paleontology 2: The emergence of the skeleton (v1.0)

In the previous essay I traced the evolution of life from four billion years ago to about six hundred million years ago.

The Cambrian period stretched from 541 million years to 485 million years. There was a rapid diversification of animals in that period called the Cambrian explosion. Preceding the Cambrian period is the Precambrian era with the Ediacaran period as the last period of that era stretching for ninety-five million years (from 635 million years ago).

As described in the previous essay, multicellular animals first emerged in the Ediacaran period about six hundred million years ago. There was also an explosion of multicellular animals in this period called the Avalon explosion about 575 million years ago (about thirty-five million years before the Cambrian explosion). During this time, many animals developed bilateral symmetry with a clear anterior and posterior side, along with increasing complexity. Many animals during this time fit into the annelid (segmented worms), arthropod (invertebrate animals), echinoderm (radially symmetrical, spiny skin), and cnidarian (radially symmetrical and simple organization around a central body cavity) phyla.

Skeletons in animals emerged about 550 million years ago and is the next major evolutionary change. There are two types of skeletons – exoskeleton (on the outside) and endoskeleton (on the inside). This is incredibly significant because it is much easier to find and study skeletons than traces of animals who are soft bodied that preceded that. Why did skeletons emerge? Scientists have proposed atmospheric oxygen levels rose and the chemistry of the oceans changed in such a way that animals could harness the minerals required to build hard structural parts.

In the Cambrian explosion (about 538 million years ago), many of the major simpler animal phyla that exist today started appearing in the fossil record. It lasted for 13 to 25 million years. Trilobites (means three lobes – these are now extinct) form one of the earliest known groups of arthropods that appeared in this period. The distinguishing feature of arthropods is the presence of a jointed exoskeletal covering. The body is usually segmented, and the segments bear paired jointed appendages (examples are lobster, crab, and insects today). Mollusks (example snails today) appeared in this period also.

The transition to vertebrates was a major evolution change. Animals with endoskeletons are called vertebrates, although some vertebrates also have a dermal exoskeleton.  Vertebrates are a well-known group of animals that includes today’s mammals, birds, reptiles, amphibians, and fish. The defining characteristic of vertebrates is their backbone, an anatomical feature that first appeared in the fossil record during the Ordovician period immediately after the Cambrian period. The Ordovician period began 485 million years ago, following the Cambrian period and ended 444 million years ago, when the Silurian period began.  Animal endoskeletons starts with the cranium and proceeds on to jaw and then the vertebrae. The earliest known vertebrates are jawless fish that appeared in the Ordovician period that had a cranium but no vertebral column. These are similar to today's lampreys and hagfish. 

During the Ordovician period, the area north of the tropics was almost entirely ocean, and most of the world's land was collected into the southern supercontinent Gondwana. Throughout the Ordovician, Gondwana shifted towards the South Pole and much of it was submerged underwater. More recently tetrahedral spores that are like those of primitive land plants have been found, suggesting that plants invaded the land at this time.

Until mammals and birds, all vertebrates were ectothermic. Ectothermic (commonly called cold blooded) means regulating body temperature from the outside through behavioral changes. For example, an ectotherm might stay under a rock in the shade to keep cool on a hot, sunny day. Almost all living fish, amphibians, and reptiles are ectothermic. Their metabolic rate and level of activity depend mainly on the outside temperature.

The transition to vertebrates continued for 300 million years. About 450 million years ago cartilaginous fish appeared with a complete vertebral column. These fish also had jaws. About 400 million years ago, bony fish appeared. Early bony fish evolved into modern ray-finned and bone-finned fish. The study of fish is called Ichthyology. 

Just a quick rundown of how the story went from there. The first amphibians evolved from a lobe-finned fish ancestor (bony fish with fleshy, lobed, paired fins) about 365 million years ago. Reptiles emerged from an amphibian ancestor about 300 million years ago. Mammals and birds evolved from reptilian ancestors. Mammals emerged about 200 million years ago and birds about 150 million years ago.

This essay covers the two key major evolutions between six hundred million years and about 400 million years, and also cover the Cambrian and Avalon explosions.