Saturday, 22 August 2015

On The Origin Of Angiosperms

Angiosperms are the most successful plant group on the planet
Angiosperms, better known as the flowering plants, are the most diverse group on the planet. It is the combination of flowers and seeds enclosed within a capsule - commonly called the fruit - which has made the them so successful, enabling them to infiltrate virtually all of Earth's terrestrial environments.

Some fruits are colourful and sweet and so attract animals to eat them. In turn the plants can use the guts of animals as a freight system for widespread dispersal. A few fruits are designed to anchor on to fur using a series of microscopic hooks. Others are simply designed to be robust - palm trees can propagate across oceans owing to the thick shell of the coconut.

Similarly, flowers have evolved to attract a wide range of animals from insects to hummingbirds. During feeding pollen is transferred to the animal and pollination occurs during the animal's next encounter with another member of the same plant species. The elaborate and diverse flower and fruit-based strategies for pollination and dispersal have enabled angiosperms to become a globally successful group. Yet their evolutionary origins are rather more humble. The New Caledonian flower Amborella trichopoda has petals which are very similar to leaves, giving us a glimpse of where flowers may have come from.

The 125 to 130 million year old fossil of Montsechia
, the oldest known angiosperm on the planet
Flowers are common to, yet do not characterise, angiosperms. The identifying feature is the eponymous encapsulated seed. Previously, the oldest angiosperm fossil was Archaefructus, a 125 million year old species from China. A new species, Montsechia, has pushed back the angiosperm fossil record by 5 million years.

'A first flower is technically a myth, like the first human,' said David Dilcher from Indiana University's Bloomington College of Arts and Sciences' Department of Geological Sciences. 'But based on this new analysis, we know now that Montsechia is contemporaneous, if not more ancient, than Archaefructus.'

Specimens of Montsechia vidalii were first discovered more than 100 years ago in the early Cretaceous limestone of the Iberian and Montsec ranges in Spain, but only recently has Dilcher's reanalysis revealed its angiosperm anatomy.

By carefully dropping hydrochloric acid onto the specimen Dilcher was able to separate the stems of leaf structures from the rock. They were then cleaned with nitric acid and bleached with potassium chlorate before a detailed examination under stereo, light and electron microscopes.

'Montsechia possesses no obvious 'flower parts,' such as petals or nectar-producing structures for attracting insects, and lives out its entire life cycle under water,' said Dilcher. 'The fruit contains a single seed which is borne upside down' - the defining characteristic of an angiosperm. 'Indeed the plant's appearance would have been most similar to a distantly related dark green aquatic plant called Ceratophyllum. 'There's still much to be discovered about how a few early species of seed-bearing plants eventually gave rise to the enormous, and beautiful, variety of flowers that now populate nearly every environment on Earth,' Dilcher concludes.