Fossil Fishes and Early Vertebrates

The earliest vertebrates and fossil fishes record the origin of the backbone, jaws, and paired fins, and the dramatic transition from water to land.

Definition

Fossil fishes and early vertebrates comprise the aquatic and transitional members of the vertebrate lineage, from Cambrian jawless forms through the lobe-finned fishes that gave rise to land animals.

Scope

This topic covers jawless fishes (agnathans), armored placoderms, cartilaginous and bony fishes, and the lobe-finned ancestors of tetrapods, along with the fish-to-tetrapod transition and the origins of jaws, teeth, and limbs.

Core questions

• How and when did the vertebrate backbone and head evolve?
• What was the origin of jaws and paired appendages?
• Which fossils document the fish-to-tetrapod transition?
• How do early fish groups relate to living fishes?

Key concepts

• Agnathans and the origin of the head
• Gnathostomes and the origin of jaws
• Sarcopterygian lobe fins
• Tetrapodomorph transitional forms

Key theories

Origin of jaws

Jaws are interpreted as modified anterior gill arches, an innovation that transformed feeding and is documented across early gnathostome fossils.

Fish-to-tetrapod transition

Lobe-finned fishes and early tetrapodomorphs such as Tiktaalik show a mosaic of fish and tetrapod features marking the move onto land.

Clinical relevance

The deep ancestry of vertebrate anatomy revealed by fossil fishes illuminates the evolutionary origins of features shared by all later vertebrates, including humans, such as limbs, jaws, and sensory systems.

History

Early fossil fishes were central to nineteenth-century work by Louis Agassiz, and twentieth-century debates over tetrapod origins focused on lobe-finned fishes. The 2004 discovery of Tiktaalik refined the picture of the water-to-land transition.

Debates

Sequence of limb and digit origins

The order in which weight-bearing limbs, digits, and air-breathing evolved during the fish-tetrapod transition is debated as new fossils emerge.

Key figures

• Philippe Janvier
• Erik Jarvik
• Neil Shubin

Seminal works

• janvier1996
• benton2015

Frequently asked questions

What were the first vertebrates?

The first vertebrates were small, jawless fish-like animals that appeared in the Cambrian and Ordovician, lacking the jaws and paired fins of later fishes.

How did animals first move onto land?

Lobe-finned fishes evolved sturdy limb-like fins and the ability to breathe air, and their tetrapod descendants gradually adapted to life on land in the Devonian.