From Fins to Limbs in the Fossil Prep Lab

Behind the Lab Window

1On Saturdays, eighth grader Hollis Mbeki volunteers at the county natural history museum. Hollis helps in the fossil preparation lab, a room with a big window where visitors can watch staff clean rock away from fossils with tiny tools. This month, paleontologist Dr. Annika Toledo is teaching Hollis to read the fossil record: all the fossils that have been found, together with the ages of the rocks they came from.

2Dr. Toledo started with the chart on the lab wall, shown in Table 1. Rock ages come from measuring natural radioactive elements in volcanic layers near the fossils, and they are given in millions of years ago (mya). For each group, the chart shows the age of the oldest rocks where its fossils have been found. For a group that is no longer alive, it also shows the age of the youngest rocks where its fossils appear.

3Next she opened the lab's teaching drawer, which holds casts of three fossils from rocks of different ages, described in Table 2. Eusthenopteron lived in water and breathed with gills. Tiktaalik had a flat head with its eyes on top, a little like a crocodile's, and lived in shallow streams. Acanthostega had limbs with digits, but its body was still built for life in water, and its limbs were better for paddling than for walking.

4Dr. Toledo told the story behind the Tiktaalik cast. In the late 1990s, a team of paleontologists hoped to find fossils from the time when some fish-like animals were giving rise to four-limbed ones. They searched geology maps for rocks about 375 million years old that had formed in shallow streams, and they found such rocks in the Canadian Arctic. After several summers of digging, the team found Tiktaalik in 2004.

5Fossils form only rarely, usually when a living thing is buried quickly in mud or sand before it rots away. Hard parts such as bones, teeth and shells become fossils far more often than soft parts. Many rock layers of a given age are also buried deep underground or have been worn away. Because of this, the fossil record has gaps, and new finds keep filling some of them in.