Tuesday, May 14, 2019

Big Crushing Bite for Tiny Creature

A tiny creature had a big bite. The creature was a forerunner of modern amphibians and mammals. 

Micro-CT scanning of a tiny snake-like fossil discovered in Scotland has shed new light on the elusive creature, thought to be one of the earliest known tetrapods to develop teeth that allowed it to crush its prey.

Detailed scans of Acherontiscus caledoniae showed a unique combination of different tooth shapes and sizes as well as a deep lower jaw which scientists believe would have given the creature the ability to pierce, cut and grind the hard-shelled crustaceans that made up its diet.
Reconstructed skull and lower jaw of Acherontiscus caledoniae


Credit: University of Lincoln


Scientists led by the University Museum of Zoology in Cambridge alongside the University of Lincoln, the Natural History Museum in London and the University of Southampton, found that the dental pattern of Acherontiscus is at odds with that of several other tetrapods of this period, which tended to have uniform rows of cone-like teeth sometimes curved backwards at the tip. The variation in the shape and size of teeth shown in this fossil displays a level of dental adaptation that is unprecedented in such an early tetrapod.

As co-author Dr Marcello Ruta from the University of Lincoln’s School Of Life Sciences explains: “We found that Acherontiscus preceded the origin of modern tetrapod lineages and joined an array of primitive groups that independently acquired long and often miniaturized bodies, and exhibited either reduced or no limbs.”

The fossil is the only known specimen of this limbless tetrapod, which measured just 6 inches long and existed in swampy marshlands on the outskirts of Edinburgh some 330 million years ago. The delicate nature of the fossil meant that scientists were unable to use mechanical or chemical methods to free its skeleton from the surrounding rock, or study the specimen under a microscope.

Acherontiscus caledoniae holotype specimen NMS 1967.13.1. (a) Photograph of the skull and anterior postcranium. Scale bar 10 mm. (b) View of micro-CT image of the visible surface (approximately dorsal) of the skull. (d) Close-up of the crowns of two of the left dentary teeth showing apicobasal ridges. Scale bar 0.25 mm. (c) View of micro-CT image of the matrix-embedded (approximately ventral) surface of the skull. Abbreviations: crthy, ceratohyal; fro, frontal; hyob, hyobranchial; jug, jugal; L, left; lac, lacrimal; lr, lower; max, maxilla; ?par, ?parietal; popar, postparietal; porb, postorbital; premax, premaxilla; pteryg, pterygoid; R, right.
File:Acherontiscus fossil and CT.jpg
Credit: Jennifer A. Clack , Marcello Ruta , Andrew R. Milner , John E. A. Marshall , Timothy R. Smithson and Keturah Z. Smithson / Wikimedia Commons

Lead author Professor Jennifer Clack from the University Museum of Zoology in Cambridge said: “Using advanced techniques of micro-CT scanning, we were able to make sense of Acherontiscus’ complex skull, revealing minute anatomical details that allowed us to produce a greatly revised and much more complete reconstruction.

“We were particularly surprised to realize the great variety of shapes and sizes of its teeth. Acherontiscus is the earliest known tetrapod showing a crushing dentition, a feature with a rather discontinuous distribution in fossil and modern tetrapods.”

Fragments in the surrounding matrix have also revealed more about Acherontiscus’ habitat which will inform further research into the area as co-author Professor John Marshall from the University of Southampton’s School of Ocean and Earth Science explains: “Our study provided impetus for exploring the ecology and environments of the Scottish wetlands where Acherontiscus lived. Analysis of the content of fossil spores from about 0.2 grams of the matrix surrounding the creature suggests that this animal lived close to or within a still water body surrounded by herbaceous plants related to clubmosses. A more distant forest of larger, tree-like relatives of modern quillworts was also present.”

During the early Carboniferous, the number of digits on hands and feet of stem-tetropods became standardized at no more than five, as lineages with more digits died out. By mid-Carboniferous times, the stem-tetrapods had radiated into two branches of true ("crown group") tetrapods. Modern amphibians are derived from either the temnospondyls or the lepospondyls (or possibly both), whereas the anthracosaurs were the relatives and ancestors of the amniotes (reptiles, mammals, and kin). The first amniotes are known from the early part of the Late Carboniferous. Amphibians must return to water to lay eggs; in contrast, amniote eggs have a membrane ensuring gas exchange out of water and can therefore be laid on land.C

The Carboniferous is a geologic period and system that spans 60 million years from the end of the Devonian Period 358.9 million years ago (Mya), to the beginning of the Permian Period, 298.9 Mya.

Contacts and sources:
Sophie Belcher.
University of Lincoln


Citation:  r Acherontiscus caledoniae: the first heterodont and durophagous tetrapod  Jennifer A. Clack , Marcello Ruta , Andrew R. Milner , John E. A. Marshall , Timothy R. Smithson  and Keturah Z. Smithson   Royal Society Open Science journal. The full paper is available at:
https://royalsocietypublishing.org/doi/10.1098/rsos.182087




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