The origin of fingers in the LRT

This is the first time the four tiny scattered fingers
of Trypanognathus were collected and added to a reconstruction (Fig 1). They were too hard to find earlier. This taxon is the most primitive tetrapod in the large reptile tree (LRT, 2168 taxa) despite its late appearance (Latest Carboniferous, 300 mya) in the fossil record. An earlier origin of fingers is documented in the Middle Devonian (390 mya) based on ichnite (footprint) data.

Figure 1. Origin of fingers according to taxa in the LRT. Trypanognathus is featured as the first tetrapod with fingers.

The Bauplan of the Trypanognathus manus
falls neatly into a phylogenetic slot provided by precursor and descendant taxa (Fig 1). Trypanognathus has been ignored and omitted by tetrapod workers perhaps due to its late appearance in the fossil record. Workers have favored the eight-finger hypothesis based on discoveries and the fascinating, but invalid stories told by including unrelated taxa and omitting pertinent taxa. Simply adding taxa will let the cladogram tell you which is which.

That omission problem
also happens in origin hypotheses for turtles, pterosaurs, whales, Vancleavea, dinosaurs, birds (e.g. omitting all but one specimen attributed to Archaeopteryx), reptiles, diapsids, mesosaurs, sharks, bony fish, sturgeons, etc. etc. as longtime readers of this blogpost have seen.

Figure 2. Trypanognathus in situ. Colors added here. Manus reconstructed from matching colors.

Trypanognathus remigiusbergensis
(Schoch and Voigt 2019; latest Carboniferous) This is a late-surviving specimen of the first taxon in the LRT with fingers, toes and dorsal nares. It nests in a more basal position than originally thought, between Panderichthys and Laidleria. Derived states are the penetration of vomerine tusks through the splenial and symphyseal tusks through the premaxilla. The body is elongate with well-ossified, but small limbs, the presacral count is ca. 28, The pleurocentra are large and reached ventrally almost as far as the intercentrum.

First appearing several years ago, this still appears to be a novel hypothesis
of interrelationships not taught in vertebrate paleontology textbooks. If not, please provide a citation so I can promote it here.

References
Schoch RR and Voigt S 2019. A dvinosaurian temnospondyl from the Carboniferous-Permian boundary of Germany sheds light on dvinosaurian phylogeny and distribution. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2019.1577874.

wiki/Laidleria
wiki/Trypanognathus – not posted yet except the Netherlands version

reptileevolution.com/trypanognathus.htm
reptileevolution.com/trypanognathus2.htm

Revising Rainerichthys

After tracing and reconstructing the insitu fossil
of Rainerichthys using DGS methods (Fig1), a less bizarre, though still bizarre, reconstruction of this iniopterygian was recovered.

Figure 1. Rainerichthys in situ and reconstructed. Colors added here. Original reconstruction shown for comparison. Yes, the pectoral girdle raises the pectoral fin high over the skull, as shown here and in figures 2 and 3, likely in order to hitch a ride on a larger faster swimming fish in the manner of remoras.

The original reconstruction
(Fig 1) introduced novel skull-wrist connections that were untenable.

This is a typical example
of the sort of housekeeping that needs to be done on any cladogram you build. Hope all readers are building their own cladogram or plan to do so soon.

Figure 2. Gregorius, Iniopteryx and several tall skull iniopterygians including Rainerichthys.

Rainerichthys zangerli
(Grogan and Lund 2009; Mississipian, 320mya; 20 cm long) is newly reconstructed here. Distinct from related taxa, Rainerichthys has a dual forehead, a narrow oval cross-secition, a post orbital forming the lower half of the circumorbital ring and it lacks a premaxilla. Two clades of iniopterygians are recovered by the large reptile tree (LRT, 2168 taxa). One clade (Fig 2) has a taller, narrower skull. The other clade (Fig 3) has a lower, wider skull. All are derived from a late survivor of an Early Silurian radiation in the Early Carboniferous, Gregorius, (Fig 2, 3) in the LRT.

Figure 3. The low, wide skull clade of iniopterygids like Cervifurca and Helodus. Note the male claspers in Cervifurca, absent in the female Helodus. The Helodus pectoral fin may have been anchored higher on the torso than shown here.

Sometimes
published illustrations and reconstructions are unreliable. Often not. Testing is always appropriate. Applying colors to images greatly simplifies identification, reconstruction and comparison.

References
Grogan ED and Lund R 2009. Two new iniopterygians (Chondrichthyes) from the
Mississippian (Serpukhovian) Bear Gulch Limestone of Montana with evidence of a new form of chondrichthyan neurocranium. Acta Zoologica (Stockholm) 90 (Suppl. 1): 134–151.
Lund R and Grogan E 2004. Five new euchondrocephalan Chondrichthyes from the Bear Gulch Limestone (Serpukhovian, Namurian E2b) of Montana, USA. Recent Advances in the Origin and Early Radiation of Vertebrates 505-531.
Zangerl R 1997. Cervifurca nasuta n. gen. et sp. : an interesting member of the Iniopterygidae (Subterbranchialia, Chondrichthyes) from the Pennsylvanian of Indiana, USA. Fieldiana, Geoloy new ser. no 35. Pub 1483. 24pp. PDF
Zangerl R and Case GR 1973. Iniopterygia : a new order of Chondrichthyan fishes from the Pennsylvanian of North America. University of Illinois Urbana-Champaign. Chicago : Field Museum of Natural History.

wiki/Gregorius_rexi
wiki/Iniopteryx
wiki/Cervifurca – not yet posted
wiki/Rainerichthys – not yet posted
wiki/Papilionichthys – not yet posted

Ancestors of the grizzly bear, Ursus arctos

The following blogpost was prompted by
the rescoring of Paleocene Protictis (Fig 1), now a basal member of Carnivora, a clade that still lacks a single basalmost member. Instead are the mouse-like marsupial without a pouch, Monodelphis, (Fig 1) and Alcidedorbignya one of the last common ancestors of placentals. Unfortunately, it’s still pretty far off the main track transitional to herbivorous Pantolambda.

At its current genesis
Carnivora splits into the civets on one branch and the rest of the Carnivora. Protictis nests at its base, followed by Nandinia (the palm civet), Nasua (the coatimundi) and Ursus arctos (the grizzly bear. Fig 1), still nesting apart from the many other bears in the large reptile tree (LRT, 2168 taxa).

Figure 1. Basal members of the clade Carnivora, including Nandinia, Protictis, Nasua and Ursus arctos. Monodelphis is a marsupial without a pouch. Alcidedorbignya is likely a marsupial without a pouch, too. Without a better last common ancestor that is a placental and a member of the Carnivora, Alcidedorbignya and Monodelphis will have to do.

On the civet branch of basal Carnivora,
Eupleres and Fossa (Fig 1, both extant Madagascar taxa) nest to the side of Protictis from Middle Paleocene North America. That geographical split also suggests a deep Mesozoic (perhaps Jurassic, pre-Madagascar split from Africa 150mya) origin for these taxa.

Revisiting Gregorius rexi, another soft placoderm basal to sharks and rays

More housekeeping
in the large reptile tree (LRT, 2168 taxa) and more experience with more fish to compare are bringing new insights to transitional taxa like Gregorius rexi (Fig 1). This is only one of many odd fish found in Early Carboniferous sediments at the Bear Gulch Formation of Montana and no where else on the planet.

Figure 1. Gregorius rexi in situ. DGS colors added here and used to create the reconstruction at upper right. Purple is ventral plastron inherited from placoderm ancestors. Note the extensive gill branchials. This taxon is basal to sharks without a gill cover (operculum). Blue thick outlines represent lateral fins. Yellow green cones represent claspers that elongate in Iniopterygians in figure 1b. This image updates prior attempts at reconstruction.

Figure 1b. Iniopteryx and Cervifucan, two iniopterygids. Note the large high pectoral fins and elongate claspers found in male specimens. This clade finds its origin in Gregorius (Fig 1). This figure added October 28, 2022.

Gregorius rexi
(Lund and Grogan 2004; Bear Gulch Fm. Serpukhovian, Latest Mississipian) is traditionally considered a type of small,two-spined ratfish. In the LRT Gregorius nests basal to basal toothless sharks like Chondrosteus. So it is a late survivor of an Early Silurian radiation of soft placoderms like Loganellia. Gregorius retains a ventral plastron, like the placoderm Entelognathus and displays the dorsal spines seen later in Palaeobates and Hybodus. along with sister spiny sharks, like Climatius. The pectoral fin was large and had a central series of bones anchoring radials. The tail was heterocercal with subequal dorsal and ventral portions.

Figure 2. Gregorius and related taxa to scale and full scale on a 72-dpi monitor. Note the ventral plate on Loganellia. The bones in Coccosteus are retained and modified in the human body and all tetrapods. It’s time to start relabeling traditional fish bones with tetrapod labels.

This appears to be a novel hypothesis of interrelationships.
If not, please provide a citation so I can promote it here.

References
Lund R and Grogan E 2004. Five new euchondrocephalan Chondrichthyes from the Bear Gulch Limestone (Serpukhovian, Namurian E2b) of Montana, USA. Recent Advances in the Origin and Early Radiation of Vertebrates 505-531.

wiki/Gregorius_rexi
wiki/Iniopteryx
wiki/Iniopterygiformes

Tiny Bellairsia gracilis enters the LRT as a Middle Jurassic survivor of a Late Permian radiation of tiny protosquamates

Tatanda et al 2022 reported,
“a deficiency of fossil evidence, combined with serious conflicts between molecular and morphological accounts of squamate phylogeny, has caused uncertainty about the origins and evolutionary assembly of squamate anatomy.”

Not true. The large reptile tree (LRT, 2168 taxa) documents squamate origins back to Ediacaran worms. It’s common for paleontologists to say how poorly their science has performed in order to boost the value of their own discoveries and insights.

“Here we report the near-complete skeleton of a stem squamate, Bellairsia gracilis, from the Middle Jurassic epoch of Scotland, documented using high-resolution synchrotron phase-contrast tomography.”

In the LRT protosquamates (= stem squamates) appear in the Late Permian. Bellairsia is a late survivor in Middle Jurassic Scotland.

“Phylogenetic analyses return strong support for Bellairsia as a stem squamate, suggesting that several features that it shares with extant gekkotans are plesiomorphies, consistent with the molecular phylogenetic hypothesis that gekkotans are early-diverging squamates.”

Please avoid molecular studies. Too often they deliver false positives and they do not include fossil taxa. Stick with traits and avoid taxon exclusion, a problem once again in the Tatanda et al paper. These authors borrowed three prior cladograms. They should have built their own, rather than trust the work of others.

“We also provide confident support of stem squamate affinities for the enigmatic Oculudentavis.”

The day after Oculudentavis was published it was nested with the fenestrasaur lepidosaur, Cosesaurus in the LRT.

The day after the specimen mistakenly referred to Oculudentavis (Fig 1) was published it was nested with basal protosquamates like Tijubina. The two specimens assigned to Oculudentavis do not nest together in the LRT. Don’t be lazy. Test them both.

Phylogenetic results reported by Tatanda et al
“Phylogenetic analyses of a modified version of the matrix of refs.  14,23,24 recovers strong support (posterior probability = 1.0) for B. gracilis as a stem squamate, as part of a sister clade to the squamate crown group that also includes the mid-Cretaceous taxa Huehuecuetzpalli mixtecus (Albian, Mexico) and Oculudentavis naga (Cenomanian, Myanmar).”

In the LRT Huehuecuetzpalli and the holotype of Oculudentavis are not squamates, but relatives of Macrocnemus, a mid-sized tanystropheid lepidosaur, and a sister to a tiny tanystropheid lepidosaur, Cosesaurus respectively. None of these are related to Bellairsia.

Figure 1. Bellairsia nests with the GRS 28627 specimen mistakenly referred to Oculudentavis. Diagram and Bellairsia images from Tatanda et al. Colors added here.

In the LRT
Bellairsia (Fig 1) nests with the referred specimen wrongly assigned to Oculudentavis, GRS 28627 (Fig 1), close to the basalmost protosquamates, Tijubina, Saurosternon and Palaegama and far from the holotypes of Huehuecuetzpalli and Oculudentavis and far from the origin of the Squamata.

This tiny specimen (BMNH R12678) was originally described
by Evans 1998 based on an exposed dentary. The rest of the buried specimen was revealed by µCT scanning by Tatanda et al 2022.

References
Evans SE 1998. Crown group lizards (Reptilia, Squamata) from the Middle Jurassic of the British Isles. Palaeontographica Abteilung A 250:123-154.
Tatanda M, Fernandez V, Panciroli E, Evans SE and Benson RJ 2022. Synchrotron tomography of a stem lizard elucidates early squamate anatomy. Nature Oct 26 2022

Publicity
newscientist.com/what-lizards-ancestors-looked-like-167-million-years-agophys.org/news/2022-10-scottish-fossil-lizards.
newsweek.com/evolution-secrets-240-million-year-snake-ancestor

Moray eels move in with snakeheads

More housekeeping
in the large reptile tree (LRT, 1267 taxa) moves Gymnothorax, the moray eel (Fig 1) and kin, close to another cave-dwelling pair of fish with sharp teeth, the snakeheads, Channa and Aenigmachanna. This resolves a long-standing error in the LRT. Corrections are part of the scientific process as the LRT has been building, taxon by taxon over the last decade without a mentor.

Figure 1. This is a clade of increasingly eel-like taxa, led by Channa, the snakehead, that are most famous for their odd and rapacious mouths. Members go back at least to the Carboniferous.

A better understanding of the skull of Gymnothorax
(Fig 2) shed light on this solution. The moray eel maxilla was gone, as in Aenigmachanna (Fig 4). Distinct from all other tested taxa, the premaxilla was anteriorly fused to the vomers and posteriorly separated, becoming a distinct bone traditionally labeled the maxilla. That only made sense – except when confronted with all the other trait data. That’s the label I had to relabel.

Figure 2. The skull of the moray eel, Gymnothorax, appears to have a premaxilla and a separate maxilla. Turns out the anterior portion of the premaxilla fused to the vomers and the posterior portion separated. All sister taxa have an equally long premaxilla and reduce or eliminate the maxilla.

According to Wikipedia
moray eels are a kind of eel related to the European eel, Anguilla. According to the LRT moray eels are eel-like by convergence, not by a close interrelationship.

Figure 3. Skull of Channa, the snakehead from Digimorph.org and used with permission.

Earlier the LRT nested moray eels
in a separate clade basal to all other bony fish. That error has now been repaired, making it only the latest repair in a very long list of repairs. Unfortunately the bones don’t come with labels or colors. That has to be done here.

Figure 4. The dragon snakehead, Aenigmachanna. Note the vestigial maxilla (bright green) and the elongate premaxilla (yellow). This is the transitional taxon (figure 1) between Channa (figure 3) and Gymnothorax (figure 2).

Another clade member making the phylogenetic jump
is Early Carboniferous, Harpacagnathus, famous for its moray eel-like and Ridley Scott’s ALIEN-like second set of jaws (Fig 5), traditionally identified on Harpacanthus an odd set of rostral ornaments.

Figure 5. Harpacanthus, an Early Carbonifermous relative of the extant moray eel shown in situ and reconstructed.

Gymnothorax funebris
(originally Lycodontis funebris Ranzani 1839) is the extant green moray eel, which has no limbs or fins and traditionally nests within the Teleostei. Here this ‘eel’ is derived from Channa, another cave dweller. Pharygneal jaws (former gill bars) race anteriorly to double capture prey and drag it back to the digestive system. This species lays 10,000 eggs, which are fertilized externally. Gymnothorax afer (Bloch 1795, type genus; 2m) is the extant dark moray eel.

References
Bloch ME 1795. Naturgeschichte der ausländischen Fische. Berlin. v. 9. i-ii + 1-192, Pls. 397-429.
Leidy J 1856. Indications of five species, with two new genera, of extinct fishes. Proc Acad Nat Sci Philadelphia 7:414.
Traquair RH 1886. On Harpacanthus, a new genus of Carboniferous Selachian Spines. Journal of Natural History. Series 5. 18(108): 493–496.
Valliant LL 1882. Sur un poisson des grandes profondeurs de l’Atlantique, l’Eurypharynx pelecanoides. Comptes Rendus Hebdomadaires des Séances de l’Académie des Sciences, Série D, Sciences Naturelles 95: 1226-1228.

wiki/Eurypharynx
wiki/moray eel
wiki/Harpacanthus

Promissum, a ‘giant’ conodont, enters the LRT with primitive soft skull bones

Between Metaspriggina
and lampreys (Pteromyzon) is where conodonts nests in the large reptile tree (LRT, 2165 taxa). Promissum pulchrum (Kovacs-Endrody 1986, Late Ordovician, est 40cm in length, Figs 1, 2) is the ‘giant’ conodont. It is one of the very few specimens preserving a body outline. Most other conodonts are known only from scattered complex, sometimes microscopic, feeding apparatus structures. These are both common enough and distinct enough to identify strata and formations. What these structures belonged to was a major paleo question until recently.

Figure 1. Promissum, a conodont, compared to Lasanius, a birkeniid, to scale and to length.

According to Gabbot, et al 1995, who described Promissum, the ‘giant’ conodont,
“Conodonts are extinct Cambrian/Triassic vertebrates, soft bodied apart form the phosphatic elements of their feeding apparatuses.” Their cladogram nested conodonts between hagfish + lampreys and anaspids + ostracoderms. That is slightly different from the LRT, which employs generic taxa, not suprageneric taxa. Gabbot et al were unable to see or score skull traits (Fig 2).

Figure 2. The skull of Promissum as originally traced by Gabbot et al 1995 DGS colors added here. Note the origin of vertebrate skull bones at this grade. Blue dots are homologous with gill openings.

We looked at the feeding apparatus
of Promissum (Fig 3) earlier here without entering this taxon into the LRT. Comparative anatomy became possible only by including more basal vertebrates and by finally applying DGS methods to the Promissum data (Figs 1, 2).

Figure 3. Conodont feeding mechanism in dorsal and anterior views, retracted and extended.

Very few vertebrates are known from the Ordovician.
Even so, conodonts like Promissum are not outliers. After DGS tracing and analysis in the LRT Promissum looks like precursors and descendants. It fits right into a slot between better known Metaspriggina (Fig 1) and lampreys. Given this tight fit, the morphological gamut of basal vertebrates in the LRT is not expanding, suggesting that most, if not all basal vertebrate Bauplans are already known. All are variations of each other.

Figure 4. Birkenia skull for comparison to Jamoytius. Note the oralcavity has several mouth parts here identified with tetrapod homologies, but this was done at a time before many bssal vertebrates were added to the LRT. Perhaps these are closer in homology to conodont feeding elements. In any case, the skull elements here find closer homologs in more primitive soft conodont tissues.

Birkenia skull elements
(Fig 4) provide homologs for similar conodont skull elements. Generally that includes the deep short rostrum, dorsal orbits and long line of gill openings and specifically the individual skull bones. Smaller, simpler homologs of the complex conodont feeding apparatus elements (Fig 3) are also be present in Birkenia, shown here as slender struts mislabeled Vo, Pa, Ect and Pt, plus the stiff circum-oral elements mislabeled cilia.

Figure 5. The oral cavity of three lampreys.

Lamprey circum-oral rasping ‘teeth’
(Fig 5) likewise provide homologs to vertebrates at this grade of morphology, but not to gnathostome teeth.

Figure 6. Medial section of Acipenser (sturgeon) larva with temporary teeth from Sewertzoff 1928.

Conodonts and the LRT answer the enigma of baby sturgeon teeth
(Fig 6) which represent horny (not dentine nor enamel) vestiges of this ancestral condition in conodonts and likely sturgeon ancestors like Early Cambrian Haikouichthys (Fig 7).

Figure 7. Early Cambrian Haikouichthys nests basal to sturgeons, like baby Acipenser, in the LRT.

References
Gabbot SE, Aldridge RJ and Theron JN 1995. A giant conondont with preserved muscle tissue from the Upper Ordovician of South Africa. Nature 374:800–803.

wiki/Promissum

The long-legged maned wolf, Chrysocyon, enters the LRT

The extant maned wolf
Chrysocyon brachyurus (Illiger 1815 originally Canis brachyurus, Smith 1839; 90 cm at the withers, Figs 1, 2) has elongate limbs adapted to the tall grasslands of South America. Derived from Canis (the wolf, Fig 2), Chrysocyon is related to a smaller extant canid from South America, Speothos, the bush dog (Fig 2).

Figure 1. The maned wolf, Chrysocyon, in vivo.

Canids arriving from North America
adapted to new environs in South America. Chyrsocyon, evolved taller limbs. Speothos developed shorter limbs.

Figure 2. Chrysocyon skeleton compared to Canis (wolf) and Speothos (bear dog).

A South American bear,
Tremarctos ornatus and an Indian bear, Melursus ursinus, nest closer to these canids than to North American bears in the LRT. Their present geographic separation is a topic worthy of further study.

Figure 2. The South American bush dog, Speothos, nests with the South American spectacled bear, Tremactos, in the LRT.

As documented earlier
polar and black bears (genus: Ursus) originated with South American kinkajous (genus: Potos) while grizzly bears (genus: Ursus) originated with coatimundis (genus: Nasua), all apart from Tremarctos and Melursus.

Figure 3. Tremarctos ornatus, the spectacled bear of South America (the Andes), nests with the bush dog (Speothora) also from South American (Figs 1, 2) and the sloth bear from India.
How did they got separated?

References
Illiger JKW 1815. Ueberblick der Säugthiere nach Ihrer Vertheilung Über die Welttheile – Abhandl. König. Akad. Wiss. Berlin 1804-18811: p39-159.
Smith CEH 1839. Jardine’s Naturalist Library 9:241–247.

wiki/Maned_wolf
wiki/Bush_dog – Speothos

The origin of gnathostomes in the LRT — updated

Recent papers have shed light on the origin(s) of jaws
in vertebrates transitioning to the clade Gnathostomata. Freshman mistakes were made earlier in the large reptile tree (LRT, 2164 taxa) which recovered a dual origins of jaws: one in tiny placoderms, like Shenacanthus (Fig 5), and another between sturgeons and Chondrosteus (Fig 6). After the addition of new taxa and recent housekeeping in the LRT, that hypothesis is no longer valid. Corrections are part of the process as new taxa shed new new light.

Now sturgeons nest much more basally in the LRT, far from the origin of jaws. Chondrosteus nests much later, following placoderms.

Here are the current steps in broad brush strokes
for the origin of jaws in the LRT (Figs 1–6) starting with a flat, jawless placoderm, Qilinyu, from the Late Silurian. That’s far after the origin of jaws prior to the Early Silurian. So this was a late surviving taxon from an earlier radiation.

Figure 1. Jawless placoderm Qilinyu in palatal view. Animated Meckel’s cartilage here.

Late Devonian Bothriolepis
was also jawless (Fig 2), but had a mobile upper lip created by the (pink) nasals and bordered by a (yellow) toothed premaxilla. Perhaps these were used for scraping benthic sediments or corals.

Figure 2. Bothriolepis palate view animated.

Fragile mobile mandibles
appear with tiny Latest Silurian Bianchengichthys (Fig 3). This was another late survivor of an earlier Early Silurian genesis of more substantial jaws (Figs 4, 5).

Figure 3. Tiny Bianchenichthys documents the first appearance of jaws in the gnathostome lineage of placoderms, distinct from the convergent ptyctodont origin of jaws in figure 3.

It is either ironic or unfortunate
that the phylogenetic acquisition of jaws does not chronologically follow fossil examples from rare sediments. Bearing that in mind, toothless Coccostesus (Fig 4) from the Middle Devonian represents the next stage in mandible evolution with sharp, strong biting elements.

Figure 4. Austroptyctodus animated. Here the bones are labeled according to Coccosteus (above), the outgroup taxon of the Ptyctodontidae. Note the fusion of nasal elements (pink) with palatine elements (indigo). In front of the lateral view is a central nasal bone in anterior view flanked by two hypothetical nares as in Coccosteus at the top of the image.

Coccosteus ancestors gave rise to famous giant predatory placoderms
along with smaller, less famous ptyctodontids, like Austroptyctodus (Fig 4) This clade is characterized by a narrower skull with fewer facial and temporal bones.

Figure 5. Tiny Early Silurian Shenacanthus documents the earliest appearance of gnathostome jaws.

Stepping back to the Early Silurian
is wasp-sized Shenacanthus. It had a dentary, but lacked a premaxilla and maxilla. The upper tooth plate (palatine) erupted from the arching lacrimal, medial to the postorbital (amber), as in Coccosteus, the ANU V244 specimen and probably Entelognathus.

Figure 6. The Early Silurian ‘soft placoderm’, Loganellia is a transitional taxon from placoderms to sharks.

By the Early Silurian gnathostomes were radiating.
One of these was Loganellia (Fig 6), best described as ‘soft placoderm’ transitional to basal sharks, like Chondrosteus (Fig 7) and acanthodians like Climatius.

Figure 7. Chondrosteus revised and animated. This toothless taxon is derived from soft placoderms like Loganellia (Fig 6). Apparently all teeth were absent in this Early Jurassic late survivor of an Early Silurian radiation.

Housekeeping continues
on the LRT. Traditional problems are finding new solutions by trait analysis.

References
Li Q et al. 2021. A new Silurian fish close to the common ancestor of modern gnathostomes. Current Biology 31:3613–3620. doi.org/10.1016/j.cub.2021.05.053
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.
Zhu M et al. 2016. A Silurian maxillate placoderm illuminates jaw evolution. Science 354.6310 (2016): 334-336.

wiki/Bianchengichthys
wiki/Gnathostomata

Tiny pre-snakes of the Early Cretaceous – to scale

Once again,
a picture (Fig 1) pretty much tells this story.

Here
(Fig 1) are the tiny snake ancestors tested presently in the large reptile tree (LRT, 2164 taxa, subset Fig 2), shown together at full scale on a 72 dpi monitor. These taxa are derived from basal geckos.

Not shown
are the larger aquatic taxa (e.g. Adriosaurus) that transition between Eichstaetisaurus gouldi and yet another round of phylogenetic miniaturization represented by the tiny proximal snake ancestors, Tetrapdophis and Barolcherosaurus (Fig 1).

Figure 1. Retinosaurus, Jucraseps, Barlochersaurus are just a few of the several tiny taxa nesting in the pre-snake liineage of the LRT. Based on its skull sutures, Retinosaurus was a likely juvenile trapped in amber. These are all shown full size @72dpi monitors.

Evidently snakes learned how to be snakes
while tiny during this Early Cretaceous radiation. These taxa have been traditionally omitted from most snake origin studies.

Figure 2. Subset of the LRT focusing on snakes and their ancestors. Hoyalacerta separates the two Eichstaettisaurus species. Blue taxa are aquatic. Juvenile Retinosaurus nests with one of them.

Taxon exclusion was minimized in the LRT hypothesis of interrelationships.
All other candidate taxa from prior studies were tested and nest elsewhere. Rescoring moves the juvenile amber taxon Retinosaurus (Fig 3) with slightly earlier Eichstaettisaurus schroederi (Fig 1).

Figure 3. The juvenile amber taxon, Retinosaurus. Tetrapod colors added here. Note the loose and open skull sutures marking this as a juvenile.

Ardeosaurus brevipes
(Meyer, 1860) Late Jurassic, ~150 mya, ~20 cm in length, was originally and subsequently (Mateer 1982) considered a gekkotan. Here Ardeosaurus and its sister, Eichstaettisaurus, were derived from a sister to Tchingisaurus and it nested at the base of all snakes, including Adriosaurus and Pachyrhachis. This is one of the earliest known stem snakes that is not also a gecko.

Retinosaurus hkamtiensis
(Cernansky et al. 2021; Early Cretaceous; GRS 29689) is a tiny lizard found in Burmese amber and considered a juvenile. Here it nests with Eichstaettisaurus, a snake ancestor. Tiny taxa, like this, typically nest at the base of new clades.

Eichstaettisaurus schroederi
Broili 1938, Hoffstetter 1953, Kuhn 1958) Late Jurassic, ~150 mya, ~10 cm in length was originally considered another Ardeosaurus, but has several distinct characters (note the feet). Originally and traditionally it was considered a relative of Gekko, but nests near the base of geckos as a snake ancestor.

Eichstaettisaurus gouldi
(Evans et al. 2004) Early Cretaceous, has a longer torso, shorter legs and a very snake-like palate, nesting it closer to snakes than either of these two taxa. Bipedal Cretaceous lizard tracks (Lee et al. 2018) are best matched to Eichstattisaurus.

References
Bolet A and Evans SE 2012. A tiny lizard (Lepidosauria, Squamata) from the lower Cretaceous of Spain. Palaeontology 55:491-500.
Broili F 1938. Ein neuer fund von ?Ardeosaurus H. von Meyer. Sitzungs− berichte der Bayerischen Akademie der Wissenschaften, München 1938: 97–114.
Caldwell M, Nydam R, Palci A and Apesteguía S 2014. The oldest known fossil snakes: a tempera range extension of 70 million years. Journal of Vertebrate Paleontology abstracts.
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