Fossil Focus

Fossil Focus: Marsupial evolution – A limited story?

Fossil Focus
by Verity Bennett*1 Introduction: There are three groups of mammals alive today: the egg-laying monotremes (echidnas and platypuses); the marsupials (those with pouches); and the placentals (those that develop a placenta in the womb and give birth to comparatively developed young). Marsupials and placentals are sister groups, more closely related to each other than to monotremes. Along with their closest fossil ancestors, marsupials belong to the clade metatheria, whereas placentals belong to the clade eutheria. Together, metatheria and eutheria comprise the therian mammals. Marsupials are much less diverse than placental mammals in terms of numbers of different groups, range of lifestyles, range of body shapes and where they live. Why this is the case is still not well understood, and a

Fossil Focus: Pterosaurs

Fossil Focus
by David W. E. Hone*1 Introduction: Pterosaurs are often mistakenly called flying dinosaurs, but they are a distinct, although related, lineage. They are an extinct group of reptiles from the Mesozoic era (251 million to 66 million years ago) and were the first vertebrates to evolve powered flight (Figs 1 and 2). Pterosaurs were first described as early as 1783 and recognized as flying reptiles shortly afterwards, and more than 150 species are now known. Fossil pterosaurs have been found around the world, with every continent yielding specimens. Adult pterosaurs ranged in size from around 1 metre in wingspan to more than 10 metres; the largest species were the biggest flying animals of all time. They occupied the skies for much of the Mesozoic era and had the air to themselves unt...

Fossil Focus: Using Plant Fossils to Understand Past Climates and Environments

Fossil Focus
by Leyla J. Seyfullah*1 Introduction: Fossils provide us with our only direct record of prehistoric life. Studying them can help us to reconstruct the anatomy, behaviour and evolution of long-extinct organisms. Perhaps less obviously, fossils are also among the most important sources of information for scientists attempting to learn about past (palaeo) climates and environments — a major focus of research in Earth and environmental sciences, motivated in part by concerns over future climate change. Fossil plants (Fig. 1), in particular, can be useful for decoding past climate signals. Most plants are terrestrial (meaning that they live on land). They are generally incapable of moving around, and so are totally dependent on the atmosphere and the soil or rock (substrate) on which they gro

Fossil Focus: Xiphosura

Fossil Focus
by Jason A. Dunlop*1 Introduction: The Xiphosura are commonly known as horseshoe crabs because the front part of their bodies is horseshoe-shaped. They have sometimes been called king crabs, although this name is also used for a group of large true crabs. Despite their various common names, xiphosurans are not crustaceans. Older studies assumed that they were some sort of crab, mostly because they have gills and live in the sea, but careful anatomical studies towards the end of the nineteenth century showed that they are actually more closely related to arachnids. The name Xiphosura means ‘sword tail’ and refers to another obvious feature of these animals: a long, pointed tail spine. Horseshoe crabs — especially earlier fossil ones — also look quite a lot like trilobites. This has led to

Fossil Focus: The preservation of colour

Fossil Focus
by Holly E. Barden*1 Introduction: Colour is important in modern ecosystems, but the colours of extinct organisms are very rarely preserved in the fossil record. Colouration is most commonly seen in fossilized brachiopod shells and arthropod carapaces; however, establishing that these colours are original and not artefacts of fossilization processes is difficult. Until recently, few studies have attempted to do so, but within the past few years the subject has become an active area of research, with significant developments. There have been several studies investigating the morphological and geochemical evidence of pigments in birds and dinosaurs, as well as work on the colouration of insects. Such analyses have paved the way for major leaps forward in our understanding of the behaviour ...

Fossil Focus: Animal embryos

Fossil Focus
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

Fossil Focus: Vertebrate tracks and trackways

Fossil Focus
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...

Fossil Focus: Arachnida

Fossil Focus
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

Fossil Focus: Chasmataspidida

Fossil Focus
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

Fossil Focus: Coal swamps

Fossil Focus
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 ...