| 1. |
- Lindgren, Johan, et al.
(författare)
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Soft-tissue evidence for homeothermy and crypsis in a Jurassic ichthyosaur
- 2018
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 564:7736
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Tidskriftsartikel (refereegranskat)abstract
- Ichthyosaurs are extinct marine reptiles that display a notable external similarity to modern toothed whales. Here we show that this resemblance is more than skin deep. We apply a multidisciplinary experimental approach to characterize the cellular and molecular composition of integumental tissues in an exceptionally preserved specimen of the Early Jurassic ichthyosaur Stenopterygius. Our analyses recovered still-flexible remnants of the original scaleless skin, which comprises morphologically distinct epidermal and dermal layers. These are underlain by insulating blubber that would have augmented streamlining, buoyancy and homeothermy. Additionally, we identify endogenous proteinaceous and lipid constituents, together with keratinocytes and branched melanophores that contain eumelanin pigment. Distributional variation of melanophores across the body suggests countershading, possibly enhanced by physiological adjustments of colour to enable photoprotection, concealment and/or thermoregulation. Convergence of ichthyosaurs with extant marine amniotes thus extends to the ultrastructural and molecular levels, reflecting the omnipresent constraints of their shared adaptation to pelagic life.
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| 2. |
- Bazzi, Mohamad, et al.
(författare)
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Feeding ecology has shaped the evolution of modern sharks
- 2021
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Ingår i: Current Biology. - : Cell Press. - 0960-9822 .- 1879-0445. ; 31:23, s. 5138-5148.e4
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Tidskriftsartikel (refereegranskat)abstract
- Sharks are iconic predators in today’s oceans, yet their modern diversity has ancient origins. In particular, present hypotheses suggest that a combination of mass extinction, global climate change, and competition has regulated the community structure of dominant mackerel (Lamniformes) and ground (Carcharhiniformes) sharks over the last 66 million years. However, while these scenarios advocate an interplay of major abiotic and biotic events, the precise drivers remain obscure. Here, we focus on the role of feeding ecology using a geometric morphometric analysis of 3,837 fossil and extant shark teeth. Our results reveal that morphological segregation rather than competition has characterized lamniform and carcharhiniform evolution. Moreover, although lamniforms suffered a long-term disparity decline potentially linked to dietary “specialization,” their recent disparity rivals that of “generalist” carcharhiniforms. We further confirm that low eustatic sea levels impacted lamniform disparity across the end-Cretaceous mass extinction. Adaptations to changing prey availability and the proliferation of coral reef habitats during the Paleogene also likely facilitated carcharhiniform dispersals and cladogenesis, underpinning their current taxonomic dominance. Ultimately, we posit that trophic partitioning and resource utilization shaped past shark ecology and represent critical determinants for their future species survivorship.
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| 3. |
- Ahlberg, Per E., 1963-, et al.
(författare)
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The smallest known Devonian tetrapod shows unexpectedly derived features
- 2020
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Ingår i: Royal Society Open Science. - : ROYAL SOC. - 2054-5703. ; 7:4
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Tidskriftsartikel (refereegranskat)abstract
- A new genus and species of Devonian tetrapod, Brittagnathus minutus gen. et sp. nov., is described from a single complete right lower jaw ramus recovered from the Acanthostega mass-death deposit in the upper part of the Britta Dal Formation (upper Famennian) of Stensio Bjerg, Gauss Peninsula, East Greenland. Visualization by propagation phase contrast synchrotron microtomography allows a complete digital dissection of the specimen. With a total jaw ramus length of 44.8 mm, Brittagnathus is by far the smallest Devonian tetrapod described to date. It differs from all previously known Devonian tetrapods in having only a fang pair without a tooth row on the anterior coronoid and a large posterior process on the posterior coronoid. The presence of an incipient surangular crest and a concave prearticular margin to the adductor fossa together cause the fossa to face somewhat mesially, reminiscent of the condition in Carboniferous tetrapods. A phylogenetic analysis places Brittagnathus crownward to other Devonian tetrapods, adjacent to the Tournaisian genus Pederpes. Together with other recent discoveries, it suggests that diversification of 'Carboniferous-grade' tetrapods had already begun before the end of the Devonian and that the group was not greatly affected by the end-Devonian mass extinction.
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| 4. |
- Bazzi, Mohamad, et al.
(författare)
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Static Dental Disparity and Morphological Turnover in Sharks across the End-Cretaceous Mass Extinction
- 2018
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Ingår i: Current Biology. - : CELL PRESS. - 0960-9822 .- 1879-0445. ; 28:16, s. 2607-2615
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Tidskriftsartikel (refereegranskat)abstract
- The Cretaceous-Palaeogene (K-Pg) mass extinction profoundly altered vertebrate ecosystems and prompted the radiation of many extant clades [1, 2]. Sharks (Selachimorpha) were one of the few larger-bodied marine predators that survived the K-Pg event and are represented by an almost-continuous dental fossil record. However, the precise dynamics of their transition through this interval remain uncertain [3]. Here, we apply 2D geometric morphometrics to reconstruct global and regional dental morphospace variation among Lamniformes (Mackerel sharks) and Carch-arhiniformes (Ground sharks). These clades are prevalent predators in today's oceans, and were geographically widespread during the late Cretaceous-early Palaeogene. Our results reveal a decoupling of morphological disparity and taxonomic richness. Indeed, shark disparity was nearly static across the K-Pg extinction, in contrast to abrupt declines among other higher-trophic-level marine predators [4, 5]. Nevertheless, specific patterns indicate that an asymmetric extinction occurred among lamniforms possessing lowcrowned/triangular teeth and that a subsequent proliferation of carcharhiniforms with similar tooth morphologies took place during the early Paleocene. This compositional shift in post-Mesozoic shark lineages hints at a profound and persistent K-Pg signature evident in the heterogeneity of modern shark communities. Moreover, such wholesale lineage turnover coincided with the loss of many cephalopod [6] and pelagic amniote [5] groups, as well as the explosive radiation of middle trophic-level teleost fishes [1]. We hypothesize that a combination of prey availability and post-extinction trophic cascades favored extant shark antecedents and laid the foundation for their extensive diversification later in the Cenozoic [7-10].
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| 5. |
- Bazzi, Mohamad, et al.
(författare)
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Tooth morphology elucidates shark evolution across the end-Cretaceous mass extinction
- 2021
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Ingår i: PLoS biology. - : Public Library of Science (PLoS). - 1544-9173 .- 1545-7885. ; 19:8
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Tidskriftsartikel (refereegranskat)abstract
- Sharks (Selachimorpha) are iconic marine predators that have survived multiple mass extinctions over geologic time. Their prolific fossil record is represented mainly by isolated shed teeth, which provide the basis for reconstructing deep time diversity changes affecting different selachimorph clades. By contrast, corresponding shifts in shark ecology, as measured through morphological disparity, have received comparatively limited analytical attention. Here, we use a geometric morphometric approach to comprehensively examine tooth morphologies in multiple shark lineages traversing the catastrophic end-Cretaceous mass extinction-this event terminated the Mesozoic Era 66 million years ago. Our results show that selachimorphs maintained virtually static levels of dental disparity in most of their constituent clades across the Cretaceous-Paleogene interval. Nevertheless, selective extinctions did impact apex predator species characterized by triangular blade-like teeth. This is particularly evident among lamniforms, which included the dominant Cretaceous anacoracids. Conversely, other groups, such as carcharhiniforms and orectolobiforms, experienced disparity modifications, while heterodontiforms, hexanchiforms, squaliforms, squatiniforms, and dagger synechodontiforms were not overtly affected. Finally, while some lamniform lineages disappeared, others underwent postextinction disparity increases, especially odontaspidids, which are typified by narrow-cusped teeth adapted for feeding on fishes. Notably, this increase coincides with the early Paleogene radiation of teleosts as a possible prey source, and the geographic relocation of disparity sampling "hotspots," perhaps indicating a regionally disjunct extinction recovery. Ultimately, our study reveals a complex morphological response to the end-Cretaceous mass extinction and highlights an event that influenced the evolution of modern sharks.
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| 6. |
- Chen, Dong Lei, 1985-, et al.
(författare)
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A partial lower jaw of a tetrapod from "Romer's Gap"
- 2018
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Ingår i: Earth and environmental science transactions of the Royal Society of Edinburgh. - 1755-6910 .- 1755-6929. ; 108:1, s. 55-65
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Tidskriftsartikel (refereegranskat)abstract
- The first half of the Mississippian or Early Carboniferous (Tournaisian to mid-Visean), an interval of about 20 million years, has become known as "Romer's Gap" because of its poor tetrapod record. Recent discoveries emphasise the differences between pre-"Gap" Devonian tetrapods, unambiguous stem-group members retaining numerous "fish" characters indicative of an at least partially aquatic lifestyle, and post-"Gap" Carboniferous tetrapods, which are far more diverse and include fully terrestrial representatives of the main crown-group lineages. It seems that "Romer's Gap" coincided with the cladogenetic events leading to the origin of the tetrapod crown group. Here, we describe a partial right lower jaw ramus of a tetrapod from the late Tournaisian or early Visean of Scotland. The large and robust jaw displays a distinctive character combination, including a significant mesial lamina of the strongly sculptured angular, an open sulcus for the mandibular lateral line, a non-ossified narrow Meckelian exposure, a well-defined dorsal longitudinal denticle ridge on the prearticular, and a mesially open adductor fossa. A phylogenctic analysis places this specimen in a trichotomy with Crassigyrinus and baphetids + higher tetrapods in the upper part of the tetrapod stem group, above Whatcheeria, Pederpes, Ossinodus, Sigournea and Greererpeton. It represents a small but significant step in the gradual closure of "Romer's Gap".
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| 7. |
- Chen, Dong Lei, 1985-, et al.
(författare)
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Development of cyclic shedding teeth from semi-shedding teeth : the inner dental arcade of the stem osteichthyan Lophosteus
- 2017
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Ingår i: Royal Society Open Science. - : ROYAL SOC. - 2054-5703. ; 4:5
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Tidskriftsartikel (refereegranskat)abstract
- The numerous cushion-shaped tooth-bearing plates attributed to the stem-group osteichthyan Lophosteus superbus, which are argued here to represent the ancient form of inner dental arcade, display a unique and presumably primitive way of tooth shedding by basal hard tissue resorption. They carry regularly spaced, recumbent, gently recurved teeth arranged in transverse tooth files that diverge towards the lingual margin of the cushion. Three-dimensional (3D) reconstruction from propagation phase contrast synchrotron microtomography (PPC-SRμCT) reveals remnants of the first-generation teeth embedded in the basal plate that have never been discerned in any taxa. These teeth were shed by semi-basal resorption with the periphery of their bases retained as dentine rings. The rings are highly overlapped, which evidences tooth shedding prior to adding the next first-generation tooth. Later teeth at the same sites underwent cyclical replacing and shedding through basal resorption, producing stacks of buried resorption surfaces separated by bone of attachment. The number and spatial arrangement of resorption surfaces elucidates that basal resorption of replacement teeth had taken place at the older tooth sites before the addition of the youngest first-generation teeth at the lingual margin. Thus the replacement tooth buds cannot have been generated by a single permanent dental lamina, but must have arisen either from successional dental laminae associated with the predecessor teeth, or directly from the dental epithelium of these teeth. The virtual histological dissection of these Late Silurian microfossils broadens our understanding of the development of the gnathostome dental systems and the acquisition of the osteichthyan-type of tooth replacement.
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| 8. |
- Chen, Donglei, 1985-, et al.
(författare)
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The developmental relationship between teeth and dermal odontodes in the most primitive bony fish Lophosteus
- 2020
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Ingår i: eLIFE. - 2050-084X. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- The ontogenetic trajectory of a marginal jawbone of Lophosteus superbus (Late Silurian, 422 Million years old), the phylogenetically most basal stem osteichthyan, visualized by synchrotron microtomography, reveals a developmental relationship between teeth and dermal odontodes that is not evident from the adult morphology. The earliest odontodes are two longitudinal founder ridges formed at the ossification center. Subsequent odontodes that are added lingually to the ridges turn into conical teeth and undergo cyclic replacement, while those added labially achieve a stellate appearance. Stellate odontodes deposited directly on the bony plate are aligned with the alternate files of teeth, whereas new tooth positions are inserted into the files of sequential addition when a gap appears. Successive teeth and overgrowing odontodes show hybrid morphologies around the oral-dermal boundary, suggesting signal cross-communication. We propose that teeth and dermal odontodes are modifications of a single system, regulated and differentiated by the oral and dermal epithelia.
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| 9. |
- Gai, Zhikun, et al.
(författare)
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The Evolution of the Spiracular Region From Jawless Fishes to Tetrapods
- 2022
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Ingår i: Frontiers in Ecology and Evolution. - : Frontiers Media S.A.. - 2296-701X. ; 10
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Tidskriftsartikel (refereegranskat)abstract
- The spiracular region, comprising the hyomandibular pouch together with the mandibular and hyoid arches, has a complex evolutionary history. In living vertebrates, the embryonic hyomandibular pouch may disappear in the adult, develop into a small opening between the palatoquadrate and hyomandibula containing a single gill-like pseudobranch, or create a middle ear cavity, but it never develops into a fully formed gill with two hemibranchs. The belief that a complete spiracular gill must be the ancestral condition led some 20th century researchers to search for such a gill between the mandibular and hyoid arches in early jawed vertebrates. This hypothesized ancestral state was named the aphetohyoidean condition, but so far it has not been verified in any fossil; supposed examples, such as in the acanthodian Acanthodes and symmoriid chondrichthyans, have been reinterpreted and discounted. Here we present the first confirmed example of a complete spiracular gill in any vertebrate, in the galeaspid (jawless stem gnathostome) Shuyu. Comparisons with two other groups of jawless stem gnathostomes, osteostracans and heterostracans, indicate that they also probably possessed full-sized spiracular gills and that this condition may thus be primitive for the gnathostome stem group. This contrasts with the living jawless cyclostomes, in which the mandibular and hyoid arches are strongly modified and the hyomandibular pouch is lost in the adult. While no truly aphetohyoidean spiracular gill has been found in any jawed vertebrate, the recently reported presence in acanthodians of two pseudobranchs suggests a two-step evolutionary process whereby initial miniaturization of the spiracular gill was followed, independently in chondrichthyans and osteichthyans, by the loss of the anterior pseudobranch. On the basis of these findings we present an overview of spiracular evolution among vertebrates.
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| 10. |
- Jeffery, Jonathan E., et al.
(författare)
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Unique pelvic fin in a tetrapod-like fossil fish, and the evolution of limb patterning
- 2018
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : NATL ACAD SCIENCES. - 0027-8424 .- 1091-6490. ; 115:47, s. 12005-12010
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Tidskriftsartikel (refereegranskat)abstract
- All living tetrapods have a one-to-two branching pattern in the embryonic proximal limb skeleton, with a single element at the base of the limb (the humerus or femur) that articulates distally with two parallel radials (the ulna and radius or the tibia and fibula). This pattern is also seen in the fossilized remains of stem-tetrapods, including the fishlike members of the group, in which despite the absence of digits, the proximal parts of the fin skeleton clearly resemble those of later tetrapods. However, little is known about the developmental mechanisms that establish and canalize this highly conserved pattern. We describe the well-preserved pelvic fin skeleton of Rhizodus hibberti, a Carboniferous sarcopterygian (lobe-finned) fish, and member of the tetrapod stem group. In this specimen, three parallel radials, each robust with a distinct morphology, articulate with the femur. We review this unexpected morphology in a phylogenetic and developmental context. It implies that the developmental patterning mechanisms seen in living tetrapods, now highly constrained, evolved from mechanisms flexible enough to accommodate variation in the zeugopod (even between pectoral and pelvic fins), while also allowing each element to have a unique morphology.
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