The Rise of the Vertebrates

FAS Astronomers Blog, Volume 31, Number 11.

The Earth’s history is described by geologic time, which is broken into eons, eras, periods, and epochs. The first three eons (Hadean, Archean, and Phanerozoic) are often combined into the Precambrian supereon. Today, we find ourselves in the Cenozoic era of the fourth (Phanerozoic) eon. However, as we look back, it was during the first (Paleozoic) era of the Phanerozoic eon that things began to get really interesting with the diversification of life and the rise of the vertebrates.

The Beginning

Life began back during the Precambrian supereon. We really don’t know exactly when, but it was at least 3 ½ billion years ago. Life slowly evolved from single cell organisms to multicell organisms. Early life was known as prokaryotes (containing simple cells without a nucleus). More complex life evolved later as eukaryotes (built from cells with a nucleus). By the end of the Precambrian, life was well entrenched on the Earth.

A huge increase in the diversity of life began with the Cambrian Explosion at the opening of the Cambrian period and the Paleozoic era, around 539 million years ago. Life was primarily aquatic and included many invertebrates along with the first of the vertebrates. Trilobites (early three lobed arthropods) became dominant and would last for 250 million years throughout the entire Paleozoic era. The land, on the other hand, was mostly barren at this time.

The complexity of ocean life continued to rise during the Ordovician period with the Great Ordovician Biodiversification Event (aka the Ordovician Radiation). Invertebrates dominated the seas during this period.

Marine Vertebrates

The first of the vertebrates were fish (or is it fishes – I’m never sure). Jawless fish were the first to appear as early as the Cambrian period, although they mostly disappeared by the late Devonian. The modern-day lamprey is one of the few remaining.

Fish soon developed jaws, which allowed them to eat rather than filter their food. Cartilaginous fish (Chondrichthyes) and Bony fish (Osteichthyes) appeared during the late Silurian period.

Then came the Devonian period (aka the “age of fishes”) when fish topped the food chain. It was the bony fish that spurred on evolution. The bony fish split into ray-finned fish (Actinopterygii – having fins with a ray of bones) and lobe-finned fish (Sarcopterygii – having fins attached by a single bone). Many of the common fish today are ray finned.

“Fish” crawled out onto land

As the Earth moved from the Devonian into the Carboniferous period, the lobe-finned fish adapted and began to form lungs. Their fins evolved toward true legs allowing them to move across the bottom of shallow inlets. Before the four-limbed tetrapods arrived, there was tiktaalik – an animal nicknamed “fish-a-pod” that bridged the gap between fish and land-dwelling tetrapods. Eventually, true tetrapods appeared. These were the amphibians, which had to lay their eggs in a water environment, so their young continued to be tied to the sea.

Amphibians

As you probably know amphibians are still with us today. They split into a few different branches and over time evolved into the lissamphibians, which are the modern frogs and toads. Two other branches were the smaller lepospandyls and larger temnospondyls, although these two branches eventually died out.

Land dwelling tetrapods

More importantly, amphibians evolved into land-dwelling amniotes, which had amniotic eggs with membranes that allowed them to remain on the land.

Amniotes split into three major groups: anapsids, synapsids, and diapsids. It all has to do with the holes in their heads behind their eye sockets. Anapsids and Diapsids are often grouped together as Sauropsids.

• Anapsids didn’t have a hole. There is some thought that anapsids were the ancestors of turtles and tortoises.
• Synapsids had a single hole and were the ancestors of mammals.
• Diapsids had two holes and were the ancestors of reptiles.

Mammal ancestors take over

For a while, the mammals (or at least their ancestors) dominated the land. During the Permian period, the synapsids developed into pelycosaurs such as the sail finned Dimetrodon. Later they gave way to therapsids such as the herbivore Moschops and the saber-tooth looking carnivores goronopsians (aka goronopsids). The latter were not quite dinosaur size, but at around ten feet in length, they were some of the more dominant animals of the time.

Almost everything dies off

The first potential reign of mammals ended abruptly with the Permian-Triassic (P-T) extinction. This is called the Great Dying when life nearly came to an end. The extinction was most likely caused by volcanic activity that formed the Siberian Traps and resulted in a global increase in CO₂, which led to reduced oceanic oxygen levels, acid rain, and global warming (ref. Michael Benton, When Life Nearly Died Out and Andrew Knoll, A Brief History of Earth).

Mammals sneak by

Most of the larger therapsids disappeared. An exception was the dog/pig sized two-tusked dicynodont Lystrosaurus. They eventually spread across Pangaea and accounted for a majority of the land vertebrates during the early Triassic period before dying off by the middle Triassic. There were also the smaller cynodonts – the ancestors of modern mammals.

Reptiles take over

The Great Dying was a boon to the reptilian ancestors. Life slowly recovered during the Triassic period as diapsids branched off into lepidosaurs – the ancestors of lizards and snakes. However, they really took off with another branch, the “ruling reptiles” known as archosaurs, who dominated the Triassic period eventually leading to the first dinosaurs. Archosaurs were also the beginning of flying pterosaurs and crocodylomorphs (the ancestors of the modern crocodiles).

Of course, the dinosaurs soon came to rule the Earth during the Jurassic and Cretaceous periods. More about them in future articles.

Marine Reptiles

Yes, fish were dominant during much of the Paleozoic era. For a time, they became the most feared sea creatures. Armored fish (Ostracoderms) appeared during the late Silurian and eventually led to the huge (30-foot) armored placoderms such as Dunkleosteus that terrorized the seas during the Devonian but died off by the end of the period.

It took a while, but some land-dwelling reptiles decided to go back into the seas becoming the large marine reptiles of the Mesozoic era (ichthyosaurs, plesiosaurs, pliosaurs, and mosasaurs). These were not dinosaurs, but like pterosaurs, they lived alongside the Jurassic and Cretaceous dinosaurs.

Sharks

Cartilaginous fish (Chondrichthyes) led to the ancestors of the modern sharks that emerged during the Ordovician period. Ancient sharks had to compete with (and were preyed upon by) the armored placoderms. However, sharks diversified during the Carboniferous period after the Devonian-Carboniferous extinction killed off their larger competitors.

For a time, there were also Spiny Sharks (Acathodians), with features from both cartilaginous and bony fish, but they died off by the end of the Paleozoic era.

Sharks continued to evolve and compete with the larger reptiles of the Mesozoic and Cretaceous periods. Eventually, they became one of the dominant sea creatures of the Cenozoic era – especially the 60-foot long Megalodon.

Invertebrates

The vertebrates are the largest of the creatures to walk (or swim) the earth. However, invertebrates, including insects, actually make up over 90% of the animal species today. So, we shouldn’t skip over a couple of the larger invertebrates. During the Carboniferous period, oxygen was well about the levels we see today leading to an abundance of insects. This included giant 2 ½ foot dragonflies (Meganeura) and 10-foot millipedes (Arthropleura).