FEATURE ARTICLE
Virtual Fossils from 425 Million-year-old Volcanic Ash
A set of exceptionally preserved but difficult-to-extract fossils reveals the diverse creatures from a Silurian sea-floor community
Derek E. Briggs, Derek J. Siveter, David J. Siveter, Mark D. Sutton
A Diversity of Crustaceans
Ostracodes (sometimes called "seed shrimps") are crustacean relatives of barnacles and water fleas. These tiny aquatic arthropods are typically only a few millimeters long as adults. Most species live on or close to the bottom.
Ostracodes are by far the most common group of arthropods in the fossil record, known from thousands of species and countless empty shells. They are also abundant today. They have been very successful in adapting to a wide range of marine, brackish and freshwater environments, from the deep ocean to garden ponds.
Although ostracodes have been studied extensively for more than 150 years, the earliest forms were known to paleontologists only from their shells. And without being able to study how their limbs were configured, these investigators struggled to understand the creatures' affinities—could the tiny fossils simply be another group of arthropods masquerading in an ostracode-like shell? With the discovery of fossil ostracodes from Herefordshire, this question was answered at a stroke.
Inside the protective, hinged shell of the first ostracode we investigated, we discovered the head and trunk with their specialized appendages for feeding and locomotion, and a pair of lateral eyes. The gut of this creature is also preserved, together with gills for breathing and evidence of its circulatory system. This anatomy proved remarkably similar to that in some living forms, indicating that these creatures were indeed ostracodes and that little evolutionary change has taken place over 425 million years. The male copulatory organ is even preserved in this Silurian fossil, providing the earliest unequivocal testimony for gender in animals.
Colymbosathon ecplecticos—meaning "swimmer with a large penis"—of course attracted extensive media coverage when we announced its discovery in 2003. The International Herald Tribune noted, "He's 425 million years old and clearly virile." MSNBC declared that the "Oldest known male fossil bares all." And although the New York Times thoughtfully informed its readership that "It's a boy," the UK's Guardian reported on this story under the headline, "Well Hung Scientists' Big Find." It goes to show what heady intellectual considerations drive most of the world's science reporting.
A second exceptionally well-preserved ostracode not only turned out to be a female, but one containing eggs and possible juveniles. We called it Nymphatelina gravida—"pregnant young woman of the sea"—because this fossil provides an unequivocal and unique view of maternal care in a fossil invertebrate and demonstrates that an egg-brooding reproductive strategy was passed down from the Silurian to the present day.
The evidence of the soft parts shows that both these remarkable Herefordshire ostracodes belong to a living group called the myodocopes. Surprisingly, however, the shell of the female in particular resembles that of a different group of fossil ostracodes, the palaeocopes, which are known only from their preserved hard parts. This observation demonstrates that the shell alone may be a poor clue to the true nature of the animal within!
The Herefordshire deposit also preserves a number of other crustaceans, including barnacles, which are remarkable in that they metamorphose from a small free-swimming larva (the cyprid) to a stalked "goose" barnacle or to the more familiar balanomorphs, which are often seen attached to rocks in the intertidal zone. The adult barnacle develops a mineralized shell of several parts, which tend to separate when the animal dies and may be preserved as individual fossils. The tiny larva is much less likely to become fossilized. Indeed, we discovered the first fossil cyprid known, Rhamphoverritor reduncus, in the Herefordshire deposit.
The most familiar living crustaceans, at least to the restaurant goer, are the malacostracans, the group that includes the shrimps, lobsters and crabs. Crabs have a good fossil record because of the thick calcified covering on their backs, which is called the carapace. In simpler malacostracans, however, such as the phyllocarids, even the carapace was soft and prone to decay; hence their fossil record, which holds the key to the origin and early evolution of the group, is poor.
Fortunately we discovered the earliest completely preserved phyllocarid, which we named Cinerocaris magnifica ("splendid shrimp from the ashes"), among the Herefordshire fossils. It has two pairs of antennae and prominent eyes projecting forward from the head, which also bears a suite of feeding appendages. The body is divided into a thorax and abdomen, the latter being muscular (and no doubt good to eat!) and ending in the tail fork typical of this group.
Not all the crustaceans in the Herefordshire deposit are clearly recognizable as belonging to still-living groups. Tanazios dokeron, for example, has the two pairs of antennae and mandible characteristic of crustaceans, but its rear head appendages were not specialized for feeding—they appear very similar to those of the trunk. The head shield of Tanazios is a strange horned structure, and the long trunk is made up of more than 60 short segments, each with a similar pair of unusual appendages shaped a bit like a catcher's mitt. A pair of antenna-like appendages projects from the tail. Given these characteristics, we concluded that Tanazios was an early offshoot of the crustacean lineage.
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