Polish Footprints…

A research paper presented in the Proceedings of the Royal Society has revealed tiny dinosaur footprints in the sedimentary rocks of central Poland. These rocks are 250 million years old, laid down in the Early–Middle Triassic period, which is 5 million years older than any previous dinosaur tracks. It is just a couple of million years after the Permian-Triassic extinction when much of the previous life on Earth was wiped out (probably the result of climate change combined with an asteroid collision or super-volcano or both; an unfortunate coincidence!). Despite the unimaginably long timescale, in comparison with the history of life on Earth it is fairly dramatic action. Geologists divide history up into Eons, Eras, Periods and Epochs. The Archean Eon runs from the earliest recorded rocks to 2.5 billion years ago when the first evidence of simple life begins to appear. This marks the beginning of the 2 billion year Proterozoic Eon; you may know that protozoa means “first animals”. The Phanerozoic Eon covers the last 500 million years

The Earth condensed out of a swirling mass of dust about 4.5 billion years ago. At some point over the following 2 billion years some organic molecules developed that could copy themselves. From these simple beginnings life emerged and for the next 2 billion years the dominant life forms on Earth were very simple single-celled or small multi-celled plants and animals. The processes of evolution are slow but about 540 million years ago we entered the Cambrian Period (part of the Paleozoic Era), which was a 50 million year period of extraordinary diversity in plant and animal life; it is often referred to as the Cambrian Explosion because so many new and complex forms appeared in such rapid succession. The first vertebrates appeared in the Cambrian and then Ordovician Periods. Scorpions evolved in the Silurian Period, land invertebrates and the first amphibians evolved during the Devonian Period. The Carboniferous Period is marked by the appearance of the huge forests that have since become coal beds. Later in this period the first reptiles appeared on land then in the Permian Period reptiles returned to the sea.

The Permian-Triassic boundary also marked the transition to the Mesozoic Era which spanned the time from 251 million years ago to 67 million years ago. This comprises the Triassic, Jurassic and Cretaceous Periods. The huge extinction opened up a large number of ecological niches allowing a wide variety of species to evolve, and within 30 million years there was a great diversity of dinosaurs and other marine and land vertebrates. During the Jurassic Period the first birds and mammals evolved and at some point during the Cretaceous Period the first fruit bearing plants appeared. Towards the end of the Cretaceous Period, as global temperatures began to increase, the larger animals started to decrease in number. About 65.5 million years ago, something pretty dramatic happened to trigger another mass extinction event. It was probably a meteor impact or increased volcanic activity or, possibly, both.

This extinction marks the boundary between the Mesozoic Era and the Cenozoic Era. This Era includes the Paleogene and Neogene Periods, which are sub-divided into five epochs called the Paleocene, Eocene & Oligocene and the Miocene and Pliocene (respectively). This is the Era when mammals and birds diversified. The oceans contained great whales and the woolly mammoths and sabre-toothed tigers evolved on land. The end of the Pliocene (2.58 million years ago) was accompanied by a cooling and drying of the planet. This also marked the start of the Quaternary Period, the first part of which was called the Pleistocene Epoch and the second part (from 12,000 years ago), called the Holocene Epoch. Modern humans appeared during the Pleistocene about 200,000 years ago. The transition to the Holocene is indicated by the retreat of the Baltic-Scandinavian Ice Age.

I stitched the diagram below together from various bits I found on the Internet – it probably explains things much more clearly.