Aurore Canoville & Anusuya Chinsamy (2016)
Bone microstructure of pareiasaurs (Parareptilia) from the Karoo Basin, South Africa: implications for growth strategies and lifestyle habits.
Anatomical Record (advance online publication)
Numerous morphological studies have been carried out on pareiasaurs; yet their taxonomy and biology remain incompletely understood. Earlier works have suggested that these herbivorous parareptiles had a short juvenile period as compared to the duration of adulthood. Several studies further suggested an (semi-) aquatic lifestyle for these animals, but more recent investigations have proposed a rather terrestrial habitat.
Bone paleohistology is regarded as a powerful tool to assess aspects of tetrapod paleobiology, but few studies have been conducted on pareiasaurs. The present study assesses intra and inter-specific histovariability of pareiasaurs and provides fresh insights into their paleobiology, thereby permitting a re-evaluation of earlier hypotheses. Our sample comprises various skeletal elements and several specimens covering most of the taxonomic and stratigraphic spectrum of South African pareiasaurs, including large and basal forms from the Middle Permian, as well as smaller and more derived forms from the Late Permian.
Our results concerning size of elements and histological tissues show that for pareiasaurs, element size is not a good indicator of ontogenetic age, and furthermore, suggest that the specific diversity of the Middle Permian pareiasaurs may have been underestimated. The bone histology of these animals shows that they experienced a relatively rapid growth early in ontogeny. The periosteal growth later slowed down, but seems to have been protracted for several years during adulthood. Pareiasaur bone microanatomy is unusual for continental tetrapods, in having spongious stylopod diaphyses and thin compact cortices. Rigorous paleoecological interpretations are thus limited since no modern analogue exists for these animals.
E.M. Bordy, L. Sciscio, F. Abdala, B.W. McPhee & J.N. Choiniere (2016)
First Lower Jurassic vertebrate burrow from southern Africa (upper Elliot Formation, Karoo Basin, South Africa).
Palaeogeography, Palaeoclimatology, Palaeoecology (advance online publication)
The first Lower Jurassic burrow casts from South Africa is described.
The host Elliot Formation contains body fossils that show fossorial adaptations.
Burrow casts are found in a pedogenically modified crevasse splay sandstone.
Burrow casts are attributed to advanced tritheledontid cynodonts.
Fossoriality in non-mammalian cynodonts persisted for > 50 million years.
Vertebrate burrows are common ichnofossils in the Permo-Triassic of the main Karoo Basin in South Africa. They are generally attributable to one of several lineages of therapsid, including the derived clade known as cynodonts. Despite the presence of cynodont species in the Upper Triassic and Lower Jurassic of the Karoo Supergroup, vertebrate burrows have never been reported from this part of the succession. Recent fieldwork recovered a semi-elliptical burrow cast in the Lower Jurassic upper Elliot Formation (Stormberg Group) on the farm Edelweiss 698 (Free State). The horizontal and vertical diameters of the burrow cast are ~ 18 and ~ 7 cm, respectively. This semi-horizontal, straight to slightly sinuous tunnel is ~ 50 cm long with a ramp angle of < 5°. The tunnel lacks branching, terminal chambers, and associated fossil bones. The burrow cast consists of medium, massive sandstone and very rare, faint, horizontal to slightly inclined lamination. The burrow cast is hosted in fine-grained, palaeo-pedogenically altered, crevasse splay sandstone that is 10–20 cm thick and is under- and overlain by a massive, red, bioturbated floodplain mudstone unit with large-scale (> 20 cm deep) desiccation cracks, invertebrate trace fossils, calcareous rhizoconcretions, and spherical-to-elongated carbonate nodules. These and other associated sedimentary features provide evidence for a semi-arid, fluvio-lacustrine palaeoenvironment during the burrowing activity. Based on comparisons to fossil and modern burrows, this burrow cast is interpreted as a vertebrate burrow, and is the first record of vertebrate fossorial activity within the Lower Jurassic of southern Africa. The ancient burrow architect has yet to be positively identified. However, given the size and morphology of the burrow and the occurrence of similar sized fossil cynodont therapsids that inhabited the main Karoo Basin in the earliest Jurassic, the potential burrow-maker may be tentatively linked to the Cynodontia (e.g., Pachygenelus - an advanced tritheledontid).
Aurélie Kapusta and Alexander Suh (2016)
Evolution of bird genomes—a transposon's-eye view.
Annals of the New York Academy of Sciences (advance online publication)
Birds, the most species-rich monophyletic group of land vertebrates, have been subject to some of the most intense sequencing efforts to date, making them an ideal case study for recent developments in genomics research. Here, we review how our understanding of bird genomes has changed with the recent sequencing of more than 75 species from all major avian taxa. We illuminate avian genome evolution from a previously neglected perspective: their repetitive genomic parasites, transposable elements (TEs) and endogenous viral elements (EVEs). We show that (1) birds are unique among vertebrates in terms of their genome organization; (2) information about the diversity of avian TEs and EVEs is changing rapidly; (3) flying birds have smaller genomes yet more TEs than flightless birds; (4) current second-generation genome assemblies fail to capture the variation in avian chromosome number and genome size determined with cytogenetics; (5) the genomic microcosm of bird–TE “arms races” has yet to be explored; and (6) upcoming third-generation genome assemblies suggest that birds exhibit stability in gene-rich regions and instability in TE-rich regions. We emphasize that integration of cytogenetics and single-molecule technologies with repeat-resolved genome assemblies is essential for understanding the evolution of (bird) genomes.
Lorenzo Alibardi (2016)
Review: cornification, morphogenesis and evolution of feathers.
Protoplasma (advance online publication)
Feathers are corneous microramifications of variable complexity
derived from the morphogenesis of barb ridges. Histological and ultrastructural
analyses on developing and regenerating feathers clarify the three-dimensional
organization of cells in barb ridges. Feather cells derive from folds of the
embryonic epithelium of feather germs from which barb/barbule cells and
supportive cells organize in a branching structure. The following degeneration
of supportive cells allows the separation of barbule cells which are made of
corneous beta-proteins and of lower amounts of intermediate filament
(IF)(alpha) keratins, histidine-rich proteins, and corneous proteins of the
epidermal differentiation complex. The specific protein association gives rise
to a corneous material with specific biomechanic properties in barbules, rami,
rachis, or calamus. During the evolution of different feather types, a large
expansion of the genome coding for corneous feather beta-proteins occurred and
formed 3–4-nm-thick filaments through a different mechanism from that of 8–10
nm IF keratins. In the chick, over 130 genes mainly localized in chromosomes 27
and 25 encode feather corneous beta-proteins of 10–12 kDa containing 97–105
amino acids. About 35 genes localized in chromosome 25 code for scale proteins
(14–16 kDa made of 122–146 amino acids), claws and beak proteins (14–17 kDa
proteins of 134–164 amino acids). Feather morphogenesis is periodically
re-activated to produce replacement feathers, and multiple feather types can
result from the interactions of epidermal and dermal tissues. The review shows
schematic models explaining the translation of the morphogenesis of barb ridges
present in the follicle into the three-dimensional shape of the main types of
branched or un-branched feathers such as plumulaceous, pennaceous, filoplumes,
and bristles. The temporal pattern of formation of barb ridges in different
feather types and the molecular control from the dermal papilla through
signaling molecules are poorly known. The evolution and diversification of the
process of morphogenesis of barb ridges and patterns of their formation within
feathers follicle allowed the origin and diversification of numerous types of
feathers, including the asymmetric planar feathers for flight.
Aditya Saxena, Matthew Towers & Kimberly L. Cooper (2017)
Review article: The origins, scaling and loss of tetrapod digits.
Philosophical Transactions of the Royal Society B 2017 372 20150482
Many of the great morphologists of the nineteenth century marvelled at similarities between the limbs of diverse species, and Charles Darwin noted these homologies as significant supporting evidence for descent with modification from a common ancestor. Sir Richard Owen also took great care to highlight each of the elements of the forelimb and hindlimb in a multitude of species with focused attention on the homology between the hoof of the horse and the middle digit of man. The ensuing decades brought about a convergence of palaeontology, experimental embryology and molecular biology to lend further support to the homologies of tetrapod limbs and their developmental origins. However, for all that we now understand about the conserved mechanisms of limb development and the development of gross morphological disturbances, little of what is presented in the experimental or medical literature reflects the remarkable diversity resulting from the 450 million year experiment of natural selection. An understanding of conserved and divergent limb morphologies in this new age of genomics and genome engineering promises to reveal more of the developmental potential residing in all limbs and to unravel the mechanisms of evolutionary variation in limb size and shape. In this review, we present the current state of our rapidly advancing understanding of the evolutionary origin of hands and feet and highlight what is known about the mechanisms that shape diverse limbs.
This article is part of the themed issue ‘Evo-devo in the genomics era, and the origins of morphological diversity’.