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Monday, July 10, 2023

Why do we find so much amber in Cretaceous rocks?

An open window to the vanished world of the Cretaceous

UNIVERSITY OF BARCELONA

Why do we find so much amber in Cretaceous rocks? — IMAGE: INSECTS PRESERVED IN AN AMBER PIECE FROM EL SOPLAO IN CANTABRIA (SPAIN). view more
CREDIT: XAVIER DELCLÒS, UB-IRBIO

What would a traveler from the future think if one day s/he could analyze the rocks that are currently forming on the planet? Surely, this person would find quite a few plastic fragments and wonder why this material was so abundant in rocks of a certain age on Earth. This is the same question that geologists and palaeontologists have asked themselves after many years of studying another material: amber, the fossilized resin from the Cretaceous that helps us reconstruct what the forests inhabited by dinosaurs were like.

We know the reason for the abundance of so many plastics in today's ecosystems, "but we can only estimate the natural causes that would explain the production of large quantities of resin in the Cretaceous," says Xavier Delclòs, professor at the Faculty of Earth Sciences of the University of Barcelona and first author of an article published in the journal Earth-Science Reviews that addresses this enigmas of modern palaeontology.

"The stories of plastic and fossil resins are very different, but they have one thing in common: the curiosity involved in observing that some new and relevant phenomenon arose at some point in Earth's history and was recorded in rocks", says Delclòs, member of the Department of Earth and Ocean Dynamics and the Biodiversity Research Institute (IRBio) of the UB.

"Amber, and in particular its abundance, would be of little interest were it not for the fact that it contains in its interior many organisms that inhabited the forests of the past, which have been perfectly preserved as fossils and which today allow us to know the forests of the Cretaceous with a detail that seems unreal sometimes" says Enrique Peñalver, a member of the Geological and Mining Institute of Spain, a national centre of the Spanish National Research Council (CN IGME-CSIC) and also co-author of the study.

How were the large amber deposits formed?

The Cretaceous, a period extending from 145.5 to 66 million years ago, represents a time of rapid evolutionary change and diversification of organisms. Today, the dominant conditions that in the Cretaceous allowed the mass formation of abundant resin deposits all over the planet are not present, nor is it known why there was, at the time of the dinosaurs, such an extremely abundant production of resin.

"For about 54 million years, and for the first time in Earth's history, there was a mass production of resin by plants, and we still don't know why", Delclòs and Peñalver point out. "Production quantities that could have formed fossil resin deposits of what we know today as amber had never been reached. From the Barremian to the Campanian, and thanks to the conditions existing on the planet, certain groups of conifers were able to originate large deposits of fossil resin that open a real window to the ecosystems of the past and today provide very important palaeobiological information. We have called this time span the Cretaceous Resinous Interval (CREI)".

The formation of large amber deposits requires the existence of trees with the ability to produce a lot of resin. During the Cretaceous, only gymnosperms —e.g., conifers— which are evolutionarily older than flowering plants, could produce resin. Moreover, the resin had to be trapped in a sedimentary environment without oxygen to preserve it for millions of years. But what environmental or biological factors could have conditioned such resin production in the Cretaceous?

"Our study shows that, during the Cretaceous, coniferous forests were widely distributed across the planet. These amber deposits formed during the CREI shared these characteristics: high resin production exclusively by conifers; the presence of fusain, a material derived from plant material burnt by forest fires; fossils preserved in amber that correspond to similar fauna and flora among different deposits; and resin accumulation in transitional sedimentary environments under subtropical and temperate paleoclimates that coincide with the onset of sea-level rise stages.

The study also indicates that the mass production of resin was not continuous during the CREI nor was it equal everywhere: there were times of higher and lower production. In the study, carried out by a large multidisciplinary group of experts, the participation of Ricardo Pérez de la Fuente, from the Oxford University Museum (United Kingdom), is particularly noteworthy.

An open window to the vanished world of the Cretaceous

Pieces of amber recovered by palaeontologists in different sites around the world provide new insights into the Cretaceous. This period saw the emergence of large terrestrial ecosystems dominated by angiosperms — flowering plants — and many of the evolutionary lines of present-day organisms. The distribution of continents and ocean currents was altered, the climate was warmer and more humid than today's, and sea levels rose more than 200 meters above today's coastlines.

"In the atmosphere there were high levels of carbon dioxide (CO2) due to intense volcanism, but also of oxygen (O2) due to the great extension of forests to latitudes now covered by ice, a feature that also enhances large-scale fires", Delclòs and Peñalver note.

This is the global landscape and environment that dominated the Earth during much of the Cretaceous. The environmental factors conditioned the life and evolution of the organisms that existed on the planet, especially the terrestrial ones, from the smallest to the great dinosaurs, and the relationships between the different species.

In this scenario, the CREI emerges as a global phenomenon, with amber outcrops distributed everywhere during the Cretaceous, and concentrated especially in Laurasia and the northern margin of Gondwana. Environmental factors may have affected on a global scale, while biological factors — interaction between plants and arthropods, etc. — may have acted on a regional scale.

"CREI represents a great window to a vanished world, at the beginnings of modern ecosystems dominated by flowering plants, where dinosaurs lived, and where the lineages of the first birds and mammals evolved. Studying this period allows us to obtain many data of maximum scientific interest on phylogenetic relationships, extinct organisms, the beginning of behaviours that we can recognize today in many groups, intra- and interspecific relationships of extinct organisms (parasitism, pollination, parental care, swarming, forestry, reproduction, etc.) of the inhabitants of a terrestrial environment —the forest— that are not usually fossilized", the experts conclude.

This research aims to unravel evolution mysteries about how the forest ecosystems were 110 million years ago.

Amber from El Soplao (Cantabria, Spain) is providing traces of new insect species key to understand how was life in Cretaceous forests

CREDIT

Xavier Delclòs, UB-IRBio

JOURNAL

Earth-Science Reviews

DOI

10.1016/j.earscirev.2023.104486

METHOD OF RESEARCH

Observational study

SUBJECT OF RESEARCH

Not applicable

ARTICLE TITLE

Amber and the Cretaceous Resinous Interval

ARTICLE PUBLICATION DATE

30-Jun-2023

Madagascar hippos were forest dwellers

Research demonstrates the important role woodlands have played on the island

Peer-Reviewed Publication

UNIVERSITY OF CINCINNATI

Extinct dwarf hippos that once roamed Madagascar lived in forests rather than open grasslands preferred by common hippos on mainland Africa, researchers at the University of Cincinnati discovered.

The findings suggest grasslands that now cover much of the enormous island off the eastern coast of southern Africa were a relatively recent change facilitated by people rather than a natural habitat sustained in part by these famously large vegetarians.

The study was published in the journal Plants, People, Planet.

When Madagascar broke away from Africa’s mainland 150 million years ago, its plants and animals evolved in geographic isolation in the Indian Ocean. Madagascar had no elephants, giraffes, rhinos or other big mammals like those found on the mainland today.

But it did have hippos.

About the size of a cow, the dwarf or Malagasy hippo was far smaller than its four-ton cousin, the common hippopotamus. Even so, the Malagasy hippopotamus was among the largest land animals on the island along with Nile crocodiles and the flightless and enormous elephant bird.

These hippos likely resembled today’s secretive and endangered pygmy hippos found in the forests and swamps of West Africa’s Liberia and Guinea, said Brooke Crowley, a UC professor of geosciences and anthropology and lead author of the study.

“Ecologically, we think the Malagasy dwarf hippos were pretty close to the pygmy hippos that live in forests in West Africa,” Crowley said.

Crowley and her research colleagues conducted an isotopic analysis of stable carbon and nitrogen found in the bones of extinct Malagasy dwarf hippos that roamed the island more than 1,000 years ago. These isotopes, found in the bones of animals, leave behind a fingerprint of the foods they ate. And this provides clues about their preferred habitats.

Researchers took samples from the bones of dwarf hippos at museums along with those the team collected on the island. They found that dwarf hippos did not regularly graze on grass in dry, open habitats, even in regions dominated by grassland today. Instead, they preferred plants found in the wetter, more forested landscapes. This suggests forest was more abundant before people began changing the landscape to grow cultivated plants, graze domesticated cows and goats and obtain firewood and building materials.

Common hippos on the mainland love grass. Their name derives from the Greek words for “river horse.” Each night they leave the safety of rivers and waterholes to find fresh pasture, cropping grass like a horse, before returning in the morning.

But the researchers’ analysis found that grass represented only a small part of the diet of Malagasy dwarf hippos. Instead, they behaved more like browsers, feeding on sedges and leaves. As a result, hippos likely had little influence on maintaining or expanding grasslands on the island.

“For years we’ve seen evidence that these animals were not grazers,” said Laurie Godfrey, a study co-author and professor emerita at the University of Massachusetts Amherst.

Godfrey said there is evidence to suggest that people caused the extinction of hippos on the island when they created permanent communities and moved from hunting and gathering to raising domestic animals and crops. She calls her idea the “Subsistence Shift Hypothesis,” which she said is an elaboration on a similar idea first proposed by noted archaeologist Robert Dewar.

“There is pretty compelling convergent evidence showing that many of the extinct animals disappeared in a short window of time coinciding with the transition of people from hunting and gathering to pastoralism,” UC’s Crowley said.

Crowley thinks restoring native forests is key to helping conserve wildlife on the island. Based on their study, expansive grasslands were not a critical habitat, at least for the island’s hippos.

“Some colleagues argue that grasslands are ancient and that we need to protect and manage them like we do forest,” Crowley said. “I would argue that forests are far more important. We are not contending that grasses did not exist in the past, but pointing out that there is no evidence for large grasslands devoid of trees prior to about 1,000 years ago.”

It’s a point the researchers make in the study as well.

“It is clear that Madagascar faces a biodiversity crisis much greater than that which it has already endured. Preventing this crisis will demand new conservation actions,” the study concluded.

The study was supported by grants from the National Science Foundation, the African Regional Research Program Fulbright and the National Geographic Society.

Researchers conducted an isotopic analysis on the bones of extinct dwarf hippos and learned that they preferred sedges and leaves to grass.