by John Cunningham*1 Introduction: Animal embryos are small (typically less than 1 millimetre across), soft and squidgy, so it was traditionally considered impossible for them to be preserved in the fossil record. However, over the past 15 years or so a series of remarkable discoveries have shown that embryos can indeed be fossilized under exceptional circumstances. The microscopic fossils that have been identified as embryos are almost exclusively from the Ediacaran and Cambrian periods, around 635 million to 488 million years ago. This spans the period of time when the major groups of animals are thought to have first appeared, so these fossils allow palaeontologists to study the embryology of some of the earliest animals, shedding light on the evolution of development. The first fo
by Peter Falkingham*1 Introduction: The fossilized footprints and trackways of vertebrates are often overlooked in favour of the skeletal remains of the animals that made them. At museums, for instance, many more people will crowd around the dinosaur skeletons than around the dinosaur tracks nearby, and yet fossilized tracks can provide us with information about extinct animals that is simply not available from the bones alone. A track is the result of an interaction between an animal and a surface, or substrate. The final track shape (morphology) is directly determined by three factors: Producer: the shape of the track-maker's foot Behaviour: the motion and loading (kinematics and kinetics) of that foot Substrate: the conditions of the surface when the track is made (sandy, mudd...
by Jason A. Dunlop*1 Introduction: Arachnida is one of the major arthropod groups. It includes spiders (Araneae), scorpions (Scorpiones), mites (Acari) and harvestmen (Opiliones), as well as a number of rarer and less familiar groups (Fig 1). The name Arachnida was introduced by the French zoologist Jean-Baptise Lamarck and is derived from Greek mythology: in one story, the maiden Arachne challenged the goddess Athene to a weaving contest, and was subsequently transformed into a spider — condemned to weave for evermore. There are about 100,000 living species of arachnids, with mites and spiders representing the most diverse and species-rich groups. Fossil arachnids are considerably rarer, with more than 1,700 described species (well over half of which are spiders) and a record that exten
by Jason A. Dunlop*1 Introduction: Chasmataspidida (Fig 1) are rare, extinct arthropods known only from the early to mid Palaeozoic Era. They are probably closely related to either xiphosurans (horseshoe crabs; Fig 2) or eurypterids (sea scorpions; Fig 1); some chasmataspid fossils were originally misinterpreted as members of one of these two groups. They were first discovered in the 1950s, and were only recently recognized as a group distinct from horseshoe crabs. Chasmataspids are currently the oldest known examples of the Euchelicerata lineage —all Chelicerata except Pycnogonida (sea spiders) — and palaeontologists date the origins of these euchelicerates back to at least the late Cambrian Period. Morphology: Most chasmataspids are a few centimetres long. The body is divided int
by Phil Jardine*1 Introduction: The Paleocene–Eocene Thermal Maximum (PETM) is one of the most intense and abrupt intervals of global warming in the geological record. It occurred around 56 million years ago, at the boundary between the Paleocene and Eocene epochs. This warming has been linked to a similarly rapid increase in the concentration of greenhouse gases in Earth’s atmosphere, which acted to trap heat and drive up global temperatures by more than 5 °C in just a few thousand years. The fossil record gives us the means of understanding how life was affected by the PETM, and so provides an excellent opportunity to study the relationships between evolution, extinction, migration and climate change. The early Palaeogene world: At the time of the PETM, the world was already much w
by Ben Slater*1 Introduction: Coal swamps are the classical terrestrial (land-based) ecosystems of the Carboniferous and Permian periods. They are forests that grew during the Palaeozoic Era (encompassing the Carboniferous and Permian) in which the volume of plant biomass dying and being deposited in the ground was greater than the volume of clastic (grains of pre-existing rock) material, resulting in a build-up of peat. This was subsequently buried, and eventually turned into coal over geological time. These swamps gave rise to most of the major, industrial-grade coal reserves that are mined today. The palaeontology of these coal-forming ecosystems is well known from the Carboniferous rocks of Euramerica (modern day Europe and North America), owing to the history of coal exploitation ...
by Jason A. Dunlop*1 Introduction: Pycnogonida, or sea spiders, are not true spiders at all. They are in fact a group of — probably rather primitive — marine arthropods, characterized by a small, slender body and in many cases by correspondingly long legs (Fig. 1). So unusual is their morphology that many of their internal-organ systems have been displaced into the legs. Because of their strange appearance, older studies occasionally referred to them as ‘nobody crabs’ (literally crabs without a body) — although it is important to stress that they are not crustaceans, any more than they are spiders. Pycnogonids are thought either to have evolved right at the very base of the arthropod tree — and thus not to be closely related to any particular group of arthropods — or to be related to ara
by Alistair J. McGowan*1 Introduction: Biological diversity, or biodiversity, shot to prominence among non-specialists in 1992, after the Rio Earth Summit (Fig. 1). Media coverage of the summit did a tremendous amount to raise awareness of the need to gather baseline data on species, and of the spectre of extinction hanging over some of them. The international Convention on Biodiversity declared 2010 the International Year of Biodiversity, and 2011–20 the Decade of Biodiversity. The use of the term biodiversity in the media has increased greatly, and the word is now in general use. Many countries now have biodiversity action plans that start locally and move through various levels and habitat types to the national level (for example, see the United Kingdom’s Biodiversity Action Plan).
by Jason A. Dunlop*1 Introduction: Chelicerata is one of the main divisions of the arthropods, and essentially consists of arachnids and their closest relatives. The name was coined in 1901 by the Berlin-based zoologist Richard Heymons (Fig. 1). It means the ‘claw-bearers’, in reference to the claw- or fang-shaped mouthparts that characterize the group. In addition to the arachnids, Chelicerata also includes the horseshoe crabs (Xiphosura), the extinct sea scorpions (Eurypterida) and little-known chasmatapaids (Chasmataspidida), and the sea spiders (Pycnogonida). The inclusion of sea spiders within this group is controversial, as we shall see below, and arachnids, horseshoe crabs, eurypterids and chasmataspids are sometimes grouped together as the Euchelicerata. The name Merostoma