They became fast from small to large predators: Surprisingly, some of the early representatives of the terrestrial vertebrates apparently grew extremely quickly. This emerges from examining the bone structures of an ancient tetrapod that hunted for prey around 330 million years ago. The finding challenges previous assumptions about the characteristics of our distant ancestors, the paleontologists say.
The deep branching points in the tree of life are often the focus of research. As far as the early evolutionary history of land vertebrates (tetrapods) is concerned, the focus is on the period from 385 to 320 million years ago. It is believed that creatures from this era formed the basis for the evolution and diversification of amphibians, reptiles, birds and mammals including humans. Paleontologists are therefore particularly interested in fossils of early representatives of the tetrapods. But they are rare and often there are very few fossil records of a species. An exciting exception, however, is an animal that a team of US paleontologists is now putting in the spotlight: Hundreds of bones from several individuals of different sizes of Whatcheeria were discovered at a site in the US state of Iowa.
How did early land vertebrates grow?
These predators, which are up to about two meters long, lived there around 331 to 326 million years ago in the area of a lake. "Whatcheeria would appear to us today as a large salamander with crocodile features -- with a narrow head and lots of teeth," says co-author Ben Otoo of the University of Chicago. The animals also had sturdy limbs that could support their weight on land, but they also had anatomical features that suggest a life in and around the water. As the researchers explain, it was a so-called stem tetrapod - a representative of the group to which the evolutionary history of today's land animals can be traced back. "Whatcheeria is one of the best represented early tetrapods in the fossil record, and the abundance of material allows us to ask questions about its biology that are impossible for almost all of its contemporaries," says co-author Ken Angielczyk of the Field Museum in Chicago . In the current study, the researchers have now investigated the extent to which traces of the growth processes in these animals can be detected in the fossil bones.
Basically, it was previously assumed that the early tetrapods grew slowly and continuously larger, similar to today's primitive-looking representatives of the vertebrates. In modern tetrapods, on the other hand, juveniles usually grow relatively quickly and then stop growing when they reach the adult level. To see how Whatcheeria evolved, the researchers targeted femurs of individuals ranging from hatchling to the known maximum size. To do this, they prepared thin slices of bone and examined these transparent structures under the microscope. As an animal grows, it forms new layers of bone with each growing season, explains Otoo. "By examining how thick the growth rings are over an animal's life, you can find out how the animal has grown throughout its life," says the paleontologist.
Whatcheeria grew up quickly
As the researchers report, when examining the Whatcheeria bone slices, they encountered a surprise: "I can well remember jumping up and saying that the findings break rules that we use for the development of growth in these early tetrapods,” recalls first author Megan Whitney. Because it became apparent that Whatcheeria initially grew very quickly, which then weakened later. Traces of so-called fibrolamellar bone tissue were particularly interesting. Until now, it was thought that only amniotes (reptiles, birds, and mammals) exhibited these structures that accompany their rapid growth patterns. But apparently this type of bone tissue developed early in the evolutionary history of the tetrapods, the findings show.
However, it remains unclear how widespread these characteristics were among the stem tetrapods. The growth pattern may also have been specifically important to whatcheeria's lifestyle: "If you're a large apex predator, it can be beneficial to grow large quickly as it makes it easier to hunt other animals rather than become prey yourself." , says co-author Stephanie Pierce of Harvard University in Cambridge. However, the concept also has disadvantages: in order to grow quickly, there must be enough food and resources for the growing animal. In some cases, it can therefore be better to grow more slowly and continuously. That is why both concepts still exist today. "Evolution is about trying out different ways of life and combinations of traits," says Angielczyk.
The paleontologists now want to stay on the ball: They plan to study the bone structures of other early tetrapods to uncover their growth strategies and their possible relationship to the ecological niches of these animals.
Source: Harvard University, Field Museum, professional article: Communications Biology, doi: 10.1038/s42003-022-04079-0