Complex green organisms emerged a billion years ago

Of all the organisms that photosynthesize, land plants have the most complex bodies. How did this morphology emerge? A team of scientists led by the University of Göttingen has taken a deep dive into the evolutionary history of morphological complexity in streptophytes, which include land plants and many green algae. Their research allowed them to go back in time to investigate lineages that emerged long before land plants existed. Their results revise the understanding of the relationships of a group of filamentous algal land colonizers much older than land plants. Using modern gene sequencing data, researchers pinpoint the emergence of multicellularity to almost a billion years ago. The results were published in the journal Current Biology.

The study focused on Klebsormidiophyceae, a class of green algae known for its ability to colonize diverse habitats worldwide.

The team of researchers conducted extensive sampling, investigating habitats ranging from streams, rivers, and lake shores to bogs, soil, natural rocks, tree bark, acidic post-mining sites, sand dunes, urban walls, and building façades.

"It's really fascinating that these tiny robust little organisms have such a high diversity in their morphology and also are extremely well adapted to live in sometimes very harsh environments," says Dr Tatyana Darienko, University of Göttingen's Institute for Microbiology and Genetics.

This comprehensive sampling aimed to create a global distribution map for Klebsormidiophyceae, emphasizing their adaptability, ecological significance, and hidden diversity.

Based on genetic data calibrated by fossils, the researchers performed "molecular clock analyses."

While delving into the complex evolutionary history of Klebsormidiophyceae, the researchers faced challenges in resolving phylogenetic relationships using traditional markers.

To overcome this, they employed hundreds of genes obtained from the transcriptomes of 24 isolates from different continents and habitats.

"Our approach, known as phylogenomics, was to reconstruct the evolutionary history taking into account whole genomes or large fractions of genomes," explains Dr Iker Irisarri, Leibniz Institute for the Analysis of Biodiversity Change.

"This extremely powerful method can reconstruct evolutionary relationships with very high precision."

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Multiple evolutionary trajectories in aquatic crocodiles

In the geological past, several groups of crocodiles evolved towards a morphology adapted to marine life. However, the extent of these adaptations and their evolutionary trajectories remained unknown. An exhaustive study of their morphology by a scientific team from the Evolution & Diversity Dynamics Lab (EDDyLab) at the University of Liège has now shed light on the evolutionary mechanisms at work, thanks to three-dimensional reconstructions.

Contrary to what its few current species might suggest, the crocodile group was highly diversified in the past, with herbivorous, arboreal, and even totally marine species. Thalattosuchians and dyrosaurs, two crocodile species, colonised the marine environment independently in the geological past. "These two groups of crocodiles are also very interesting to study because they managed to survive major biological crises," explains Isaure Scavezzoni, a doctoral student at the Evolution & Diversity Dynamics Lab and principal author of the study. Thalattosuchians survived the Jurassic-Cretaceous transition (145 million years ago) and dyrosaurs the mass extinction at the end of the Cretaceous (66 million years ago)." However, the extent and diversity of these animals' adaptations to marine life are still very poorly understood because their body anatomy has been relatively little studied. We do not know the evolutionary trajectories underlying these evolutionary successes. Are they similar or did these groups take different routes to marine life? Researchers at the University of Liège's EDDyLab have attempted to answer this question using 3D modelling.

"The scale of the task involved in answering these questions is immense," explains Valentin Fischer, palaeontologist and director of the EDDyLab. We have carried out hundreds of scans and high-definition 3D reconstructions of the limb, shoulder and pelvis bones of a wide range of species of thalattosuchians, dyrosaurs and even modern crocodiles." These data enabled the team to analyse the evolutionary trajectories of these two species in order to detect possible convergences, i.e. cases of independent evolution of similar morphologies. To do this, several dozen reference points were placed on each bone; the resulting 3D coordinates were then compared between species and tested in a phylogenetic framework, i.e. taking into account the kinship links between the species analysed.

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Scientists develop new index based on functional morphology to understand how ancestors of modern birds used their wings

Scientists at Nagoya University in Japan have developed an index to estimate how a bird uses its wings for flight or other locomotion by measuring the strength of the coracoid bone and the animal's body mass. It should improve our understanding of how extinct animals used their wings and the different patterns of wing-propelled locomotion that emerged as birds evolved. Their findings were published in the Journal of Anatomy.

The presence of a wing alone does not tell us whether an animal can fly. For example, penguins evolved wings to propel them through water whereas feathered dinosaurs may have used their wings for other purposes, such as thermoregulation and intraspecific display. Therefore, to better understand how animals evolved the ability to fly, an index must take into account both the presence of wings and the ability to perform powerful wing-beats.

"We wanted to create a new index because people think that if an animal has wings, then it can fly," said the study's second author, Assistant Professor Shin-ichi Fujiwara. "But this is not always true. An animal can also use its wings for other purposes, such as thermal insulation in flightless animals. Our research team focused on how changes in skeletal morphology can lead to changes in locomotion. Subsequently, these changes can lead to major ecological transitions such as a shift in lifestyle from a terrestrial environment to an aerial, aquatic, arboreal, or subterranean environment. The origin of flight in birds has been an important topic in this field. We, therefore, needed to develop an alternative index, based on biomechanics, to determine the flapping ability of birds and which we could also use to measure skeletal remains."

To create this index, the researchers used the avian coracoid bone. The coracoid bone acts as a strut to prevent the thoracic skeleton from deforming when an animal's powerful flight muscles, which connect the wings to the sternum, contract. Doctoral student Takumi Akeda of the Department of Earth and Planetary Sciences, Graduate School of Environmental Studies, at Nagoya University, and Fujiwara of the Nagoya University Museum, measured the size of a cross section of the coracoid bone in relation to the body mass of 220 bird specimens. Their sample of 209 species included extinct birds such as the dodo and the great auk.

The researchers then divided the birds into four groups based on how they used their wings. These groups were those that used flapping flight (e.g., pigeons); those that used wing-propelled diving (e.g., penguins); those that were flightless with no flapping ability (e.g., ostriches); and those that used thermal and dynamic soaring (e.g., albatrosses and vultures). Based on the strength of the coracoid bone and flapping ability, the researchers could create a new index to analyze flight patterns.

They found that the strength of the coracoid in relation to body mass may reflect the force exerted by the flight muscles, which counteract the lifting force on the wings. This helps to estimate how a bird uses propulsion. Soaring birds had increased coracoid strength, probably to enable them to withstand the greater bending forces caused by the contraction of the flapping muscles. In contrast, non-flapping birds had lower coracoid strength. These findings show that coracoid strength in relation to body mass reflects the lifting force on the wings, therefore, it is a useful tool for reconstructing the type of propulsion used by the animal.

Akeda and Fujiwara's index should allow future researchers to assess the flight styles and flapping abilities of not only extinct birds but also other flying animals, including the Pteranodon and Quetzalcoatlus of "Jurassic World" fame. The index could also allow them to estimate the origin of flight in winged theropods, the ancestors of birds.

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