpals.

First considered a microsaur
(Cope 1868), then a gephyrostegid (Romer 1930, 1950; Carroll 1970), Eusauropleura was identified as more primitive than Gephyrostegus (Carroll 1970), but still terrestrial, not aquatic and close to the ancestry of reptiles, but not itself a reptile.

So what is a reptile?
As determined here in 2011, there is no list of traditional reptile skeletal traits that upholds the reptile status of Gephyrostegus. There is a new list. Irregardless of skeletal traits, only the nesting of Gephyrostegus as the last common ancestor of all reptiles in the LRT tells us it was laying eggs with an amnion, the ONLY trait needed to determine its reptile status. Silvanerpeton, from the earlier Viséan, was likely also a reptile because phylogenetic descendants of late-surviving Gephyrostegus are also found in coeval Viséan strata. Reptiles are that old. Given that the last common ancestor of Silvanerpeton and Gephyrostegus must also be a late-surviving member of that basalmost reptile radiation, whether the amnion was fully developed or not, something we may never know given the fragility of an amniotic membrane over 300 million years in stone. Earlier workers did not enter Eusauropleura, Silvanerpeton and Gephyrostegus into a wide gamut phylogenetic analysis and so did not recover a last common ancestor status for these amphibian-like reptiles.

Another specimen attributed to Eusauropleura
AMNH 6860 (Moodie 1909, Carroll 1970), is a bit more jumbled, more incomplete and more difficult to reconstruct. A complete ilium with an elongate posterior process is easy to see in this specimen. Such a process provides attachment points for more than one sacral rib, a traditional reptile trait, but this is difficult to determine in the scattered remains of the fossil. And is this really Eusauropleura?

Yet another specimen attributed to Eusauropleura
PU 16815 is an isolated pectoral girdle, bones lacking in the other specimens and therefore not readily comparable.

Scales
According to Carroll 1970, “Scales, both dorsal and ventral, are conspicuous in these specimens [Gephyrostegus and Eusauropleura]. The body was protected by heavy, oblong scales, overlapping to form a chevron pattern, between the pelvic and pectoral girdles. Were they not associated with the skeleton, they would be difficult to distinguish from those of [more primtive] embolomeres. Laterally the scales assume a more oval outline, become thinner, smaller and less extensively overlapping. The dorsal scales are small, thin and round. Where worn, all the scales exhibit a pattern of fine ridges, running parallel with the margins. These form a pattern of concentric ridges in the dorsal scales, similar to that of [more primitive] discosauriscids. Except for the heavier ossification of the dorsal scales, those of Eusauropleura are generally similar to those of Gephyrostegus.”

References
Carroll RL 1970. The ancestry of reptiles. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. 257 (814):267–308. DOI: 10.1098/rstb.1970.0026
Cope ED 1868. Synopsis of the Extinct Batrachia of North America. Proceedings of the Academy of Natural Sciences of Philadelphia 1868:208-221.
Romer AS 1930. The Pennsylvanian tetrapods of Linton, Ohio. Bulletin of the American Museum of Natural History. 59 (2):144–147.
Romer AS 1950. The nature and relationships of the Paleozoic microsaurs: American Journal of Science 248:628-654.

wiki/Eusauropleura

SVP 2018: new Whatcheeria data from nearly 100 specimens

Last one for this year.
This post finishes up an inundation of about 40 2018 SVP abstract reviews. We’ll get back to a regular one-a-day look at paleo news later today.

Otoo, Bolt and Lombard 2018
bring us new information on Whatcheeria (Fig. 1), a basal tetrapod (Fig. 2) now known from nearly 100 specimens. In the large reptile tree (subset Fig. 2, LRT) the Whatcheeria clade nests between the primitive Ossinodus clade and the Ichthyostega clade plus all higher tetrapods. The authors report, “Whatcheeria is key for establishing character polarities on the tetrapod stem, particularly in the context of recent controversies about age of the tetrapod crown group and the timing and pattern of the lissamphibian/amniote split.” In the LRT Tulerpeton and Eucritta have taken over that role.

Figure 1. Pederpes is a basal taxon in the Whatcheeria + Crassigyrinus clade.

After a short description
of key Whatcheeria traits, and without describing a phylogenetic analysis, Otoo, Lombard and Bolt conclude: “This combination of features in the femur emphasizes the moasic of characters present in Whatcheeria, and, in conjunction with recent Tournaisian discoveries, emphasizes the complexity of post-Devonian tetrapod evolution.”

A subset of the LRT
(Fig. 2) portrays post-Devonian tetrapod evolution rather differently and rather simply

Figure 2. 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.

References
Otoo BK, Bolt JR, Lombard E 2018. A leg up: Whatcheeria and its new contributions to tetrapod anatomy. SVP abstracts.

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. 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.

 

 

Cacops: Temnospondyl or Lepospondyl?

In order to understand
the interrelationships of reptiles, one needs to known where to begin and what came before the beginning. Earlier the large reptile tree (LRT) recovered the Viséan Silvanerpeton and the Late Carboniferous Gephyrostegus bohemicus at the base of the Amniota (= Reptilia) with origins in the early Viséan or earlier (340+mya).

Reptiles were derived from the clade Seymouriamorpha, 
close to Utegenia, which also nests at the base of the Lepospondyli, + Seymouria + Kotlassia. These, in turn, were derived from the reptilomorphs, Proterogyrinus and Eoherpeton.

Reptilomorphs, in turn, were derived from Temnospondyls,
at present, Eryops (unfortunately too few taxa to be more specific at present), and temnospondyls, in turn, were derived from basal tetrapods, like Pederpes.

Figure 1. Cacops and its sisters in the LRT.

A recent objection
by Dr. David Marjanovic suggested that the basal tetrapod, Cacops, was not a lepospondyl, but actually a temnospondyl.

Figure 1. Sclerocephalus in situ and reconstructed. To no surprise, this taxon nests with Eryops among the temnospondyls. Note the expanded ribs.

That’s worth checking out.
So I added taxa: Sclerocephalus and Broiliellus (Fig. 2). The former nested with Eryops as a temnospondyl. The latter nested with Cacops and the lepospondyls. The new taxa did not change the topology. So… either the present topology is correct, or I’ll need some taxon suggestions to make the shift happen.

Figure 1. Broiliellus skull. This taxon nests with Cacops among the lepospondyls, derived from a sister to the Seymouriamorph, Utegenia. Note the ‘new’ bone between the lacrimal and jugal. That’s a surface appearance of the palatine!

 

The large reptile tree tells us
that reptiles and lepospondyls are all seymouriamorphs with Utegenia at the last base of the lepospondyls, but known only form late-surviving taxa at present. Lepospondyls continue to include Cacops and Broiliellus, along with extant amphibians and microsaurs, which mimic basal reptiles. Most of these taxa should be found someday in Romer’s Gap prior to the Viséan in the earliest Carboniferous or late Devonian.

Seymouriamorpha
Wikipedia reports, “[Seymouriamorpha] have long been considered reptiliomorphs, and most paleontologists may still accept this point of view, but some analyses suggest that seymouriamorphs are stem-tetrapods (not more closely related to Amniota than to Lissamphibia) aquatic larvae bearing external gills and grooves from the lateral line system have been found, making them unquestionably amphibians. The adults were terrestrial.

The LRT finds
seymouriamorphs basal to reptiles + lepospondyls. The latter includes lissamphibians (all extant amphibians , their last common ancestor and all of its descendants) and several other clades, including Microsauria, Nectridea, and several very elongate taxa.

Dissorophididae
Wikipedia reports, “It has been suggested that the Dissorophidae may be close to the ancestry of modern amphibians (Lissamphibia), as it is closely related to another family called Amphibamidae that is often considered ancestral to this group, although it could also be on the tetrapod stem. The large reptile tree also recovers this relationship. Cacops and Broiliellus are both considered dissorophids.

References
Lewis GE and Vaughn PP 1965. Early Permian vertebrates from the Cutler Formation of the Placerville area, Colorado, with a section on Footprints from the Cutler Formation by Donald Baird: U.S. Geol. Survey Prof. Paper 503-C, p. 1-50.
Moodie RL 1909. A contribution to a monograph of the extinct Amphibia of North America. New forms from the Carboniferous. Journal of Geology 17:38–82.
Reisz RR, Schoch RR and Anderson JS 2009. The armoured dissorophid Cacops from the Early Permian of Oklahoma and the exploitation of the terrestrial realm by amphibians. Naturwissenschaften (2009) 96:789–796. DOI 10.1007/s00114-009-0533-x
Williston SW 1910. Cacops, Desmospondylus: new genera of Permian vertebrates. Bull. Geol. Soc. Amer. XXI 249-284, pls. vi-xvii.
Williston SW 1911. Broiliellus, a new genus of amphibians from the Permian of Texas. The Journal of Geology 22(1):49-56.

wiki/Cacops
wiki/Platyhystrix
www/Broiliellus
wiki/Dissorophidae

Ianthodon: a basal edaphosaur without tall neural spines

A new paper
by Spindler, Scott and Reisz (2015) brings us new data on the basal pelycosaur Ianthodon schultzei (Fig. 1; Garnet locality, Missourian Age; 305-306 mya, Middle Pennsylvanian, Late Carboniferous). The authors reported that Ianthodon represented a more basal sphenacodontid than Haptodus. In the large reptile tree Ianthodon was derived from a sister to Haptodus and nested at the base of Edaphosaurus + Ianthasaurus + Glaucosaurus, all edaphosaurids.

Figure 1. Ianthodon schultzei (image modified from Spindler, Scott and Reisz 2015) was considered a basal pelycosaur, and it is, but here nests as a basal edaphosaur. And it has no tall neural spines. So pelycosaur sails were convergent, not homologous. Spindler, Scott and Reisz considered this specimen a juvenile due to its incomplete ossification.

Notably Ianthodon does not have tall neural spines. Earlier we wondered whether the tall neural spines of Edaphosaurus and Dimetrodon were convergent or homologous. Now it is clear, via Ianthodon, and Sphenacodon (sorry I did not notice this yesterday) that the tall neural spines of Edaphosaurus and Dimetrodon were convergent.

Most well-known pelycosaurs
were Early Permian in age. Ianthodon demonstrates an earlier origin for their carnivore/ herbivore split. And it retains carnivore teeth! Therapsids likewise originated in the Late Carboniferous according to this new data.

Phylogenetic history
Spindler, Scott and Reisz (2015) report, “In the original description and phylogenetic analysis of Kissel and Reisz (2004), Ianthodon was found to nest surprisingly high within Sphenacodontia, as a sister taxon to the clade that included Pantelosaurus, Cutleria and sphenacodontids. In a subsequent, large-scale analysis, Ianthodon was found to be more basal, near the edaphosaurid–sphenacodont node (Benson, 2012), but its exact position remained poorly resolved. In the latter analysis, Benson (2012) extensively revised the character list and included all known “pelycosaur” grade synapsids, while Kissel and Reisz (2004) used data and taxa derived from Laurin (1993), which mainly followed Reisz et al. (1992). Another recent analysis of sphenacodont synapsids by Fröbisch et al. (2011), as part of a description of a new taxon, recovered Ianthodon, Palaeohatteria and Pantelosaurus in an unresolved polygamy.”

The Spindler, Scott and Reisz (2015) analysis
used 122 characters (vs. 228 in the large reptile tree). Their tree shows 12 taxa, 4 of which are suprageneric. In their tree Ianthodon nested between Edaphosauridae and Haptodus. (So close, but no cigar.) Their tree also nested two therapsid taxa (Biarmosuchus and Dinocephalia) with Cutleria, Sphenacodon, Ctenospondylus and Dimetrodon. Thus Spindler, Scott and Reisz appear to be excluding several key taxa and their tree topology differs significantly from the large reptile tree at the base of the Therapsida, with or without Ianthodon.

References
Spindler F, Scott D. and Reisz RR 2015. New information on the cranial and postcranial anatomy of the early synapsid Ianthodon schultzei (Sphenacomorpha: Sphenacodontia), and its evolutionary significance. Fossil Record 18:17–30.

Purbicella, a basal scleroglossan from the Purbeck Limestone

Figure 1. Purbicella in situ (palatal view) and traced using DGS, then reconstructed using those tracings. Gray areas are unknown. If you think this looks like a generalized, plesiomorphic scleroglossan, you’re right! Here colorizing the bones helps identify sutures and paired elements. That’s a right pterygoid covering much of the paired frontals. The teeth are blunt.

A few years ago
a rather complete lizard skull (BGS GSb581) was described (Evans et al. 2012) from the Purbeck Limestone (Late Jurassic to Early Cretaceous) of England. It was originally excavated more than a century ago and assigned to the genus, Paramacellodus. Evans et al. renamed it Purbicella. Their cladistic analysis nested Purbicella with Lacertoidea: (Lacertidae (including Acanthodactylus), Teiidae (including Tupinambus), Gymnophthalmidae (including Gymnophthalmus), and the burrowing Amphisbaenia (including Amphisbaena)), not Paramacellodus, which nested with skinks. Evans et al. based their nesting on a partial data matrix of Conrad (2008).

The large reptile tree nested Purbicella between Acanthodactylus and Liushusaurus. The large reptile tree recovered the above listed ‘lacertoid’ taxa as members of a paraphyletic clade, some preceding Purbicella in various clades and others succeeding it.

While Purbicella is Late Jurassic/Early Cretaceous, it must have had its origins much earlier, in the Late Carboniferous, because a descendant taxon, Ascendonanus, is Early Permian.

References
Conrad JL 2008. Phylogeny and systematics of Squamata (Reptilia) based on  morphology. Bulletin of the American Museum of Natural History 310:1–182.
Evans SE, Jones MEH and Matsumto R 2012. A new lizard skull from the Purbeck Limestone Group (Lower Cretaceous) of England. Bull. Soc. géol. France, 2012, t. 183(6):517-524.