Mid Cretaceous radiation

Homepage

Mono/ Polyphyletic
origins

Terrestrial/ Aquatic
origins

Euanthial/
palaeoherb?

Purported Triassic
angiosperms

Mid Cretaceous
radiation

Environmental
considerations

Coevolution with
insects

Coevolution with
dinosaurs

The fossil record shows that angiosperms replaced their precursors, gymnosperms, as the dominant flora within 40 million years: an evolutionary heartbeat. While the reasons for this sudden take-over are still unclear, two possibilities have been suggested:

The competitive displacement theory: that angiosperms were able to outcompete their rivals for virtually every resource, shouldering them out of the way as we see at a lower, interspecific, level today. Angiosperms have a shorter life-cycle than gymnosperms, and are thus able to increase their numbers more rapidly.

Non-competitive model: The mass extinction at the end of the Triassic left many niches open when a large proportion of gymnosperm and pteridophyte lineages died out. There was no real competition; the slate was wiped clean and the angiosperms could settle in unimpeded.

When, exactly did the radiation take place?

The radiation occurred in the mid to late Early Cretaceous, when there was a massive boom in diversity, complexity and abundance. Originally, this was noted in deposits containing leaf and pollen remains.

The rise to dominance

Angiosperms originated in the fossil record 130 million years ago. At the time, there were only two continents: Laurasia and Gondwana., the result of the continuing break-up of Pangaea.

Considering the size of the continents, their interiors would have been almost untouched by water, as rain clouds would never reach the centre: they would evaporate away as they travelled miles over the hot baked earth below. This made them extremely arid environments. It is widely recognised that the first angiosperms were suited to such conditions, and this environmental setting can also explain the absence of early flowers in much of the fossil record: there was simply nothing for the flowers to sink into and die on an arid upland area. Fires swept across the supercontinents leaving a trail of destruction in their wake and with them, charcoal, which in turn was preserved, forming charcoalifications.

Why did angiosperms evolve flowers?

Flowers exist for one reason: reproduction. The consensus is that all parts of the flower are modified leaves: sepals, petals, stamens, carpels and stigmas all evolved from the same basic plan. Today, angiosperms are among the most successful organisms on earth, thanks to their modes of reproduction, and their methods of propagation. A flower is a replicating machine: its reproductive organs are perfectly positioned to give it the maximum effect. Wind pollinated flowers have vast amounts of pollen on large anthers. Those pollinated by insects have much less on smaller anthers, the better to allow the insect to brush past them and load itself with pollen to pass onto the next flower.

Dissection of a simple flower
http://naturalsciences.sdsu.edu/classes/lab2.6/lab2.6.html

Why are angiosperms so successful?

There are several reasons why angiosperms have taken the world by storm. They have many innovations that their rivals, principally the gymnosperms, did not have. Let us look at them each briefly in turn.

A giant sequoia
One of the largest living organisms on Earth
http://www.cs.wisc.edu/~hbwang/Travel/Sequoia.html
Angiosperms today can survive at the poles, in the deserts, in both freshwater and seawater, not to mention myriad intermediate habitats. Their foliage and blooms take many different shapes and sizes, but it was not always so. The evolution of flowers is not completely understood, but we know that they started small.

Early angiosperms

The oldest definite angiosperms date back to around 123 million years ago. These are not the first true angiosperms; perhaps they emerged much earlier than that, but have either not been preserved or discovered. Though there have been many reports of the ‘earliest angiosperm’ from the Triassic, no definite contenders have been discovered yet, although the enigmatic Sanmiguelia could provide an insight.

Archaefructus was discovered in 1998 in the Yixian Formation of the western Liaoning province of Northeast China. The YixianF ormation is a layer of volcanic ash some 2000-2500m thick between two sedimentary layers containing many plant and animal remains. The remains consist of reproductive axes bearing fruit in a spiral shape, where they ripen from the outside inwards. The fruits are clearly those of angiosperms because they enclose the seeds completely. No pollen or pollen-producing organs have been found, but a structure resembling a rudimentary flower has been discovered, with “crumpled leaf-structures” that may have served the same purpose as petals to attract pollinators. It has been hypothesized that both wind and insect pollination played a part in Archaefructus reproduction.

When it was found, Archaefructus was excitedly hailed as the definite Jurassic angiosperm that had been so long awaited. It was later learned that the rock was Cretaceous in age, putting paid to the theory that angiosperms had originated in Asia.

A is the specimen found, B is an enlarged view of the carpels, and C is a large view of part of a seed from one of these carpels.
http://cas.bellarmine.edu/tietjen/images/in_search_of_the_first_flower.htm