The appearance of hard teeth: rise of the predators?

An international team of palaeontologists sectioned the first skeletons of vertebrates to reveal their life cycle and the positions these marine animals occupied in the Earth’s earliest foodwebs. They focused on conodonts, animals which are already extinct but were the first among all vertebrates to start producing skeletons – hard organs permeated with minerals. The question they asked themselves was: did the appearance of hard teeth lead to the rise of predators? The team discovered that the earliest conodonts developed teeth, which were the very first hard skeleton elements, but were not predators yet. Whereas it has been commonly believed that the only incentive to develop teeth was becoming a predator.

Skeleton elements such as bone and teeth give scientists a much easier glimpse into the fossil record than soft tissues such as muscles and skin, so the appearance of the first skeletons was a major boost to our understanding of where we, the vertebrates, come from. Conodonts were abundant in the early oceans, but almost all they left behind are microscopic teeth made of material similar to our own tooth enamel. These teeth,  the first parts of skeletons that ever appeared in vertebrates, have highly diverse shapes, exhibiting simple cone- to complex platform-like morphologies which indicate different feeding strategies.

The image below shows the specimens used (light microscope and scanning electron microscope image, as well as SRXTM scan). The scale bars equal 200 µm (A–D) and 100 µm (E and F).

Enforced by minerals

Using synchrotron X-ray tomography, high-resolution scanning electron microscopy and chemical analysis, Isabella Leonhard, at the time a Masters student at Friedrich-Alexander Universität Erlangen-Nürnberg and currently a PhD student at the University of Warsaw, collaborated with Emilia Jarochowska at Utrecht University and researchers at the Oxford University Museum of Natural History and the US Geological Survey. The team looked at the simplest, most primitive forms of conodonts – shaped like simple cones or needles. These conodonts, although they developed teeth, differed from advanced forms, which developed sharp shearing organs and fed as predators and/or scavengers. These results show that hard teeth enforced by minerals pre-dated their role in crushing and grinding hard food, which is typical for predators. We can only speculate that reinforcing the first teeth with a mineral was useful also for other diets. These early conodonts were most likely filter feeders, like today’s finwhales, and preyed upon plankton. Perhaps this plankton contained some crunchy bits or filtering was more efficient through a sieve formed by hard needles, because this is how these first teeth probably functioned, says Utrecht University researcher Dr Emilia Jarochowska, who supervised the project.

Tree rings

Slicing these microscopic teeth allows us to get a glimpse into the daily life of organisms which lived nearly 500 million years ago. Conodont teeth differ from other vertebrate teeth because they kept growing throughout the whole life of the animal – adding constantly and periodically new enamel-like structures upon the youngest surface. This creates a record similar to tree rings, where each episode of the organism’s life is preserved. Advanced conodonts which fed as predators usually have surfaces of wear and damage resulting from gnawing on hard prey, but the most primitive ones did not show such traces.

Competition

The evolution of tooth-like structures before the onset of actual predatory behavior can be attributed to different environmental circumstances: the earliest conodonts were skulking around in early Palaeozoic oceans in which larger predators were still absent. With the appearance of those in the course of the Palaeozoic, conodonts may have started to adapt to an ecosystem in which they had to compete against new, larger predators. Our results bring us one step closer to unravel the long-time mystery about the rise of vertebrates as the most diverse and successful macroscopic animals on Earth, explains Isabella, the student who analysed these fossils.