Tiktaalik, (artist's impression)
From the day its discovery was announced, Tiktaalik has been a major embarrassment for creationists because not only does it refute the claim that there are no intermediate forms, but it also belies the claim that the Theory of Evolution can't make predictions.
Not only is it intermediate between fully aquatic lobe-finned fish and terrestrial tetrapod, but its discoverers predicted exactly where it would be found in the geological column and promptly went and found it there, exactly were predicted in Canada's Ellesmere Island.
But that embarrassment is about to become even more acute.
Researchers at Penn State's Eberly College have shown that Tiktaalik's ribs were attached to its pelvis and that fact helped in the evolution of walking. The research team, co-led by Tom Stewart, assistant professor of biology in the Eberly College of Science at Penn State and Neil H Shubin, one of the discoverers of Tiktaalik, have published their findings open access in the journal Proceedings of the National Academy of Science (PNAS). It is also explained in a Penn State Eberly College news release by Sam Sholtis:
Before the evolution of legs from fins, the axial skeleton — including the bones of the head, neck, back and ribs — was already going through changes that would eventually help our ancestors support their bodies to walk on land. A research team including a Penn State biologist completed a new reconstruction of the skeleton of Tiktaalik, the 375-million-year-old fossil fish that is one of the closest relatives to limbed vertebrates. The new reconstruction shows that the fish’s ribs likely attached to its pelvis, an innovation thought to be crucial to supporting the body and for the eventual evolution of walking.More detail and background are given in the abstract and introduction to the team's open access paper in PNAS:
A paper describing the new reconstruction, which used microcomputed tomography (micro-CT) to scan the fossil and reveal vertebrae and ribs of the fish that were previously hidden beneath rock, appeared April 2 in the journal Proceedings of the National Academy of Sciences.
Tiktaalik was discovered in 2004, but key parts of its skeleton were unknown. These new high-resolution micro-CT scans show us the vertebrae and ribs of Tiktaalik and allow us to make a full reconstruction of its skeleton, which is vital to understanding how it moved through the world.
Assistant Professor Thomas A. Stewart, co-lead author
Department of Biology
The Pennsylvania State University, State College, PA, USA.
Unlike most fish, which have vertebrae and ribs that are the same along the length of the trunk, the axial skeletons of limbed vertebrates show dramatic differences in the vertebrae and ribs from the head region to the tail region. The evolution of this regionalization allowed the performance of specialized functions, one of which was a mechanical linkage between ribs in the sacral region to the pelvis that enabled support of the body by the hind limbs.
New reconstruction of the skeleton of the 375-million-year-old fossil fish, Tiktaalik roseae. In a new study, researchers used Micro-CT to reveal vertebrae and ribs of the fish that were previously hidden beneath rock. The new reconstruction shows that the fish’s ribs likely attached to its pelvis, an innovation thought to be crucial to supporting the body and for the eventual evolution of walking.Credit: Thomas Stewart, Penn State
The pelvic fins of fish are evolutionarily related to hind limbs in tetrapods — four-limbed vertebrates, including humans. In fish, the pelvic fins and bones of the pelvic girdle are relatively small and float freely in the body. For the evolution of walking, the researchers explained, the hind limbs and pelvis became much larger and formed a connection to the vertebral column as a way of bracing the forces related to supporting the body.
The original description of Tiktaalik focused on the front portion of the skeleton. Fossils were meticulously prepared to remove the surrounding matrix of rock and expose the skull, shoulder girdle and pectoral fins. The ribs in this area were large and expanded, suggesting that they may have supported the body in some way, but it was unclear exactly how they would have functioned. In 2014, the fish’s pelvis, discovered in the same location as the rest of the skeleton, was also cleaned of matrix and described.Tiktaalik is remarkable because it gives us glimpses into this major evolutionary transition. Across its whole skeleton, we see a combination of traits that are typical of fish and life in water as well as traits that are seen in land-dwelling animals.
Assistant Professor Thomas A. Stewart.The researchers explained that, unlike our own hips where our bones fit tightly together, the connection between the pelvis and axial skeleton of Tiktaalik was likely a soft-tissue connection made of ligaments. The new reconstruction of the skeleton also sheds light on specializations for head mobility in Tiktaalik and new details of the fish’s pelvic fin anatomy.From past studies, we knew that the pelvis was large, and we had a sense that the hind fins were large too, but until now couldn’t say if or how the pelvis interacted with the axial skeleton. This reconstruction shows, for the first-time, how it all fit together and gives us clues about how walking might have first evolved.
Tiktaalik had specialized ribs that would have connected to the pelvis by a ligament. It’s astonishing really. This creature has so many traits — large pair of hind appendages, large pelvis, and connection between the pelvis and axial skeleton — that were key to the origin of walking. And while Tiktaalik probably wasn’t walking across land, it was definitely doing something new. This was a fish that could likely prop itself up and push with its hind fin.
Assistant Professor Thomas A. Stewart.In addition to Stewart and Shubin, the research team includes Justin B. Lemberg, Emily J. Hillan, and Isaac Magallanes at The University of Chicago, and Edward B. Daeschler at Academy of Natural Sciences of Drexel University.It's incredible to see the skeleton of Tiktaalik captured in such vivid detail. This study sets the stage for ones that explore how the animal moved about and interacted with its environment 375 million years ago.
Professor Neil H. Shubin, Co-lead author
Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy
Department of Organismal Biology and Anatomy
The University of Chicago, Chicago, IL, USA.
So, not only a striking example of a transitional form from the beginnings of terrestrial vertebrates, but evidence of the transition from fins that were probably used for pushing forward either on the seabed or on land, to the hind legs of amphibians.Significance
The origin of terrestrial vertebrates is marked by changes to the entire post-cranial skeleton. To date, information on the vertebrae and ribs of the closest relatives to limbed vertebrates has been limited, making it difficult to reconstruct how the axial skeleton was evolving. This paper describes the axial column of Tiktaalik roseae, a close relative of limbed vertebrates. The holotype specimen was µCT (micro-computed tomography) scanned, which revealed its vertebrae and posterior ribs. These data show how specialization for head mobility, body support, and pelvic fin buttressing arose in stem tetrapods, allowing for a three-dimensional reconstruction of Tiktaalik and shedding light on the antecedents to the terrestrial walking behaviors.
Abstract
The axial columns of the earliest limbed vertebrates show distinct patterns of regionalization as compared to early tetrapodomorphs. Included among their novel features are sacral ribs, which provide linkage between the vertebral column and pelvis, contributing to body support and propulsion by the hindlimb. Data on the axial skeletons of the closest relatives of limbed vertebrates are sparce, with key features of specimens potentially covered by matrix. Therefore, it is unclear in what sequence and under what functional context specializations in the axial skeletons of tetrapods arose. Here, we describe the axial skeleton of the elpistostegalian Tiktaalik roseae and show that transformations to the axial column for head mobility, body support, and pelvic fin buttressing evolved in finned vertebrates prior to the origin of limbs. No atlas–axis complex is observed; however, an independent basioccipital–exoccipital complex suggests increased mobility at the occipital vertebral junction. While the construction of vertebrae in Tiktaalik is similar to early tetrapodomorphs, its ribs possess a specialized sacral domain. Sacral ribs are expanded and ventrally curved, indicating likely attachment to the expanded iliac blade of the pelvis by ligamentous connection. Thus, the origin of novel rib types preceded major alterations to trunk vertebrae, and linkage between pelvic fins and axial column preceded the origin of limbs. These data reveal an unexpected combination of post-cranial skeletal characters, informing hypotheses of body posture and movement in the closest relatives of limbed vertebrates.
The earliest limbed vertebrates are characterized by a regionalized axial skeleton with cervical, thoracic, sacral, and caudal domains in the vertebral column and ribs (1–4). This organization corresponds to locomotor specializations that provide support for load-bearing hind limbs and increased mobility of the head (5–7). Acanthostega and Ichthyostega have specialized ribs that provide mechanical linkage between the vertebral column and pelvic girdle, connecting to the ilium by either ligament or direct articulation (1, 2, 4). These sacral ribs are absent in early tetrapodomorphs (8–10). For example, the tristichopterid Eusthenopteron has ribs that are short and generally similar across their cranio-caudal distribution and they do not approach the pelvis, which is small as compared to the pectoral girdle (8). Moreover, unlike Acanthostega and Ichthyostega, Eusthenopteron possessed to a bony linkage between the shoulder girdle and cranium that would have limited head motility (1, 2, 4, 8).
Little is known about the axial regionalization of elpistostegalian fishes (sensu Daeschler et al., 11), a paraphyletic grade within tetrapodomorpha that comprises the closest relatives of limbed vertebrates. In Panderichthys, the four most rostral vertebrae are non-rib bearing (12), and four vertebrae are described from the trunk as having broad short ribs, similar in length to the combined height of the neural arch and spine (13, 14). The vertebrae of Elpistostege are known from a series of approximately 16 that show no heterogeneity in their length or shape; ribs were not found in association with these vertebrae, and their position along the axial column is unclear (15). Since the original discovery in 2004, the axial skeleton of Tiktaalik has been largely obscured by matrix. While the rostral ribs are known to be broad and laterally expanded as compared to early tetrapodomorph conditions, the vertebral column has not been observed (11). However, the pelvis and pelvic fin of Tiktaalik are nearly the size of the pectoral appendage (16), differentiating its overall proportions from less crownward taxa such as Panderichthys (13, 17). The size and depth of the acetabulum, the general robusticity of the pubis, and the broadly expanded iliac blades of Tiktaalik are features that have not been described in other finned tetrapodomorphs (16).
Here, we present high-resolution micro-computed tomography (µCT) scans of the type specimen of Tiktaalik roseae, NUFV (Nunavut Fossil Vertebrate Collection) 108, that expose the vertebral skeleton and posterior ribs (Fig. 1 and Movies S1 and S2). These data, and the reconstruction that they allow, reveal unexpected intermediate conditions and apomorphies at the origin of limbed vertebrates and provide insight into the functional context in which they arose.Fig. 1.
Volumetric rendering of µCT scans of Tiktaalik roseae. NUFV 108 in (A) dorsal, (B) ventral, and (C) left lateral perspectives. µCT data reveal previously hidden detail on the exoccipital–basioccipital complex, vertebrae, ribs, and pelvic fin. The head, which was mechanically prepared and scanned separately (18), is positioned here slightly anterior to its preserved position to more clearly show the pectoral girdle. The preserved position of the head is depicted in SI Appendix, Fig. S1. (Scale bar, 5 cm.)Movie S1Volumetric rendering of the two blocks containing the post-cranial skeleton of NUFV 108 including matrix.Movie S2Volumetric rendering of NUFV 108 with all segmented elements in their preserved position.
Stewart, Thomas A.; Lemberg, Justin B.; Hillan, Emily J.; Magallanes, Isaac; Daeschler, Edward B.; Shubin, Neil H.
The axial skeleton of Tiktaalik roseae
Proceedings of the National Academy of Sciences (2024) 121(15) e2316106121. DOI: 10.1073/pnas.2316106121
Copyright: © 2024 The authors.
Published by PNAS. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
And another casual, unintentional refutation of multiple creationist lies and the false claims that are used by unscrupulous frauds to lure scientifically illiterate fools into their lucrative cult.
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