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They belonged to a mysterious ancient shark known as Leonodus.
Other than having names for these earliest sharks, we know almost nothing about them.
Fortunately, the shark fossil record becomes richer and more varied from the Devonian Period onward.
The ctenacanths were more typically shark-shaped than the eel-like xenacanths, with a solidly-built, tapered body, two separate dorsal fins, and a deeply-forked tail.
Yet ctenacanths are also characterized by having multi-cusped teeth (a tooth type termed , meaning "branch-toothed"), which are very unlike those of Antarctilamna and the xenacanths.
Tracing the evolution of sharks is frustrated by the nature of the beast.
Due to their characteristic cartilaginous skeleton, ancient sharks have left behind precious few clues to enable us to figure out what they were like.Shark scales have a characteristic tooth-like structure, so we can be reasonably confident that such scales did, in fact, come from some kind of shark.The oldest shark-like scales date back to the late Ordovician Period, about 455 million years ago, from what is now Colorado.Based on the form of this nearly complete braincase, many paleontologists believe that its former owner may have been a xenacanth.The oldest partially articulated fossilized shark remains were discovered by geologist Gavin Young in deposits of about the same age in the Lashley Range of Antarctica.Since many of the earliest fossil shark remains are from Antarctica and Australia, paleontologist John Long has suggested that sharks may have originated in the Southern Hemisphere.The oldest fossilized shark braincase is from mid-Devonian deposits about 380 million years old, in what is now New South Wales, Australia.The earliest known fossil shark teeth are those of Leonodus (left) dating back some 400 million years.Their overall crown shape vaguely resembles that of fossilized Xenacanthus teeth (right), possibly indicating that these early sharks were related.Antarctilamna had a stout spine in front of the long, low dorsal fin and two-pronged teeth (a tooth type termed " were almost exclusively freshwater inhabitants, and had a long, rearward-pointing fin spine just behind the cranium (the name xenacanth means "strange spine"), diplodont teeth, a slender, eel-like body, an elongate dorsal fin extending along most of the back, and a symmetrical, tapering tail.If Antarctilamna was a xenacanth, it probably had the same type of body form and tail, which may have allowed it to swim among dense lake vegetation.