Fossil spines reveal deep sea's past

Right at the bottom of the deep sea, the first very simple forms of life on earth probably emerged a long time ago. Today, the deep sea is known for its bizarre fauna. Intensive research is being conducted into how the number of species living on the sea floor have changed in the meantime. Some theories say that the ecosystems of the deep sea have emerged again and again after multiple mass extinctions and oceanic upheavals. Today's life in the deep sea would thus be comparatively young in the history of the Earth. But there is increasing evidence that parts of this world are much older than previously thought.

A research team led by the University of Göttingen has now provided the first fossil evidence for a stable colonisation of the deep sea floor by higher invertebrates for at least 104 million years. Fossil spines of irregular echinoids (sea urchins) indicate their long-standing existence since the Cretaceous period, as well as their evolution under the influence of fluctuating environmental conditions. The results have been published in the journal PLOS ONE.

The researchers examined over 1,400 sediment samples from boreholes in the Pacific, Atlantic and Southern Ocean representing former water depths of 200 to 4,700 metres. They found more than 40,000 fragments of spines, which they assigned to a group called irregular echinoids, based on their structure and shape. For comparison, the scientists recorded morphological characteristics of the spines, such as shape and length, and determined the thickness of around 170 spines from each of two time periods. As an indicator of the total mass of the sea urchins in the habitat -- their biomass -- they determined the amount of spiny material in the sediments.

What these fossil spines document is that the deep sea has been continuously populated by irregular echinoids since at least the early Cretaceous period about 104 million years ago. And they provide further exciting insights into the past: the devastating meteorite impact at the end of the Cretaceous period about 66 million years ago, which resulted in a worldwide mass extinction -- with the dinosaurs as the most prominent victims -- also caused considerable disturbances in the deep sea. This is shown by the morphological changes in the spines: they were thinner and less diverse in shape after the event than before. The researchers interpret this as the "Lilliput Effect." This means that smaller species have a survival advantage after a mass extinction, leading to the smaller body size of a species. The cause could have been the lack of food at the bottom of the deep sea.

"We interpret the changes in the spines as an indication of the constant evolution and emergence of new species in the deep sea," explains Dr Frank Wiese from the Department of Geobiology at the University of Göttingen, the lead author of the study. He emphasises another finding: "About 70 million years ago, the biomass of sea urchins increased. We know that the water cooled down at the same time. This relationship between biomass in the deep sea and water temperature allows us to speculate how the deep sea will change due to human-induced global warming."

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Scales or feathers? It all comes down to a few genes

Scales, spines, feathers and hair are examples of vertebrate skin appendages, which constitute a remarkably diverse group of micro-organs. Despite their natural multitude of forms, these appendages share early developmental processes at the embryonic stage. Two researchers from the University of Geneva (UNIGE) have discovered how to permanently transform the scales that normally cover the feet of chickens into feathers, by specifically modifying the expression of certain genes. These results, published in the journal Science Advances, open new perspectives for studying mechanisms that have enabled radical evolutionary transitions in form among species.

The skin of terrestrial vertebrates is adorned with diverse keratinized appendages, such as hair, feathers, and scales. Despite the diversity of forms within and among species, the embryonic development of skin appendages typically begins in a very similar way. Indeed, all of these structures develop from cells that produce a localized thickening on the skin surface and express particular genes. One of these genes, called Sonic hedgehog (Shh), controls a signaling pathway -- a communication system that allows the transmission of messages within and between cells. Shh signalling is involved in the development of diverse structures, including the neural tube, limb buds and skin appendages.

A common ancestor

The laboratory of Michel Milinkovitch, professor in the Department of Genetics and Evolution at the Faculty of Science of the UNIGE, is interested in the physical and biological processes that generate the diversity of skin appendages in vertebrates. In particular, his group has previously demonstrated that hair, feathers and scales are homologous structures inherited from a reptilian common ancestor.

Feathers of the chicken embryo are used by scientists as a model system to understand skin appendage development. While it is known that certain breeds of chickens, such as the 'Brahma' and 'Sablepoot' varieties, exhibit feathered legs and dorsal foot surfaces, the genetic determinism of this trait is not fully understood.

A transient modification for a permanent change

As the signaling pathways responsible for this transformation have not been fully determined, Michel Milinkovitch's group investigated the potential role of the Shh pathway. "We used the classic technique of 'egg candling', in which a powerful torch illuminates blood vessels on the inside of the eggshell. This allowed us to precisely treat chicken embryos with a molecule that specifically activates the Shh pathway, injected directly into the bloodstream,'' explains Rory Cooper, a post-doctoral researcher in Michel Milinkovitch's laboratory and co-author of the study.

The two scientists observed that this single stage-specific treatment is sufficient to trigger the formation of abundant juvenile down-type feathers, in areas that would normally be covered with scales. Remarkably, these experimentally-induced feathers are comparable to those covering the rest of the body, as they are regenerative and are subsequently and autonomously replaced by adult feathers.

After comparison with embryos injected with a 'control' solution (without the active molecule), RNA sequencing analysis showed that the Shh pathway is both immediately and persistently activated following injection of the molecule. This confirms that activation of the Shh pathway underlies the conversion of scales into feathers.

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