Monday, May 28, 2012

Horseshoe Crabs And Other Stories By Richard Fortey

OnPoint Radio had palaeontologist Richard Fortey for a talk on his new book Horseshoe Crabs and Velvet Worms: The Story of the Animals and Plants That Time Has Left Behind .

The lead-in talks about how in the history of life, species have come and gone, except for some remarkable survivors who persist today almost unchanged in appearance since they first originated hundreds of millions of years ago. These kind of creatures whose morphology resembles that of their ancestors are called "living fossils" and Richard Fortey gives quite a few examples in this entertaining talk. He is a good speaker. He qualifies the term "unchanged through time" by clarifying that these creatures have evolved, maybe not in morphology, but certainly at a molecular level. The talk has a lot of information about the habitats, life history and idiosyncrasies of life style and biochemistry of many of these organisms.

One important point is never made in the talk. What are you referring to when you use the term "survivor"? For example, the talk compares the coming and going of different species like the mastodon and musk ox with the persistence of horseshoe crabs suggesting that survivors means individual species.

The term survivor though refers not to individual species but a lineage.

Think about your own family. Say you have well kept records that traces your genealogy to 700 years ago. You could say that your family is a survivor from before the days of the black plague. But it would be absurd to suggest that you have in your family someone living today who was born in the 1300's. Every individual in your family lived within the normal range of humans lifespans. Your family lineage though contains an unbroken chain of descendants of which you are a living relative today.

Species too have lifespans just like individuals. They originate at a point in geologic time and then go extinct at some later date. The lifespan may be a few million years or so with a lot of variation amongst different types of creatures. Mammal species seem to have a lifespan of a million or two million years. Marine invertebrates may have a life span up to 10-11 million years. Horseshoe crabs are marine invertebrates and a type of arthropod. This group of arthropods originated in the Late Ordovician, about 450 million years ago. That first species i.e. the founder of the group of horseshoe crabs gave rise to descendant species and those descendant species gave rise to more recent species and so on in an unbroken chain of species until today.  As a group the horseshoe crab has survived until today but earlier species within that group are now extinct. The living representatives (species) of the persisting lineage of horseshoe crabs may have evolved just a few million years ago.

So today's horseshoe crabs are not really creatures that time left behind. They are a lineage that like any other lineage has been evolving and budding of descendant species, although it may be that the lineage of horseshoe crabs is not particularly speciose i.e. it does not bud of too many species. Today, biologists can recognize 4 living species of horseshoe crabs, which means that morphological diversification in this lineage continues, although at a slower rate than many other types of organisms.

Without this distinction, throughout the talk, the longevity of a group was being compared to the longevity of a species; for example a comment was made that sponges (the group) are 600 million years old but Homo sapiens (species) are just a hundred thousand years old. That is an unfair comparison and Richard Fortey surely knows that but he let it pass.

That is not to say that these lineages don't have an important story to tell about the reasons behind their resilience. But they are not some kind of frozen Jurassic Park.  

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