Ashok Sahni

A new genus and species of fossil pseudoscorpion (Arachnida: Pseudoscorpiones) from the Eocene amber of Western India

During the last two decades, work carried out in the Eocene lignite sequences of Rajasthan and Gujarat has thrown considerable light on the terrestrial biotas during the drift of India and radically changed concepts regarding how the Indian plate was populated, the issues of cosmopolitanism and endemism, and the nature of the mixed sal-dominated forested community that supported a diverse mammalian fauna. While Madagascar and Australia, India’s erstwhile Gondwanan neighbours, supported highly endemic faunas, the biota of India shows strong affinities with Africa, Madagascar, Europe, and North America.

We describe new skeletal elements of Vastanavis from the early Eocene Cambay Shale Formation of western India, including a small coracoid that represents an unnamed new species, and comment on the relationships between this avian taxon and the recently described Avolatavis from the early Eocene Green River Formation in North America. Like the previously described ones, the new Vastanavis bones resemble those of the late Eocene Quercypsittidae, thus strengthening psittaciform affinities of the Indian taxon. Vastanavis differs from Avolatavis in the presence of a crista medianoplantaris on the tarsometatarsus and in claw morphology, but a fossil from the early Eocene London Clay, which was previously assigned to Vastanavidae, closely resembles Avolatavis in these features and all other osteological aspects. We show that most branches in a recent phylogeny of stem group Psittaciformes collapse after modification of a single erroneous character scoring for Vastanavis. We further describe a morphologically distinctive distal humerus of a small bird resembling the stem group nyctibiid Paraprefica, which was discovered in the most recent excavation in Vastan Lignite Mine.

The present review focuses on some specific aspects of biomineralization with regard to the evolution of the first focused visioning systems in trilobites, the formation of molluscan shell architecture, dental enamel and its biomechanical properties and the structure of the calcified amniote egg, both fossil and recent. As an interdisciplinary field, biomineralization deals with the formation, structure and mechanical strength of mineralized skeletonized tissue secreted by organisms. Mineral matter formed in this way occurs in all three domains of life and consists of several mineral varieties, of which carbonates, phosphates and opaline silica are the most common. Animals and plants need mechanical support to counteract gravitational forces on land and hydrostatic pressure in the deep ocean, which is provided by a skeletonized framework. Skeleton architecture mainly consists of basic elements represented by small usually micrometer- to nanometer-sized crystallites of calcite and aragonite for carbonate systems and apatite crystallites for phosphatic ones, and then these building blocks develop into structured more complex frameworks. As selective pressures work towards optimizing stress and response, the orientation, morphology and structural arrangement of the crystallites indicates the distribution of the stress field of the biomineralized tissue. Large animals such as the dinosaurs have to deal with large gravitational forces, but in much smaller skeletonized organism such as the coccoliths, a few micrometer in diameter made up of even smaller individual crystallites, van der Waals forces play an increasingly important role and are at present poorly understood. Skeleton formation is dependent upon many factors including ambient water chemistry, temperature and environment. Ocean chemistry has played a vital role in the origins of skeletonization, 500 to 600 million years (ma) ago with the dominance of calcium carbonate as the principal skeleton-forming tissue and with phosphates and silica as important but secondary materials. The preservation of calcareous skeletons in deep time has resulted in providing interesting information: for example, the number of days in the Devonian year has been established on the basis of well-preserved lunar (annual) cycles, and isotope chemistry has led to an elaborate protocol for using O¹⁸/O¹⁶ stable isotopes for palaeotemperature measurements in the geological past. Stable isotopes of dental apatite have helped to establish ecological shifts (terrestrial to wholly marine) during the evolution of the Cetacea. Biomineralization as a field of specialization is still searching for its own independent identity, but gradually, its importance is being realized as a model for engineering applications especially at the nanometer scale.

A new tillodont, Anthraconyx hypsomylus, n. gen. n. sp., is described from the early Eocene Cambay Shale Formation at Vastan Lignite Mine, Gujarat, India. Anthraconyx hypsomylus is the smallest Eocene tillodont and is distinguished by having the most buccally hypsodont cheek teeth of any known esthonychine. The closest dental resemblances are to North American Esthonyx and Azygonyx and European Plesiesthonyx, providing further evidence of affinities between the Vastan local fauna and Euroamerican vertebrate faunas.