|
Here are the articulated cranium and lower jaws shown in oblique right lateral view (A). Right facial skeleton and skull roof shown in "exploded" view to illustrate the nature of sutural contacts (B); the left side of the cranium (braincase omitted) is shown in internal view (C). The right lower jaw in "exploded" view to illustrate sutural morphology. Individual bones shown in various colors. Artist Credit: Porro et al.. |
The
first 3D reconstruction of the skull of a 360 million-year-old near-ancestor of
land vertebrates has been created by scientists from the Universities of
Bristol and Cambridge, UK. The 3D skull, which differs from earlier 2D
reconstructions, suggests such creatures, which lived their lives primarily in
shallow water environments, were more like modern crocodiles than previously
thought.
The
researchers applied high-resolution X-ray computed tomography (CT) scanning to
several specimens of Acanthostega gunnari,
one of the 'four-footed' vertebrates known as tetrapods which invaded the land
during one of the great evolutionary transitions in Earth's history, 380-360
million years ago. Tetrapods evolved from lobe-finned fishes and display a
number of adaptations to help them survive on land.
An
iconic fossil species, Acanthostega
gunnari is crucial for understanding the anatomy and ecology of the
earliest tetrapods. However, after hundreds of millions of years in the ground
fossils are often damaged and deformed. No single specimen of Acanthostega preserves a skull that is
complete and three-dimensional which has limited scientists' understanding of
how this key animal fed and breathed -- until now.
Using
special software, the Bristol and Cambridge researchers 'digitally prepared' a
number of Acanthostega specimens from
East Greenland, stripping away layers of rock to reveal the underlying bones.
They
uncovered a number of bones deep within the skull, including some that had never
before been seen or described, resulting in a detailed anatomical description
of the Acanthostega skull.
Once
all of the bones and teeth were digitally separated from each other, cracks
were repaired and missing elements duplicated. Bones could then be manipulated
individually in 3D space. Using information from other specimens, the bones
were fitted together like puzzle pieces to produce the first 3D reconstruction
of the skull of Acanthostega, with
surprising results.
Lead
author, Dr Laura Porro, formerly of Bristol's School of Earth Sciences and now
at the Royal Veterinary College, said: "Because early tetrapods skulls are
often 'pancaked' during the fossilization process, these animals are usually
reconstructed having very flat heads. Our new reconstruction suggests the skull
of Acanthostega was taller and
somewhat narrower than previously interpreted, more similar to the skull of a
modern crocodile."
The
researchers also found clues to how Acanthostega
fed. The size and distribution of its teeth and the shape of contacts between
individual bones of the skull (called sutures) suggest Acanthostega may have initially seized prey at the front of its
jaws using its large front teeth and hook-shaped lower jaw.
Co-author,
Professor Emily Rayfield, also from Bristol's School of Earth Sciences, said:
"These new analyses provide fresh clues about the evolution of the jaws
and feeding system as the earliest animals with limbs and digits began to
conquer the land."
The
researchers plan to apply these methods to other flattened fossils of the
earliest tetrapods to better understand how these early animals modified their
bones and teeth to meet the challenges of living on land.
Digital
models of the original fossils and the 3D reconstruction are also useful in
scientific research and education. They can be accessed by researchers around
the world, without risking damage to fragile original fossils and without
scientists having to travel thousands of miles to see original specimens.
Furthermore, digital models and 3D printouts can be easily and safely handled
by students taking courses and by the public during outreach events.
Citation
Porro
LB, Rayfield EJ, Clack JC. 2015. Descriptive Anatomy and Three-Dimensional
Reconstruction of the Skull of the Early Tetrapod Acanthostega gunnari Jarvik, 1952. PLOS ONE, 2015; 10 (3):
e0118882 DOI:10.1371/journal.pone.0118882