Mesozoic Seed Dispersal

Many plants rely on wind to disperse(spread)their seeds. However, because seeds often fall into the wrong places. wind dispersal seems wasteful; it can work only in plants that produce great numbers of seeds. Wind-dispersed seeds must be light and small, so they cannot carry much energy for seedling growth. They have to germinate(grow from a seed) in relatively well-lit areas, where they can obtain energy from sunlight. Alternatively, a plant could have its seeds carried away by an animal and dropped into a good place for growth. Many animals can carry larger seeds, which can successfully germinate in darker places. Some animals visit plants to feed on pollen or nectar, and others eat parts of the plants. Others simply walk by the plant, brushing it as they pass. Small seeds may be picked up accidentally during such visits, especially if the seed has special hooks, burrs, or glues to help attach it to hairy or feathery visitor.

However, seed dispersal by animals is not cost-free. Many dispersers eat the seeds, passing only a few undamaged through their internal organs. So there is a significant wastage of seeds, depending on a delicate balance between the seed coat and the teeth and stomach of the disperser. Too strong a seed coat, and the disperser will turn to easier food or germination will be too difficult; too weak a seed coat, and too many seeds will be destroyed. Some plants’ seeds germinate well only if they are eaten by a particular disperser.

Scientists believe animals first began dispersing seeds sometime during the Mesozoic era(252-65 million years ago). Seed dispersal by animals surely evolved after insect pollination. Insects of the Jurassic period (206-145 million years ago) may have become good pollinators, but they were too small to have been large-scale seed transporters. Jurassic reptiles (including dinosaurs)were large enough, but often had low food-processing rates, so any seeds they swallowed were exposed to digestive juices for a long time. Reptiles do not even have fur in which seeds can be entangled (though feathered therapod dinosaurs might have). Seeds were undoubtedly dispersed by dinosaurs to some extent, since the huge vegetarian dinosaurs ate great quantities of vegetation. But in spite of the size of the deposits of fertilizer (in the form of waste) that must have surrounded seeds passing through dinosaur, browsing dinosaurs (ones that eat grass, leaves, and shrubs) probably damaged and trampled plants more than they helped them. It is unlikely that any Mesozoic plant would have encouraged dinosaur browsing.

Transport over a long distance can take a seed beyond the range of its normal predators and diseases and can allow a plant to become very widespread provided that there are pollinators in its new habitat. As flowering plants adapted to seed dispersal by animals, they probably dispersed into new habitats much faster than other plants did. Other things being equal, we might expect a dramatic increase in the number of flowering plants found in the fossil record as they adapted Toward seed dispersal by animals rather than by wind.

There were few effective animal seed-transporters in the Jurassic period, and dinosaurs are unlikely candidates in the Cretaceous period(the last period of the Mesozoic era). The biologist Philip Regal suggested that birds and mammals prompted the distribution of flowering plants by aiding them in seed dispersal. Birds and mammals have feathers and fur in which seeds easily become entangled; seeds pass quickly through their small bodies with their high food-processing rates and are likely to be unharmed unless they have been deliberately chewed. Flowering plant seeds would have been especially suited to transport by vertebrate (back-boned) animals because of their extra protective coating. In contrast, seeds of conifers (plants with needle-like leaves)are usually small and light, designed to blow in the wind, and conifers depend on close clusters for pollination. Isolated conifers are likely to be unsuccessful reproducers, and additional transport would make little difference to their long-term success. However. many new types of flowering plants appeared in the Early and Middle Cretaceous period, when mammals and birds were still minor members of the ecosystem, so many scientists are unconvinced by this explanation of flowering plants’ early success.?

1.Many plants rely on wind to disperse(spread)their seeds. However, because seeds often fall into the wrong places. wind dispersal seems wasteful; it can work only in plants that produce great numbers of seeds. Wind-dispersed seeds must be light and small, so they cannot carry much energy for seedling growth. They have to germinate(grow from a seed) in relatively well-lit areas, where they can obtain energy from sunlight. Alternatively, a plant could have its seeds carried away by an animal and dropped into a good place for growth. Many animals can carry larger seeds, which can successfully germinate in darker places. Some animals visit plants to feed on pollen or nectar, and others eat parts of the plants. Others simply walk by the plant, brushing it as they pass. Small seeds may be picked up accidentally during such visits, especially if the seed has special hooks, burrs, or glues to help attach it to hairy or feathery visitor.

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