Although there is terrific diversity in Crustaceamorpha, ancient crustaceans occupied the same general ecological role as do moderns ones. They are therefore boring. Insects are well known from other sources. Therefore our survey of Arthropoda focuses on two speciose and ecologically significant ancient groups that generally lack modern equivalents, Trilobita and its sister taxon, Chelicerata. Today, Chelicerates:
Trilobites and chelicerates are united in the monophyletic group Arachnomorpha: Synapomorphies are subtle, including the loss of the first antenna (recall that crustaceans have two pairs, trilobites only one and chelicerates none). Indeed, a comparison of very early trilobites and chelicerates shows their similarity to their last common ancestor.
Note, for example the post-anal telson (spine), a feature soon lost in trilobites but retained and elaborated in chelicerates.
Chelicerata. (Cambrian - Recent) Marine and terrestrial. Major synapomorphies:
As cladists have gone to work on the Burgess Shale fauna, it has emerged that many of the formerly enigmatic arthropods are either proper chelicerates of very close to the chelicerate common ancestor, including:
Xiphosura. (Silurian - Recent) Marine. Horseshoe crabs.
The living xiphosuran Limulus polyphemus nicely demonstrates the modification of the opisthosomal appendages as elements in Book gills.
Arachnida. (Silurian - Recent) Marine and terrestrial. Major groups include:
But today's focus is on.....
"Eurypterida". (Ordovician - Permian) Marine to fresh water. The name is in quotes to signify that the group may well be paraphyletic. (I.e. some eurypterids are closer to arachnids than others.)
Whatever.... Eurypterids are at least an ecologically distinctive and significant grade of chelicerate evolution.
Swimming: Eurypterids could clearly walk along the bottom, however almost all had sixth appendages modified as paddles and could surely swim to some extent. In some, the small size of the paddles or presence of other distinctly non-streamlined structures argue against routine powerful swimming. Others, such as Baltoeurypterus (pictured below) were streamlined with large paddles. Swimming was appendicular but beyond that what can we say? The issue is complicated by ontogenetic increase in lift-drag ratio due to allometry of paddles. Juveniles had high drag and probably used rowing. Adults had relatively lower drag and higher lift, consistent with underwater flight (unclear whether it was asymmetrical or symmetrical). Specimens of juveniles with exceptional three-dimensional preservation have shown that the joints were capable of achieving rowing motions. Based on joint reconstructions this tended to be used in rowing rather than subaqueous flight. Extimated top rowing speed is about 2.5 X body length per second however a large mature Baltoeurypterus using subaqueous flight might have managed 3 - 4 m/sec. - similar to sea turtles.
Walking: Depositional settings of eurypterid fossils indicate that they ranged from fresh to marine water, that individual animals seemed to be tolerant of a broad range of salinity, and that different groups tended to have preferences for a particular environment. Some have limbs that seem robust enough to allow excursions onto land. We have trackways that appear to have been made by brackish-fresh water eurypterids, indicating that, like crabs, they could emerge from the water.
A Rogue's Gallery of Eurypterid Diversity:
Baltoeurypteridae. (Ordovician - Devonian)
Hughmilleriidae. (Ordovician - Permian)
Pterygotidae. (Ordovician - Devonian)
Mixopteridae. (Ordovician - Pennsylvanian)
Carcinosomatidae. (Ordovician - Devonian)
The Chelicerate Invasion of the Land
We have seen that some eurypterids could walk on land. Starting in the Silurian, we see members an another group that we would call small eurypterids if they had gone extinct in the Devonian. Instead, we recognize them as the first scorpions.
Like the walking eurypterids, scorpions made frequent excursions onto land, and, being mostly small (the 1.5 m. Silurian Brontoscorpio notwithstanding) and having relatively large legs, they were better adapted to it. Note that the Silurian was when vascular plants began invading the land in large numbers. Thus, scorpions were taking a place in the terrestrial ecosystem almost as soon as it existed. By the Mississippian, we see the remains of scorpions that were fully equipped to breathe and feed on land, along with other terrestrial arachnids.
Problems with Arachnid monophyly: All of this rasies some creepy questions. Arachnids are diagnosed as monophyletic by only a handful of rather subtle morphological synapomorphies including:
Whatever the case, it seems clear that if scorpions hadn't emerged from the water, they would probably have been extinguished at the end of the Permian along with everyone else. What happened to the marine eurypterids? We can break their career down into this timeline:
Of course, maybe just as eurypterids are called 'sea-scorpions," maybe scorpions are "land-eurypterids." Stay tuned.