The rare sight of 40 million-year-old seeds sprouting from a pinecone fossil has been found immortalised in amber.
This unusual method of development – called precocious germination – isn’t common among plants in general and is almost unheard of in pines. The pinecone fossil provides the first evidence of the trait’s extraordinary age.
“Crucial to the development of all plants, seed germination typically occurs in the ground after a seed has fallen,” says palaeobotany expert George Poinar Jnr of Oregon State University, US.
“We tend to associate viviparity – embryonic development while still inside the parent – with animals, and forget that it does sometimes occur in plants.”
Precocious germination mostly occurs in angiosperms, which are flowering plants like apples and oranges that make up most of the fruit that we eat. Angiosperms usually have seeds that are enclosed in fruit. However, pines are a different type of plant – gymnosperms.
Gymnosperms such as conifers produce “naked” seeds that aren’t enclosed in fruit and the embryo is dormant, where the seed won’t germinate until it is in the perfect conditions.
“Seed germination in fruits is fairly common in plants that lack seed dormancy, like tomatoes, peppers and grapefruit, and it happens for a variety of reasons,” explains Poinar. “But it’s rare in gymnosperms.”
This makes precocious germination in gymnosperms incredibly rare – so rare, in fact, that Poiner says there has only been one description of this naturally occurring in literature, back in 1965.
“That’s part of what makes this discovery so intriguing, even beyond that it’s the first fossil record of plant viviparity involving seed germination,” he says.
“I find it fascinating that the seeds in this small pinecone could start to germinate inside the cone and the sprouts could grow out so far before they perished in the resin.”
The fossil is that of the extinct pine species Pinus cembrifolia, but pines in general aren’t often found preserved in Baltic amber. When they are, they are highly prized amongst collectors because the cone scales are so well-preserved they appear life-like.
Regardless, why these seeds decided to grow inside the cone is still somewhat of a mystery.
“In the case of seed viviparity in this fossil, the seeds produced embryonic stems that are quite evident in the amber,” explains Poiner.
“Whether those stems, known as hypocotyls, appeared before the cone became encased in amber is unclear. However, based on their position, it appears that some growth, if not most, occurred after the pinecone fell into the resin.
“Often some activity occurs after creatures are entombed in resin, such as entrapped insects depositing eggs. Also, insect parasites sometimes flee their hosts into the resin after the latter become trapped. In the case of the pine cone, the cuticle covering the exposed portions of the shoots could have protected them from rapid entrance of the resin’s natural fixatives.”
Pinecone fossil hints at past environment
Ancient plants preserved in amber give hints about the environment because they show the anatomy and behavior of plants in high detail, esecpially when seen at such an early stage of development.
Viviparity in other living gymnosperms suggests this trait could be linked to winter frosts. This would suggest that the ancient Baltic amber forests had a humid, warm-temperate environment with some light frosts that allowed the pine seeds to germinate.
“This is the first fossil record of seed viviparity in plants, but this condition probably occurred quite a bit earlier than this Eocene record,” says Poinar.
“There’s no reason why vegetative viviparity couldn’t have occurred hundreds of millions of years ago in ancient spore-bearing plants like ferns and lycopods.”
The pinecone fossil was descibed in a study published in Historical Biology.