A newly discovered fossil has demonstrated the origins of herbivory in land-based tetrapods (four-limbed vertebrates). The study, carried out by scientists from the University of Toronto Mississauga and Humboldt-University in Berlin, has been published in the open-access journal PLOS ONE.
As fully terrestrial vertebrates, the amniotes, began to emerge and evolve, the structure and hierarchy of land ecosystems started to drastically change. A significant part of this change was attributable to the acquisition of herbivory, which represented a major evolutionary event since it meant that tetrapods could begin to tap into the largely untouched and vast resources provided by primary producers. Herbivory broadly refers to the consumption of autotrophic (self-nourishing) organisms, but more specifically it can be used to describe nourishment from the breakdown of cellulose by microbes.
A 300-million-year old fossil discovered in Kansas, USA, has bestowed scientists with novel information on the emergence of the first herbivorous tetrapods. The fossil represents a juvenile skeleton of a previously unknown member of the caseid family, which is a branch of the group Synapsid. Synapsids were a diverse group of both early herbivores and top predators.
The 20 centimeter long fossil, Eocasea martini, consisted of part of the skull, the majority of the vertebral column, the pelvis and a hind limb. According to Jörg Fröbisch, co-author of the study, “Eocasea is one of the oldest relatives of modern mammals and closes a gap of about 20 million years to the next youngest members of the caseid family.”
Eocasea was a carnivore, but younger members of this family were herbivores. Eocasea was also very small, weighing no more than 2 kilograms when fully grown, whereas younger caseids were the largest known terrestrial vertebrates of their time. This provides evidence that these large herbivores evolved from significantly smaller non-herbivores. The acquisition of herbivory therefore also led to a dramatic increase in body size, which is consistently seen in history. This is presumably because they were not as restricted in terms of the availability of food.
Eocasea does not represent the only species that led to a transition from carnivory to herbivory with a concomitant increase in body size. Interestingly, this actually occurred independently in a temporally staggered manner at least five times, and two of these involved reptiles.
The findings are exciting because they document for the first time the earliest steps in the evolution of herbivory in land-based animals that ultimately led to a dramatic change in the terrestrial ecosystem.