Pterosaurs, the flying cousins of dinosaurs, reached immense sizes during the late Cretaceous Period (100 to 66 million years ago), with some species possibly exceeding 10 meters in wingspan.
Their ancestors during the earlier Triassic and Jurassic periods (252 to 145 million years ago) had much more modest wingspans, up to around 2 meters. But scientists have not found any fossils of these much smaller pterosaur species from the later period.
This lack of smaller forms has often been linked to the evolution of birds, which could have outperformed and replaced smaller species of pterosaurs. This explanation has prevailed for twenty years or more, but it raises questions.
For example, research suggests that pterosaurs were very early (able to fend for themselves soon after birth) and could fly soon after hatching. If this was the case, why then weren’t all pterosaurs replaced by birds which would have supplanted the young of the large forms as well as the small species of pterosaurs?
Our international research group has been working on these and other problems with pterosaurs for the past 40 years. In our new study, we found that it was probably the babies of giant pterosaurs – known as beats – that eclipsed small adult pterosaurs.
We have unearthed hundreds of pterosaur jaw specimens from the Middle Cretaceous Kem Kem group, a river deposit in southeastern Morocco. The area was home to many pterosaurs, among other species.
Most of the fossils of the Kem Kem group are remarkable because they are very large. But we selected five toothless jaw fragments for close examination due to their small size. We wanted to know more about the pterosaurs to which these fragments belonged.
We used various techniques to determine whether the jaw fragments were the jaw tips of large or giant pterosaurs, were from small adult pterosaurs, or were immature individuals of larger species (immature means anywhere, from a new -born to a teenager). These included examining the texture of the bone surface and examining the internal structure of the bone (called histology). We also looked at small openings in the jaw surface where sensory nerves emerged, called neural foramina, which were involved in detecting prey.
We found that the jawbone had a wavy fibrous texture, typical of immature individuals. More mature pterosaurs have bones with a very smooth surface. The internal structure of the bone also suggested that they belonged to young pterosaurs – the bone grew rapidly and lacked growth lines. Finally, the frequency and distribution of the foramina indicated that the samples were also from immature pterosaurs.
The general shape and characteristics of the jaws were similar to those of two already described large to giant pterosaur species of the Kem Kem group: Alanqa saharic and Apatorhamphus gyrostega. We concluded that the fragments belonged to immature individuals of these two species, some perhaps less than a year old. It is difficult to estimate the size of these pterosaurs based on the small fragments we have, but their wingspan may have been as small as 25cm.
Why is this important?
Many studies have speculated that beats (newly hatched pterosaurs) played little or no role in these ancient ecosystems. This idea would be plausible if pterosaurs were nesting – cared for by their parents after hatching – like many birds. But if pterosaurs were indeed precocious and could take care of themselves and even fly soon after hatching, they probably played a larger role in ancient ecosystems.
To prevent the adults of these large to giant pterosaurs from competing with their own young for resources, they likely occupied different ecological niches. A niche is the role that an organism plays in an ecosystem, for example, its diet, who it eats and who eats it. It is possible that the tiny pterosaurs occupied niches previously favored by small adult pterosaurs from the Triassic to Jurassic period.
Given the size spectrum of pterosaurs that likely lived side by side – as our work and previous data from the Kem Kem group show – we can say that pterosaurs looked more like crocodiles than birds in their food ecology. For example, with birds there might be 10 different species of different sizes along a bank, all feeding on slightly different prey.
Crocodiles, on the other hand, are much less diverse. Some newborn crocodiles feed on insects and as they grow up change their diet to small fish, then larger fish, then small mammals until a large adult crocodile. be able to take a zebra. It appears that the pterosaurs did something similar, occupying different niches as they grew – a much more reptilian than avian life strategy.
If the birds had truly outperformed the little pterosaurs, they surely would have outperformed those little immature beats of the giant pterosaurs as well, resulting in the pterosaurs extinction much earlier. As it stands, birds and pterosaurs coexisted for over 85 million years, until pterosaurs finally died out along with dinosaurs around 66 million years ago.
Roy Smith is a doctoral candidate in the School of Environment, Geography and Geosciences at the University of Portsmouth. David Martill is Professor of Paleobiology at the University of Portsmouth. This article first appeared on The conversation