Home About us Contact
|
Home About us Contact | ||
|
|
||
Primitive Form (primitive + form)
Selected AbstractsSix major steps in animal evolution: are we derived sponge larvae?EVOLUTION AND DEVELOPMENT, Issue 2 2008Claus Nielsen SUMMARY A review of the old and new literature on animal morphology/embryology and molecular studies has led me to the following scenario for the early evolution of the metazoans. The metazoan ancestor, "choanoblastaea," was a pelagic sphere consisting of choanocytes. The evolution of multicellularity enabled division of labor between cells, and an "advanced choanoblastaea" consisted of choanocytes and nonfeeding cells. Polarity became established, and an adult, sessile stage developed. Choanocytes of the upper side became arranged in a groove with the cilia pumping water along the groove. Cells overarched the groove so that a choanocyte chamber was formed, establishing the body plan of an adult sponge; the pelagic larval stage was retained but became lecithotrophic. The sponges radiated into monophyletic Silicea, Calcarea, and Homoscleromorpha. Homoscleromorph larvae show cell layers resembling true, sealed epithelia. A homoscleromorph-like larva developed an archenteron, and the sealed epithelium made extracellular digestion possible in this isolated space. This larva became sexually mature, and the adult sponge-stage was abandoned in an extreme progenesis. This eumetazoan ancestor, "gastraea," corresponds to Haeckel's gastraea. Trichoplax represents this stage, but with the blastopore spread out so that the endoderm has become the underside of the creeping animal. Another lineage developed a nervous system; this "neurogastraea" is the ancestor of the Neuralia. Cnidarians have retained this organization, whereas the Triploblastica (Ctenophora+Bilateria), have developed the mesoderm. The bilaterians developed bilaterality in a primitive form in the Acoelomorpha and in an advanced form with tubular gut and long Hox cluster in the Eubilateria (Protostomia+Deuterostomia). It is indicated that the major evolutionary steps are the result of suites of existing genes becoming co-opted into new networks that specify new structures. The evolution of the eumetazoan ancestor from a progenetic homoscleromorph larva implies that we, as well as all the other eumetazoans, are derived sponge larvae. [source] Early hominid hunting and scavenging: A zooarcheological reviewEVOLUTIONARY ANTHROPOLOGY, Issue 6 2003Manuel Domínguez-Rodrigo Abstract Before the early 1980s, the prevailing orthodoxy in paleoanthropology considered Early Stone Age archeological sites in East Africa to represent a primitive form of hominid campsites. The faunal evidence preserved in these sites was viewed as the refuse of carcass meals provided by hominid males in a social system presumptively characterized by sexual division of labor. This interpretation of early hominid life ways, commonly known as the "Home Base" or "Food Sharing" model, was developed most fully by Glynn Isaac.1,4 As Bunn and Stanford5 emphasized, this model was greatly influenced by a paradigm that coalesced between 1966 and 1968, referred to as "Man the Hunter."6 [source] High-resolution, monotone solution of the adjoint shallow-water equationsINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 2 2002Brett F. Sanders Abstract A monotone, second-order accurate numerical scheme is presented for solving the differential form of the adjoint shallow-water equations in generalized two-dimensional coordinates. Fluctuation-splitting is utilized to achieve a high-resolution solution of the equations in primitive form. One-step and two-step schemes are presented and shown to achieve solutions of similarly high accuracy in one dimension. However, the two-step method is shown to yield more accurate solutions to problems in which unsteady wave speeds are present. In two dimensions, the two-step scheme is tested in the context of two parameter identification problems, and it is shown to accurately transmit the information needed to identify unknown forcing parameters based on measurements of the system response. The first problem involves the identification of an upstream flood hydrograph based on downstream depth measurements. The second problem involves the identification of a long wave state in the far-field based on near-field depth measurements. Copyright © 2002 John Wiley & Sons, Ltd. [source] New hominid fossils from Woranso-Mille (Central Afar, Ethiopia) and taxonomy of early AustralopithecusAMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY, Issue 3 2010Yohannes Haile-Selassie Abstract The phylogenetic relationship between Australopithecus anamensis and Australopithecus afarensis has been hypothesized as ancestor-descendant. However, the weakest part of this hypothesis has been the absence of fossil samples between 3.6 and 3.9 million years ago. Here we describe new fossil specimens from the Woranso-Mille site in Ethiopia that are directly relevant to this issue. They derive from sediments chronometrically dated to 3.57,3.8 million years ago. The new fossil specimens are largely isolated teeth, partial mandibles, and maxillae, and some postcranial fragments. However, they shed some light on the relationships between Au. anamensis and Au. afarensis. The dental morphology shows closer affinity with Au. anamensis from Allia Bay/Kanapoi (Kenya) and Asa Issie (Ethiopia) than with Au. afarensis from Hadar (Ethiopia). However, they are intermediate in dental and mandibular morphology between Au. anamensis and the older Au. afarensis material from Laetoli. The new fossils lend strong support to the hypothesized ancestor-descendant relationship between these two early Australopithecus species. The Woranso-Mille hominids cannot be unequivocally assigned to either taxon due to their dental morphological intermediacy. This could be an indication that the Kanapoi, Allia Bay, and Asa Issie Au. anamensis is the primitive form of Au. afarensis at Hadar with the Laetoli and Woranso-Mille populations sampling a mosaic of morphological features from both ends. It is particularly difficult to draw a line between Au. anamensis and Au. afarensis in light of the new discoveries from Woranso-Mille. The morphology provides no evidence that Au. afarensis and Au. anamensis represent distinct taxa. Am J Phys Anthropol 2010. © 2009 Wiley-Liss, Inc. [source] Functional anatomy of the forelimb in Promegantereon* ogygia (Felidae, Machairodontinae, Smilodontini) from the Late Miocene of Spain and the origins of the sabre-toothed felid modelJOURNAL OF ANATOMY, Issue 3 2010Manuel J. Salesa Abstract We examine the functional anatomy of the forelimb in the primitive sabre-toothed cat Promegantereon ogygia in comparison with that of the extant pantherins, other felids and canids. The study reveals that this early machairodontine had already developed strong forelimbs and a short and robust thumb, a combination that probably allowed P. ogygia to exert relatively greater forces than extant pantherins. These features can be clearly related to the evolution of the sabre-toothed cat hunting method, in which the rapid killing of prey was achieved with a precise canine shear-bite to the throat. In this early sabre-toothed cat from the Late Miocene, the strong forelimbs and thumb were adapted to achieve the rapid immobilization of prey, thus decreasing the risk of injury and minimizing energy expenditure. We suggest that these were the major evolutionary pressures that led to the appearance of the sabre-toothed cat model from the primitive forms of the Middle Miocene, rather than the hunting of very large prey, although these adaptations reached their highest development in the more advanced sabre-toothed cats of the Plio-Pleistocene, such as Smilodon and Homotherium. Although having very different body proportions, these later animals developed such extremely powerful forelimbs that they were probably able to capture relatively larger prey than extant pantherins. [source] Middle Jurassic Coptoclavidae (Insecta: Coleoptera: Dytiscoidea) from China: a Good Example of Mosaic EvolutionACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2010WANG Bo Abstract: Adults of the aquatic coptoclavid beetles (Coleoptera: Adephaga: Dytiscoidea), described from four Middle Jurassic fossil localities in Inner Mongolia and Liaoning in northeastern China, are attributed to Daohugounectes primitives Wang, Ponomarenko and Zhang, 2009, which was previously proposed after study of larvae. The generic name Timarchopsis Brauer, Redtenbacher and Ganglbauer, 1889 is proposed as a substitute for the preoccupied and junior homonym Necronectes Ponomarenko, 1977, non Milne-Edwards, 1881. Furthermore, the subfamily name Necronectinae Ponomarenko, 1977 is substituted by the available name Timarchopsinae. Daohugounectes is placed into Timarchopsinae because its adults have long, slightly apically widened tibiae and small femoral plates. The adults of this genus differ from those of other Timarchopsinae in the following features: antennae short and widened in the middle part; basal segments of protarsi not cut apically; metaventrite with a triangular plate. The larvae look like somewhat primitive forms in the subfamily Timarchopsinae. In contrast to these primitive larvae, the adults with some advanced characters can be regarded as among the most advanced forms in the subfamily Timarchopsinae, and probably represent a transition between the Timarchopsinae and Charanoscaphinae. Such mosaic evolution within Daohugounectes indicates that the evolutionary process of aquatic beetles is far more complex than previously thought. [source] | ||||||||||