Rulers of the Sea

Fig.1. The T. rex died out 66 million years ago, along with other non-avian dinosaurs, leaving ray-finned fishes to dominate in the waters (Credit: A.E. Anderson, AMNH 5027 [left] and Magnus Manske, 2004 [right])

Even though we were excited to see a Tyrannosaurus rex in “Jurassic World,” it is probably best that they stayed in the Cretaceous. Humans and dinosaurs certainly do not mix, but ancient mammals lived side-by-side with the T. rex. That is, until the asteroid killed off all non-avian dinosaurs in the Cretaceous-Paleogene [K/Pg] extinction event, leaving an ecological void for mammals to evolve and ultimately dominate in terrestrial ecosystems. A recent study published in PNAS by paleobiologists at UC San Diego suggests a similar story unfolded with the evolution of fish in the oceans.

Nearly 99 percent of all known fish species, from salmon to seahorse, make up a class of fish called ray-finned fishes (Actinopterygii). Bony skeletal structures set them apart from their cartilaginous shark cousins, and more than 30,000 species make up the most numerous and diverse group of modern vertebrates. Until recently, scientists were unsure as to how they took this ecologically dominant position in the marine environment. Graduate student Elizabeth Sibert and Professor Richard Norris from UC San Diego’s Scripps Institution of Oceanography studied ancient fish fossils to answer this question.

Sibert and Norris removed microscopic fish teeth and mineralized shark scales from deep-sea sediment cores and compared their diversity and abundance. They found that while shark populations remained relatively steady through the asteroid impact, the proportion of fish teeth doubled and then exploded eight-fold in the following 24 million years. This in-depth study on the shift of marine vertebrate communities shows that the extinction of dinosaurs created an ecological tipping point for ray-finned fish.

The sharp population climb between 66 million to 42 million years ago is an effective lesson in basic ecology. Species in nature fill ecological niches as they they evolve and adapt to meet their extremely specific needs for food and shelter. When non-avian dinosaurs died off, along with other reptiles and the ancient invertebrate ammonites, ray-finned fishes no longer had to compete for certain resources underwater. A lack of predators also allowed populations to flourish and effectively colonize novel environments. An even more nuanced finding is illustrated by the sudden increase in larger tooth fossils preceding the prevalence of smaller teeth. This discovery suggests that large fish evolved rapidly after the asteroid impact, followed by smaller species that evolved to fill other niches.

Such science was possible through research collaborations, such as the International Ocean Discovery Program (IODP), which serve to collect ocean sediment cores and maintain repositories around the world. These cores provide information about the course of history millions of years ago, from recording climate patterns to tracking the movement of crops. This peek into ancient times allowed Sibert and Norris to map the rise of the most ecologically dominant group of vertebrates today from cylinders of dirt.




References:
Monroe, R. (2015, June 19). Exit Dinosaurs, Enter Fishes. UC San Diego News Center. Retrieved from http://ucsdnews.ucsd.edu/pressrelease/exit_dinosaurs_enter_fishes

Perkins, S. (2013, February 7). Ancestor of All Placental Mammals Revealed. Science. Retrieved from http://news.sciencemag.org/plants-animals/2013/02/ancestor-all-placental- mammals-revealed

Sibert, E. C., & Norris, R. D. (2015, June 29). New Age of Fishes initiated by the Cretaceous-Paleogene mass extinction. PNAS. doi: 10.1073/pnas.1504985112

Glossary

Cretaceous
A period on the geologic timescale lasting from around 145 to 66 million years ago following the Jurassic Period. The end of this period was marked by an asteroid impact which catalyzed a major extinction event.
Cretaceous-Paleogene (K/Pg) extinction event
Also known as the Cretaceous-Tertiary (K/T) extinction, this event was the mass extinction of around three fourths of Earth’s plant and animal species, including non-avian dinosaurs and ammonites about 66 million years ago. The fossil record suggests that an asteroid impact caused the K/Pg extinction, thus marking the end of the Cretaceous period and the beginning of the Paleogene period.
sediment cores
Cylindrical section of sediment collected by drilling into the ocean floor with a drill. As material falls to the ocean floor, it forms layers which allow scientists to create a chronology going back millions of years.
ecological niche
The position of a species relative to other species and its ecosystem. This role includes its specific needs for food and shelter.
ammonite
An extinct group of marine invertebrates in the subclass Ammonoidea. They were cephalopods, the group which includes modern day octopuses, squid, and cuttlefish.