| 1. |
- Ahlberg, Per Erik
(författare)
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Glimpsing the hidden majority
- 1990
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Ingår i: Nature. - 0028-0836 .- 1476-4687. ; 344, s. 23-
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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| 2. |
- Ahlberg, Per Erik, 1963-
(författare)
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Humeral homology and the origin of the tetrapod elbow : a reinterpretation of the enigmatic specimens ANSP 21350 and GSM 104536
- 2011
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Ingår i: Studies on fossil tetrapods. - London : The Palaeontological Association. - 9781444361896 ; , s. 17-29
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Bokkapitel (refereegranskat)abstract
- Two putative tetrapod humeri of Devonian age, ANSP 21350 from the late Famennian of Pennsylvania and GSM 104536 from the late Frasnian of Scat Craig, Scotland, are reinterpreted in the light of more recent discoveries. The morphology of ANSP 21350 can be more fully homologized with those of elpistostegids and early tetrapods than previously recognized. Unique features include distally displaced dorsal muscle attachments and a ventrally rotated distal face of the bone. This suggests that a weight-bearing ventrally directed forearm was created, not by means of a flexed elbow as in other tetrapods, but by distorting the humerus. The olecranon process on the ulna was probably poorly developed or absent. Primitive characters that are absent in other tetrapods add support to the contention that ANSP 21350 is the least crownward of known tetrapod humeri. Contrary to previous claims, Acanthostega has a characteristic tetrapod ulnar morphology with an olecranon process; it does not resemble an elpistostegid ulna and is not uniquely primitive for tetrapods. This suggests that the flexed tetrapod elbow with ulnar extensor muscles attached to the olecranon evolved simultaneously with the large rectangular entepicondyle typical for early tetrapods, probably as part of a single functional complex. GSM 104536 is denfinitely not a primitive tetrapod humerus, nor a sarcopterygian branchial bone, but cannot be positively identified at present.
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| 3. |
- Ahlberg, Per Erik, 1963-, et al.
(författare)
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Pelvic claspers confirm chondrichthyan-like internal fertilization in arthrodires
- 2009
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Ingår i: Nature. - : Nature Publishing Group. - 0028-0836 .- 1476-4687. ; 460:7257, s. 888-889
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Tidskriftsartikel (refereegranskat)abstract
- Recent finds(1,2) demonstrate that internal fertilization and viviparity (live birth) were more widespread in the Placodermi, an extinct group of armoured fishes, than was previously realized. Placoderms represent the sister group of the crown group jawed vertebrates (Gnathostomata)(3,4), making their mode(s) of reproduction potentially informative about primitive gnathostome conditions. An ossified pelvic fin basipterygium discovered in the arthrodire Incisoscutum ritchiei was hypothesized to be identical in males and females, with males presumed to have an additional cartilaginous element or series forming a clasper. Here we report the discovery of a completely ossified pelvic clasper in Incisoscutum ritchiei (WAM 03.3.28) which shows that this interpretation was incorrect: the basipterygium described previously(1) is in fact unique to females. The male clasper is a slender rod attached to a square basal plate that articulates directly with the pelvis. It carries a small cap of dermal bone covered in denticles and small hooks that may be homologous with the much larger dermal component of the ptyctodont clasper.
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| 4. |
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| 5. |
- Blom, Henning, et al.
(författare)
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Devonian vertebrates from East Greenland : a review of faunal composition and distribution
- 2007
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Ingår i: Geodiversitas. - 1280-9659 .- 1638-9395. ; 29:1, s. 119-141
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Forskningsöversikt (refereegranskat)abstract
- The Devonian vertebrate faunas of East Greenland are reviewed and their distribution discussed for the first time in the light of the most recently published stratigraphical framework for the area. The predominantly Middle and Upper Devonian continental sediments have yielded representatives of most major groups of early fossil vertebrates, including heterostracans, placoderms, acanthodians, chondrichthyans, actinopterygians, lungfishes, porolepiforms, "osteolepiforms" and tetrapods, but to date, no single publication has treated them all in their stratigraphical context. We therefore attempt to place them into the most recent, formalised lithostratigraphy, providing improved resolution for their stratigraphical distribution, as a basis for comparing East Greenland with other Devonian faunal successions worldwide. The review highlights the fact that many problems of stratigraphical correlation and dating of the East Greenland Devonian deposits remain to be resolved by further fieldwork. Several assemblages ranging from Givetian to Famennian in age can be distinguished, that correspond to older superceded subdivisional nomenclature. A possible Frasnian fauna has been recognised for the first time, adding details to an otherwise poorly dated part of the succession. Typical Devonian taxa such as Holoptychius Agassiz, 1839 and Groenlandaspis Heintz, 1932 have apparently been recorded in an otherwise unique position above the Devonian-Carboniferous boundary in the upper part of the succession. New specimens of rare elements of the fauna including an unknown arthrodire placoderm, a putative chondrichthyan spine and a patch of possibly regurgitated acanthodian spines, are illustrated for the first time.
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| 6. |
- Boisvert, Catherine Anne, 1978-, et al.
(författare)
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Comparative pelvic development of the axolotl (Ambystoma mexicanum) and the Australian lungfish (Neoceratodus forsteri) : conservation and innovation across the fish-tetrapod transition
- 2013
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Ingår i: EvoDevo. - : Springer Science and Business Media LLC. - 2041-9139. ; 4, s. 3-
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Tidskriftsartikel (refereegranskat)abstract
- Background: The fish-tetrapod transition was one of the major events in vertebrate evolution and was enabled by many morphological changes. Although the transformation of paired fish fins into tetrapod limbs has been a major topic of study in recent years, both from paleontological and comparative developmental perspectives, the interest has focused almost exclusively on the distal part of the appendage and in particular the origin of digits. Relatively little attention has been paid to the transformation of the pelvic girdle from a small unipartite structure to a large tripartite weight-bearing structure, allowing tetrapods to rely mostly on their hindlimbs for locomotion. In order to understand how the ischium and the ilium evolved and how the acetabulum was reoriented during this transition, growth series of the Australian lungfish Neoceratodus forsteri and the Mexican axolotl Ambystoma mexicanum were cleared and stained for cartilage and bone and immunostained for skeletal muscles. In order to understand the myological developmental data, hypotheses about the homologies of pelvic muscles in adults of Latimeria, Neoceratodus and Necturus were formulated based on descriptions from the literature of the coelacanth (Latimeria), the Australian Lungfish (Neoceratodus) and a salamander (Necturus). Results: In the axolotl and the lungfish, the chondrification of the pelvic girdle starts at the acetabula and progresses anteriorly in the lungfish and anteriorly and posteriorly in the salamander. The ilium develops by extending dorsally to meet and connect to the sacral rib in the axolotl. Homologous muscles develop in the same order with the hypaxial musculature developing first, followed by the deep, then the superficial pelvic musculature. Conclusions: Development of the pelvic endoskeleton and musculature is very similar in Neoceratodus and Ambystoma. If the acetabulum is seen as being a fixed landmark, the evolution of the ischium only required pubic pre-chondrogenic cells to migrate posteriorly. It is hypothesized that the iliac process or ridge present in most tetrapodomorph fish is the precursor to the tetrapod ilium and that its evolution mimicked its development in modern salamanders.
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| 7. |
- Boisvert, Catherine Anne, 1978-
(författare)
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The Origin of Tetrapod Limbs and Girdles: Fossil and Developmental Evidence
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Around 375 million years ago, the first tetrapods appeared, marking one of the most important events in vertebrate evolutionary history. The fin to limb transition saw the appearance of fingers and a weight bearing pelvic girdle. While very little research has been done on the evolution of the tetrapod pelvic girdle, a fair amount has been done on the origins of fingers but some aspects remained controversial. A combination of palaeontology, developmental biology and comparative morphology was therefore used in this thesis to better understand the fin to limb transition. The pectoral fin of Panderichthys, a sarcopterygian fish closely related to tetrapods was CT-scanned and modeled in three dimensions and its pelvic girdle and fin were examined with traditional techniques. This information from the fossil record was integrated with comparisons of the development of the Australian lungfish, Neoceratodus forsteri, our closest living fish relative and the axolotl (Ambystoma mexicanum), a salamander representing well the condition of early tetrapods. Development of bone and cartilage was studied through clearing and staining and development of skeletal muscles through immunostaining. In situ hybridizations were performed on the lungfish to study the expression of Hoxd13, associated with the formation of digits in tetrapods. This work shows that the late expression phase of Hoxd13 is present in Neoceratodus and is associated with the formation of radials. Redescription of the pectoral fin of Panderichthys reveals that distal radials are present, which, in addition to other information, lead us to conclude that digits are not novelties in tetrapods but rather have evolved from the distal radials present in the fins of all sarcopterygian fish. The earliest tetrapods lack a full set of wrist + carpals/ankle + tarsal bones. Here, we propose that this region of the limbs evolved after fingers and toes through an expansion of the region between the proximal limb bones and the digits. As for the pelvic girdle, it is very primitive in Panderichthys but comparison of its development in Neoceratodus and Ambystoma suggest that the ischium evolved through the posterior expansion of the pubis and the ilium, through an elongation of the iliac process already present in sarcopterygian fishes. The results of this thesis help to better understand the fin to limb transition and show that it is more gradual than previously believed.
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| 8. |
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| 9. |
- Botella, Hector, et al.
(författare)
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Jaws and teeth of the earliest bony fishes
- 2007
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 448:7153, s. 583-586
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Tidskriftsartikel (refereegranskat)abstract
- Extant jawed vertebrates, or gnathostomes, fall into two major monophyletic groups, namely chondrichthyans (cartilaginous fishes) and osteichthyans (bony fishes and tetrapods). Fossil representatives of the osteichthyan crown group are known from the latest Silurian period, 418 million years (Myr) ago, to the present. By contrast, stem chondrichthyans and stem osteichthyans are still largely unknown. Two extinct Palaeozoic groups, the acanthodians and placoderms, may fall into these stem groups or the common stem group of gnathostomes, but their relationships and monophyletic status are both debated. Here we report unambiguous evidence for osteichthyan characters in jaw bones referred to the late Silurian (423–416-Myr-old) fishes Andreolepis hedei and Lophosteus superbus, long known from isolated bone fragments, scales and teeth, and whose affinities to, or within, osteichthyans have been debated1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. The bones are a characteristic osteichthyan maxillary and dentary, but the organization of the tooth-like denticles they bear differs from the large, conical teeth of crown-group osteichthyans, indicating that they can be assigned to the stem group. Andreolepis and Lophosteus are thus not only the oldest but also the most phylogenetically basal securely identified osteichthyans known so far.
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| 10. |
- Clack, Jennifer A., et al.
(författare)
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Sarcopterygians : From Lobe-Finned Fishes to the Tetrapod Stem Group
- 2016
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Ingår i: Evolution of the Vertebrate Ear. - Cham : Springer Publishing Company. - 9783319466590 - 9783319466613 ; , s. 51-70
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The sarcopterygians or lobe-finned fishes is the group that gave rise to tetrapods, and they were the dominant bony fishes of the Devonian period. Their otic regions were constructed similarly to those of both the actinopterygians and chondrichthyans, their structure being the common inheritance of all jawed vertebrates. One distinguishing feature of the primitive sarcopterygian braincase was that the division between the anterior ethmosphenoid and posterior otoccipital section sof the braincase was marked by a flexible hinge joint, which is seen today in the modern coelacanth, Latimeria. The hyomandibular was long and projected ventrally with an opercular process that contacted the opercular bone and with the distal end associated indirectly with the jaw joint. It was a key component of the buccal pumping mechanism for breathing and feeding. The braincases of dipnoans (lungfishes) were the most highly modified of sarcopterygian braincases with consolidated fore and aft portions and reduction or loss of the hyomandibula. The utricle was enlarged in several fossil dipnoans, although the reason for this is not clear. The braincases of tetrapodomorph sarcopterygians differed little from the primitive condition in the group. The main modifications were to the more crownward and tetrapod-like forms from the Late Devonian. In these fishes, the hyomandibula was reduced in length, its contact with the opercular bone lost and, ultimately, the opercular bone itself disappeared. The spiracular notch and associated cleft increased in width and volume respectively, possibly resulting in increased air-breathing capacity and reduced use of the gill system.
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