Posts Tagged ‘Cretaceous spider fossils’

Pleistocene Spiders

August 5, 2015

My fascination with the ecology of southeastern North America prior to man’s colonization of the region is frustrating.  Most of the data dating to this time period has been lost to the ravages of time.  I can make educated guesses about the appearance of pristine landscapes untrammeled by man, but this is little better than conjecture based on the pollen and fossil records from a handful of sites.  Many species of plants produce so little pollen they don’t even register in samples collected by palynologists.  And the pollen that is recovered usually can’t be identified below the genera level.  So we know that pine may have produced up to 50% of the pollen at 1 site during a certain time span, but we don’t know whether it was from jack pine, a cool climate species, or longleaf pine, a denizen of warm climates.  But at least we have some data about Pleistocene plant composition.  There are thousands of species of spiders living in the south today, yet I’m unaware of a single Pleistocene-aged site in southeastern North America where the remains of spiders have been recovered.  Spiders have been a vital part of land ecosystems ever since life crept out of the sea.  Nevertheless, there is no evidence of spider species composition dating to the Pleistocene.  Spiders are soft bodied creatures, and their remains get crushed into dust over time.  The remains of hard-shelled beetles, millipedes, and snails have been found in cave deposits located in the southern Appalachians because their hard exoskeletons can remain intact for millennia.  Spiders were also certainly abundant in caves during the Pleistocene, but evidence of their presence is gone.

It’s unfortunate that we know nothing about spider species composition and distribution in the south during the Pleistocene. Even modern spiders are poorly known. There are over 120,000 species of spiders in the world, but only a tiny fraction of these have been studied in detail.  The vast majority of spider species have yet to be described and named in the scientific literature.  Spiders vastly outnumber the scientists willing to study them. Spiders have high rates of reproduction, and therefore evolve rapidly, making them an ideal candidate for an evolutionary biologist to study.  During the Pleistocene many present day species already existed, but there may have been some now extinct species.  Some were evolutionary ancestors of present day species, while others may have left no descendants.  The abundance of certain species varies annually for unknown reasons, perhaps chance.  Below are some interesting species of spiders.

drawing

Artist’s depiction of a trapdoor spider (Ummidia sp.).  I can usually find 1 of these when I dig in my garden.

Common garden spider aka black and yellow argiope (Argiope aurantia).  They build big beautiful webs capable of catching small birds and bats.  Some natives use orb-weaver webs as fishing nets.  Spider webbing is incredibly strong.

Green lynx spiders  (Peucetian viridans) are beneficial for gardeners.  This individual caught a moth.  The larva of some species of moth eat through garden vegetables.

Spined Micrathena, Micrathena gracilis

A spined micrathena (Micrathena gracilis).

Diet of elusive red widow spider revealed by MU biologist

The red widow spider (Latrodectus bishop) lives in the Florida sandhills and feeds upon beetles.

The red widow spider inhabits the pine sandhills of Florida which were islands surrounded by ocean during the marine highstands of Pliocene and Pleistocene warm climate phases.  They construct funnel webs in rolled up palmetto leaves.  This ancient species specializes in capturing flying scarab beetles. The beetles fly low to avoid bat and common nighthawk (Chordeiles minor) predation, but this strategy makes them more likely to fly into red widow spider webs.  Red widows are related to the more common black widow spiders.  They exist as a relic population today but were likely more common when sand scrub habitat prevailed in the south during the late Pliocene/early Pleistocene.

The ancestor of spiders was closely related to the horseshoe crab.  It colonized land about 400 million years ago.  Fossil remains of spiders and spider webs dating to the Cretaceous (~100 million years BP) have been found.  Some were preserved in amber, and others were fossilized in layers of ash that turned to rock.  Though the Cretaceous species are extinct, scientists classify them as belonging to extant genera.

Earth will undoubtedly host spiders long after Homo sapiens becomes extinct.  The drastic changes man has wrought to the environment have barely phased spiders.  Compared to the changes spiders have seen over the 400 million years of their existence, anthropogenic effects are but a minor blip.  Look around, there are spiders inside your house now.

Reference:

Carrell, James; and Mark Deyrup

“Red Widow Spiders (Araneae: Theridiidae) Prey Extensively on Scarab Beetles Endemic in Florida Scrub”

Florida Entomologist 97 (1) March 2014

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