Kaikaifilusaurus and the origin of the Rhynchosaurs

Kaikaifilusaurus calvoi (Simon and Kellnwer 2003, Cenomian, Cretaceous, Fig. 1) was considered a new and late surviving sphenodontoid (= rhynchocephalian) when originally described, and indeed it is.

However, and going unrecognized, Kaikaifilusaurus is also the closest sister I’ve tested (Fig. 2) to the Rhynchosauria (Hyperodapedon, Fig. 3). Thus Kaikaifilusaurus knits rhynchosaurs even closer to the Rhynchocephalia in the large reptile tree (Fig. 2).

Of course this is heresy
While a hundred years ago paleontologists associated rhynchosaurs with rhynchocephalians, all that changed after Benton (1983) when rhynchosaurs were thereafter associated with archosaurs and prolacertiforms, none of which they resembled so much. If anyone else is wondering about such strange bedfellow associations, join the club.

Sorry for the late post
I just became aware of Kaikaifilusaurus while hanging around the Geology Library at Washington University, here in St. Louis. The Simon and Kellner (2003) paper is already ten years old. The holotype MPCHv 4 consists of part of a mandible. Figure 1 portrays a later find, MPCA 300 from Apesteguía and Novas (2003).

Like a Rhynchosaur
Kaikaifilusaurus has elevated the orbit to the top of the skull. The toothless premaxilla descends sharply. The dentary anterior processes hold the premaxilla medially when the jaws are closed. The maxilla is ventrally convex. There’s more of course.

Unlike a Rhynchosaur
Kaikaifilusaurus lost the lateral temporal fenestra, which all other rhynchocephalians except Trilophosaurus, retained. The jugal expanded to fill that opening. Kaikaifilusaurus doesn’t have a wider than typical skull, nor do the nares open dorsoanteriorly.  I’d like to see images of the skull from other views to score more traits.

Chronology
Like our very own anachronistic Sphenodon, Kaikaifilusaurus was a late survivor in the Cretaceous from the early Triassic radiation that produced Hyperodapedon and the rhynchosaurs.

Funny that no online reference discusses the rhynchosaur connection of Kaikaifilusaurus, yet it seems rather obvious (Fig. 3). The previous morphological gap between Mesosuchus (Fig. 3) and Hyperodapedon in the large reptile tree was somewhat broad. I suppose it was only a matter of time (like ten years ago) for the transitional taxon to appear. It matters little that certain traits (like the loss of the lateral temporal fenestra) show that the ancestors of Kaikaifilusaurus evolved their own way for tens of millions of years after the Triassic.

A note to all detractors
All of the above is done without Photoshop, so no DGS here. I’ve simply expanded the gamut of the inclusion list to maximize the opportunity for a correct nesting of all 341 taxa and I used images form the literature. You can test these associations yourself by simply including these taxa and your own traits in any phylogenetic analysis.

Certainly sometime, someone will find a closer relative to rhynchosaurs, but at present this may be the best we have (unless there’s another paper I’m unaware of floating around now!) Don’t make me wait another ten years, if so.

Reference
Apesteguía S and Novas FE 2003. Large Cretaceous sphenodontian from Patagonia provides insight into lepidosaur evolution in Gondwana. Nature 425:609-612
Benton MJ 1983. The Triassic reptile Hyperodapedon from Elgin, functional morphology and relationships. Philosophical Transactions of the Royal Society of London, Series B, 302, 605-717.
Simon ME and Kellner AWA 2003. New sphenodontid (Lepidosauria, Rhynchocephalia, Eilenodontinae) from the Candeleros Formation, Cenomanian of Patagonia, Argentina. Boletim do Museu Nacional, Geologia, nova série 68:1-12. online here.