Fossils provide important clues regarding amphibian evolution

[rogue06]

Caecilians are odd critters. While they are amphibians they don't much resemble others in that group being worm-like (no limbs) and possessing teeth and live a life burrowing through soil using tentacles around their eyes to sense things. We don't know a whole lot about them including how they evolved or how they are related to other amphibians.

We may now finally be getting some answers thanks to the first unmistakable caecilian fossil dating back to the Triassic being discovered on what is called "Thunder Ridge" in Arizona's Petrified Forest National Park.

Previously, only 10 fossil caecilian occurrences have been discovered by scientists, and they only date back to roughly 183 mya to the Early Jurassic whereas DNA analysis estimates that they likely originated sometime back during the Carboniferous or Permian (approximately 370 million to 270 mya). That makes for at least a 87 million year gap from which no fossils had been found. The new discovery extends their fossil record back by some 35 million years thereby narrowing that gap substantially.

The material discovered included a number of postcranial fragments and several jaws. It was the jaws, some of which were barely a quarter of an inch (6.3mm) long, with their distinctive double row of teeth, that allowed the researchers to identify the remains as having come from an early caecilian. The remains from at least 70 individuals have been uncovered since the initial discovery in 2019.

The group named the creature Funcusvermis gilmorei. The genus name translates to "funky worm" and was inspired by a song by the Ohio Players called "Funky Worm," which was apparently a favorite excavation tune of the researchers.

Analysis reveals that Funcusvermis demonstrates a strong link between caecilians and other lissamphibians (frogs and salamanders) in that it shares a number of skeletal features with early members of this subclass, as well as extinct dissorophoid temnospondyls such as Gerobatrachus. which lived during the Early Permian.

As co-author Ben Kligman of the Department of Resource Management and Science, Petrified Forest National Park, Petrified Forest and Department of Geosciences, Virginia Tech, observed, "Unlike living caecilians, Funcusvermis lacks many adaptations associated with burrowing underground, indicating a slower acquisition of features associated with an underground lifestyle in the early stages of caecilian evolution."

The team also said that the discovery supports the consensus hypothesis regarding lissamphibian evolution, with Lissamphibia as a monophyletic group with a single common ancestor among the dissorophoids.

Source: One wormy Triassic fossil could fill a hole in the evolutionary story of amphibians

Sometimes the biggest finds come in the smallest (fossil) packages

When you think of an amphibian, the first thing to jump to mind is probably a springy frog or a salamander or maybe a giant cane toad. But some newly found fossils of a lesser known amphibian order called caecilians—limbless, worm-like critters that live and burrow underground—could fill in some big evolutionary gaps in how present day amphibians came to be.

Caecilians look kind of like large earthworms and there are over 124 known species of them. Like all amphibians, they find themselves at home in both water and land. Modern caecilians are now exclusively home to southern Asia, South and Central America, and parts of Africa. Their enigmatic underground lives make them difficult for scientists to study, and while they can live up to 13 years long in captivity, it’s unknown exactly how long they live in the wild.

Previously, only 10 fossil caecilian occurrences have been found by scientists, and they date back roughly 183 million years ago to the Early Jurassic Period.

Now, a team of paleontologists have unearthed a new and older caecilian fossil, extending the record of this small animal by roughly 35 million years, all the way back to the Triassic Period (roughly 250 million to 200 million years ago). This fossil has a reddish hue and offers some clues into what the weird family of creatures may have looked like. The fossil find is described in a study published January 25 in the journal Nature.

2189d4c3-2f01-443c-b44f-357da0213acc.jpg
Microscopic photograph of a lower jaw from Funcusvermis gilmorei
soon after it was recovered during microscopic sorting of sediment

"Seeing the first jaw under the microscope, with its distinctive double row of teeth, sent chills down my back," Virginia Tech doctoral student Ben Klingman and co-author of the new study said in a statement. "We immediately knew it was a caecilian, the oldest caecilian fossil ever found, and a once-in-a-lifetime discovery."

Klingman named the new fossil “Funcusvermis gilmorei.” The genus name ‘Funcusvermis’ was inspired by a song by the Ohio Players called “Funky Worm,” a favorite excavation tune for the authors.

Funcusvermisshares some skeletal features with early frog and salamander fossils and an ancient group of amphibians called dissorophoid temnospondyls.“Unlike living caecilians, Funcusvermis lacks many adaptations associated with burrowing underground, indicating a slower acquisition of features associated with an underground lifestyle in the early stages of caecilian evolution,” Klingman said.

The discovery was found in Arizona’s Petrified Forest National Park during a dig in 2019. The fossils were found in a part of the park nicknamed Thunderstorm Ridge that is well known for fossil discoveries. They were deposited in a layer of the Chinle Formation dated to approximately 220 million years ago. At the time, the state of Arizona was near the equator at the central part of the ancient supercontinent Pangaea. Like today, the region was hot, but unlike today, it was much more humid.

At least 70 individuals of Funcusvermis have been recovered as of summer 2022 at Petrified Forest National Park. Only a handful of Funcusvermis bones have been found, but until the team can find a complete skeleton, they cannot exactly determine the body length of Funcusvermis. Early inferences estimate that it was pretty tiny since remains of its lower jaws are less than a quarter inch long.

"The discovery of the oldest caecilian fossils highlights the crucial nature of new fossil evidence. Many of the biggest outstanding questions in paleontology and evolution cannot be resolved without fossils like this," said Kligman. "Fossil caecilians are extraordinarily rare, and they are found accidentally when paleontologists are searching for the fossils of other more common animals."


Source

© Copyright Original Source

The paper, Triassic stem caecilian supports dissorophoid origin of living amphibians can be read in its entirety by clicking the hyperlink, although the abstract is made available below

Abstract

Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona). The estimated Palaeozoic era gymnophionan–batrachian molecular divergence¹ suggests a major gap in the record of crown lissamphibians prior to their earliest fossil occurrences in the Triassic period^2,3,4,5,6. Recent studies find a monophyletic Batrachia within dissorophoid temnospondyls^7,8,9,10, but the absence of pre-Jurassic period caecilian fossils^11,12 has made their relationships to batrachians and affinities to Palaeozoic tetrapods controversial^1,8,13,14. Here we report the geologically oldest stem caecilian—a crown lissamphibian from the Late Triassic epoch of Arizona, USA—extending the caecilian record by around 35 million years. These fossils illuminate the tempo and mode of early caecilian morphological and functional evolution, demonstrating a delayed acquisition of musculoskeletal features associated with fossoriality in living caecilians, including the dual jaw closure mechanism^15,16, reduced orbits¹⁷ and the tentacular organ¹⁸. The provenance of these fossils suggests a Pangaean equatorial origin for caecilians, implying that living caecilian biogeography reflects conserved aspects of caecilian function and physiology¹⁹, in combination with vicariance patterns driven by plate tectonics²⁰. These fossils reveal a combination of features that is unique to caecilians alongside features that are shared with batrachian and dissorophoid temnospondyls, providing new and compelling evidence supporting a single origin of living amphibians within dissorophoid temnospondyls.

5ce7dc2b-7cf4-42c8-a6a5-73c7c66ed43c.jpg
a–c, Composite reconstruction of craniomandibular elements in lateral (a), medial (b) and dorsal (c) views. d,e, Holotype right
pseudodentary (PEFO 43891) in medial and ventral views. f, Paratype right pseudodentary (PEFO 46284) in medial view.
g–i, Referred left maxillopalatine (PEFO 46481) in medial (g), ventral (h) and dorsal (i) views. j,k, Referred left pseudoangular
(PEFO 46480) in medial and lateral views. l–o, Paratype right pseudodentary (PEFO 45800) in medial (l; expanded view in m)
and dorsal (n; expanded view in o) views. abcnV, alveolar branch cranial nerve V; adtr, adsymphyseal tooth row; af, adductor fossa;
att, attachment tissue; bp, basal pore; cnV, cranial nerve V insertions; cp, coronoid process; dpaf, dorsal pseudoangular facet;
dpdf, dorsal pseudodentary facet; dtr, dentary tooth row; dz, dividing zone; ebcnV, external branch cranial nerve V; fr, facial ramus;
hp, hamate process; imf, intramandibular foramen; jas, jaw articulation surface; lcm, lateral choanal margin; lecnV, lateral exit
cranial nerve V; mtr, maxillary tooth row; om, orbital margin; pap, posterior pseudoangular process; pc, pulp cavity; pd, pedicel;
pgp, preglenoid process; ptr, palatal tooth row; rtl, replacement tooth locus; sf, symphyseal foramen; sp, symphyseal prongs;
vpaf, ventral pseudoangular facet; vpdf, ventral pseudodentary facet. Arrows indicate anterior direction.

84b34f0f-cca8-4460-a7c2-5da18427d665.jpg
Squares denote important apomorphies (including non-mandibular features); apomorphies are optimized computationally unless followed
by an asterisk, which denotes an apomorphy suggested by our results but lacking sufficient sampling to optimize computationally. Topology
is derived from parsimony results (Extended Data Fig. 5); Yaksha peretti, Salamandra and Rana approximate conditions are found in taxa sampled
in the analysis. Illustrations represent right mandibles in medial (bottom) and dorsal (top) views for Doleserpeton annectens⁹, Eocaecilia micropodia¹²
(Illustration adapted from ref. ¹², with the permission of Museum of Comparative Zoology, Harvard University), Epicrionops petersi¹²,
Funcusvermis gilmorei, Rana, Salamandra and Y. peretti²⁸, excepting Greererpeton burkemorani³⁵ (dorsal only) and Cacops aspidephorus³⁷
(medial only). All scale bars are 2 mm except for G. burkemorani (2 cm) and C. aspidephorus (2 cm). Brackets on the branches
indicate stem groups, whereas circles indicate node groups. Crosses indicate extinct taxa.

[shunyadragon]

Great Reference! . . . but hardly understood by most.