30.2: Counting diversity in shells and bones

Sepkoski’s Phanerozoic diversity curve

Fossil diversity throughout the Archean and Proterozoic is challenging to track because of the lack of hard parts for most of the organisms alive during these times. Once paleontologists recognized the shift to skeletonized animals during the Cambrian at the beginning of the Phanerozoic, many big questions bubbled to the top of paleobiological research. Has diversity constantly been increasing through time? If periods of mass extinction (intervals when many genera or even families die off simultaneously) exist, are there similar periods of diversification, or origination, when many species or higher taxa appear at once? How long is the average species around on Earth? To tackle some of these questions, Jack Sepkoski, a paleontologist at the University of Chicago, developed what is now one of the most famous graphs in the world of paleobiology, his Phanerozoic diversity curve.

Figure \(\PageIndex{1}\): Sepkoski’s (2002) Phanerozoic marine diversity curve. Total height of graph is total number of genera. Blue shading represents the Cambrian fauna, red shading is the Palozoic fauna, green shading is the Modern fauna, and black represents microfossils.

This graph shows the number of marine genera through the Phanerozoic. It is important to note this graph does have a subtle taphonomic bias to it, as it focuses on marine animals after the development of a shelly exoskeleton. When looking at the curve, several major features can be identified: 1) the Cambrian Explosion, a time of significant diversity increase through the Cambrian; 2) the Paleozoic Diversification and Plateau, a time of extensive radiation of invertebrate groups followed by relatively stable diversity from the Ordovician through Permian; and 3) the Cenozoic Rise, a time of exponential diversity growth from the Triassic to today. The five major mass extinctions are also quite noticeable as rapid, sharp declines in diversity at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous.

After Sepkoski’s work was published, a new question arose—is the diversity curve a true representation of biological diversification through time, or is the pattern influenced by taphonomy and the quality of fossil preservation? For example, some researchers suggested that the Cenozoic Rise is simply due to the “Pull of the Recent”, meaning that there are more well-preserved, Cenozoic-aged rocks and sediments on Earth simply because there hasn’t been enough time for them to go through the geologic wringer of the rock cycle yet. In the late 1990s, a large group of paleobiologists got together to begin developing the Paleobiology Database.

Checking the data: the Paleobiology Database

The Paleobiology Database (PBDB) is an online clearinghouse of occurrence data of fossils around the world for most of the Phanerozoic. Researchers scour published scientific literature for reports of fossil occurrences at described localities. The PBDB is an amazing resource because it includes taxonomic lists from a variety of spatial scales (sample within a bed, outcrop) but also typically pairs detailed sedimentary, stratigraphic, and taphonomic descriptions with the fossil occurrences. Want to know more about Jurassic bivalves in limestones within 200 miles of Paris? The PBDB has you covered. Since 1999, the PBDB has continued to grow as researchers have added data on marine microfossils, invertebrates, and vertebrates, terrestrial reptiles, birds, mammals, and plants, and more.

After a few years of PBDB data accumulation and analyses, a consensus group of paleontologists examined the data to see if Sepkoski’s Phanerozoic diversity patterns were replicable. Even after correcting for variations in sample preservation, and chunking the geologic time scale into standardized 10 million year sections, these researchers were able to show the increase in taxonomic diversity during the Cenozoic is indeed a true biological signal in the fossil record.