CLAIMS OF POST-CRETACEOUS ICHTHYOSAUR SURVIVAL PT. 5 by Scott Mardis

In the previous installment in this series, I was critiquing creationist Chris Parker’s theory of dolphin descent from ichthyosaurs. One of the anatomical discrepancies between the two groups that I cited was the position of the orbits relative to the connection of the lower jaw to the cranium (as a general rule, the orbits occur forward of the jaw connection in reptiles and behind the jaw connection in mammals.) On closer examination, however, this idea would appear to be more ambiguous, as the figure below demonstrates.

. Clockwise from top left: (1) Hydrurga leptonyx, the Leopard Seal, a pinnipedian marine mammal, (2) Cryptocleidus oxoniensis, a plesiosaurian marine reptile, (3) Platypterygius longmani, an ichthyosaurian marine reptile, and (4) Janjucetus hunderi, a cetacean marine mammal. The claim made by W.B. Benham in 1935 that Tangarosaurus kakanuiensis from the Miocene of New Zealand was a probable ichthyosaur rested largely on the argument that the two mandibular fragments assigned to T. kakanuiensis each were

comprised of two bones as would be expected in the lower jaw of a reptile but not a mammal. In Part 3 of this series, I suggested that perhaps a furrow in the lower jaw of a cetacean might give the misleading impression that said bone was comprised of two bones and I produced a photo of a Squalodont jaw with a furrow along the jawbone to illustrate the idea ( see below)

Squalodont whale lower jaw A beaked whale fossil, Ninoziphius platyrostris, from the Pliocene of Florida, bears an even more striking resemblance to the T. kakanuiensis jaw fragments. The mandibular fragment assigned to N. platyrostris is interpreted as a fused right/ left mandible, with the suture representing the boundary between the two fragments.

Top: Fused right/left mandible fragment of Pliocene beaked whale Ninoziphius platyrostris, bottom: Right mandible fragment of Tangaroasaurus kakanuiensis

Top: Left mandible fragment of Tangaroasaurus kakanuiensis, bottom: Fused right/left mandible fragment of Ninoziphius platyrostris Despite the superficial resemblance between the putative ichthyosaur and beaked whale mandible fragments, there are anatomical discrepancies which must be addressed. In order for the beaked whale mandible fragment to display this bipartite condition, it must be either the dorsal or ventral view of the lower jaw where the right and left mandibles join together, as illustrated by the figure below of the lower jaw of Platanista gangetica, the Ganges River Dolphin.

Any teeth attached to such a fragment, even if angled obliquely away from it, would ultimately have tooth roots pointing upwards relative to the suture between the right and left mandibles. In both the right and left mandible fragments of Tangaroasaurus, tooth fragments and tooth roots clearly show the tooth root orientation to be at a 90 degree angle to the groove or suture. This is also the ichthyosaurian condition, as illustrated by a figure (below) from a paper by Maria Zammit and Benjamin Kear (2011) about healed bite marks on the lower jaw of an ichthyosaur, Platypterygius sp.. In both Tangaroasaurus jaw fragments, the suture is much more pronounced and continuous than the furrow seen on the Squalodont lower jaw figured above. If these interpretations are correct, perhaps Benham was right after all.

Illustration from Zammit and Kear (2011) showing bones of ichthyosaur lower jaw

Right mandible fragment of Tangaroasaurus with tooth fragment (t) in jaw, with (g) representing groove or suture (5 ft. in length) had a missing fossil record of 80 million years (Paleocene and Miocene Coelacanth fossils have subsequently been discovered. If Tangaroasaurus kakanuiensis from New Zealand and Ichthyosaurus gaudensis from Malta really represent ichthyosaurs from the Miocene epoch, they appear roughly 73 million years after the presumed final extinction of all ichthyosaurs. Critics of the so-called “Prehistoric Survivor Paradigm” argue that no large vertebrate has such a long “ghost lineage”, despite the fact that, when they were rediscovered alive in 1938, the Coelacanth fishes (5 ft. in length) had a missing fossil record of 80 million years (Paleocene and Miocene Coelacanth fossils have subsequently been discovered.

The Megamouth Shark, Megachasma pelagios The Megamouth Shark, Megachasma pelagios (above), was totally unknown to science until 1976. Regarding a possible ghost lineage for the Megamouth shark, here is a quotation from a paper entitled “The presence of Megachasma (Chondrichthyes: Lamniformes) in the Neogene of Belgium, first occurrence in Europe” by Pieter De Schutter (GEOLOGICA BELGICA (2009) 12/3-4:pp.179-

203): “4.2.5 Possible Cretaceous origin: Based on DNA, Shirai (1996: fig. 4) and Martin et al.(2002) speculated about a middle Cretaceous origin for the Megachasma lineage. Shimada (2007) seemed to add considerable evidence to support this hypothesis by describing the oldest known megachasmid-like tooth design to date, Megachasma comanchensis, found in a middle Cenomanian deposit (Lincoln Limestone Member, Greenhorn Formation) in Colorado, USA. This early Late Cretaceous species, only represented by four teeth, has a hooked crown with weak cutting edges and a massive root with a strong lingual protuberance. This tooth-design is certainly reminiscent of teeth of modern planktonivorous lamniforms such as Megachasma pelagios (Shimada,2007:512). However, the absence of cusplets and extremely short root lobes are in contradiction with what is observed on the Californian teeth or the

tendencies in the examined group of megachasmids (California, Belgium and extant teeth, Table 2). In addition, teeth of M. comanchensis share a remarkable resemblance with those of the Cretaceous odontaspidid Johnlongia Siverson,1996, to which they were previously attributed (Shimada et al., 2006). These observations and the time gap of 70 million years between these Cenomanian teeth and the Miocene specimens from California makes this taxonomic assignment somewhat suspect.”

Top Left: Tooth of living Megachasma pelagios, Black and white left: Presumed Megachasma comanchensis teeth, Right: Megachasma sp. teeth from the Miocene of California

Top Black and White: Presumed Megachasma comanchensis teeth, Middle and Bottom Color: Teeth of Johnlongia parvidens

Artist’s conception of Megachasma comanchensis with mosasaurs and ammonites Perhaps more directly relevant to the question of post-Cretaceous ichthyosaur survival are the real and implied ghost lineages throughout the known natural history of ichthyosaurs. One genus, Maiaspondylus, would appear to have a ghost range of around 50 million years. Lazarus ranges refer to where a known genus disappears for a significant geologic time (maybe millions of years) and then reappears.

(From “Resetting the evolution of marine reptiles at the Triassic-Jurassic boundary” by Philippa M. Thorne, Marcello Ruta, and Michael J. Benton, Publications of the Academy of Natural Sciences, PNAS, May 17 2011, vol. 108, no. 20, pp. 8339–8344) Besides the two putative Miocene ichthyosaurs, there is the discovery of mandible fragments of two different species of ichthyosaurs (Ichthyosaurus franciscanus and Ichthyosaurus californicus) in outwash gravels of the Pliocene to Holocene (4.5 Mya to Recent) Tulare Formation near Tracy, California in the San Joaquin Valley. These specimens were described by paleontologist C.L. Camp in 1942.Based on the discovery of a presumably diagnostic radiolarian (amoeboid protozoa, diameter 0.1-0.2 mm, that produce intricate mineral skeletons) called Dictyomitra varians in the matrix surrounding one of the fossils, it was claimed that the ichthyosaur fossils were “reworked” from the Franciscan Formation, representing the Tithonian stage of the Jurassic period (150-145 Mya).Reworking in this case must have been caused by tectonic activity, fluvial action or meteoric waters (rain), as glacial

scouring was impossible due to the fossil sites extreme southern location relative to Pleistocene glacial margins. Then again, maybe there is a slim chance these fossils were not “reworked”. What are the normal, non-reworked fossils to be found in the Tulare Formation? Brackish-water/freshwater invertebrates and fish. Below are photographs of some of these fossils.

Fossils from the Tulare Formation. Top from left to right: Fish tumor, fish tumor, oyster. Center from left to right: the pectin Pectin coalingensis, the sand dollar Dendraster coalingaensis, clams. Bottom: snails Were there brackish-water and freshwater ichthyosaurs? Maybe.

Ophthalmosauridae incertae sedis OUM J.13795, Purbeck Limestone Group, Lower Cretaceous; ‘Swanage’, Dorset, England. (from Ensom et al. 2009) “It remains open to speculation whether the ichthyosaur either(a) was a rare visitor to these marginal marine environments when a more fully marine incursion was temporarily established, perhaps searching for food, and later dying, or (b) was washed in while alive or as a carcass, perhaps during a storm surge, or (c) was simply one small individual of a population that like its apparently ‘marine’ invertebrate neighbours had become adjusted to the reduced salinity, but relatively stable conditions of the Purbeck ‘open’ lagoon. The third and simplest of these is our preferred scenario. We leave open to future investigation the question as to why this ichthyosaur’s remains came to be found within an essentially non-marine group of rocks, although we have sympathy with the view it may simply have been living there.”- P.C. Ensom et al., “The age and identity of an ichthyosaur reputedly from the Purbeck Limestone Group, Lower Cretaceous, Dorset, southern England”, Cretaceous Research, 30 (2009), pp. 699–709. So, make of it what you will: there are two disputed possible Miocene ichthyosaurs in the scientific literature (though the general consensus is that they are probably misidentified and do not represent late surviving ichthyosaurs), as well as two “reworked” ichthyosaur

specimens, probably of Jurassic age, in Pleistocene deposits from California.

THE END