WEIRD SHARKS
In a way, at the very core of the trilobite definition, Schlotheim seems right. The remains of the animal he encoun- tered in the Permian of Germany (or what was at the time the Duchy of Saxe-Gotha), had three very unique regions, and the middle region, the washboard of horizontal cristae, looked similar to the thoracic ridges of some trilobites. The problem then arises from every other part of the definition of a trilobite: that it had jointed segments, a hard shell not made from apatite (the mineral in bones and teeth), and a hypostome for eating. Another issue is that a trilobite head is not the featureless hemisphere of a Janassa crown. Trilobite heads have eyes, cranial sutures, glabella, and all kinds of other anatomical structures that combine to indicate that a trilobite fossil is what remains of a small arthropod, and that a tooth lacking these features is… a tooth.
The study of fossil animals, and of the earth in general, was
still very new at this time. The idea that trilobites represented bottom-dwelling marine arthropods was a somewhat recent discovery when Schlotheim was alive, and the whole body of Janassa would not be described until 1899.
When Janassa bodies were finally found, especially in the Carboniferous Bear Gulch locality in the 1990s, they displayed a very flat animal. It had eyes on the top of its body, a mouth that faced downward -towards the sea bed- and large flat fins to propel it along. Janassa, it would seem, was a bottom dweller after all; one of the first vertebrates to ever take advantage of the sea floor in the same way that stingrays, skates, and flounders do today. Janassa may have been the first bottom dwelling “flat fish” in the literal sense of the word.
But this revelation isn’t what makes the story ironic. For that, we must look to Edward Lhwyd. In 1698 he published a landmark paper in British paleontology. In it he described several fossils he recovered from Wales in the UK, includ- ing the first published reference to trilobite fossils since the ancient Greeks and Romans. The concept that these strange
stones were, with certainty, the remains of long dead animals was new and hotly debated in Lhwyd’s time. The idea that any animal could go extinct was also new (and generally thought to be false, heretical, or both) in his era. The combination of these two 17th century perspectives led Lhwyd to describe the first ever discovery of a trilobite (outside of the ancient Romans and Greeks) as the “Sceleton [sic] of some flat fish.” This means that the history of our early scientific understand- ing of Janassa is the mirror opposite of our understanding of trilobites: Lhwyd’s flat fish became an arthropod while Schlotheim’s arthropod became a flat fish.
Paige’s entry: Cladoselache and its
missing claspers. Cladoselache was a small shark that went extinct about 350 million years ago. It was about four feet in length, and the body plan of this species is very similar to modern sharks, with a few key differences. These ancient sharks had a terminal mouth and a very weak, flexible jaw joint. They also lacked a solid column of vertebra, a feature of modern-day sharks. Cladoselache also had scales only around its eyes, gills, and mouth. Modern sharks, on the other hand, have dermal scales over most of their bodies.
Some paleontologists suggest that Cladoselache had two primary tooth structures: either smooth teeth or teeth with several sharp cusps. The cusped teeth each have five needle- like points. These cusps improved the grip that the shark would have had on its prey. Cladoselache teeth lead researchers to believe that this species didn’t tear or chew their prey, but rather swallowed their prey whole. Many fossils found show that the shark would digest its prey tail first, leading research- ers to believe that Cladoselache chased down its prey, biting the tail
first.
The diet of the Cladoselache consisted of ray-finned fishes, conodonts, small shrimp-like organisms, and possibly other smaller species of shark.
Some of best specimens of this species come from the Cleveland Shale, located on the southern shore of Lake Erie in Ohio. These specimens are so well pre- served that many fossils have soft body parts intact, such as kidneys, muscles, and skin. There are a few hypotheses as to why this species is so well pre- served. One is that the deeper water was anoxic and aided in preservation by slowing or preventing the decay of soft tissues. Another hypothesis is that Cladoselache’s blood was urea-rich and this, like the anoxic water conditions, helped prevent the decay as the shark was covered in sea floor sediment.
To me, the most fascinating charac-
Lhwyd’s original sketch of the skeleton (A) of “some flat fish” which he recovered from rocks near Llandeilo in Wales. When compared to this Janassa tooth (B) in lingual view from the Pennsylvanian of Nebraska, the horizontal cristae of the tooth appear similar to the thorax of the trilobite (but only vaguely).
www.aipg.org
teristic about this species is the repro- ductive system. Modern male sharks have organs called claspers which trans- fer sperm to the female during mating.
Oct.Nov.Dec 2020 • TPG 35
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