Nichollsaura enters the LRT basal to pliosaurs

Druckenmiller and Russell 2008 described a complete plesiosaur
they named Nichollsia. That name was preoccupied by a segmented invertebrate, so the plesiosaur was renamed Nichollsaura (Figs. 1, 2).

Figure 1. Nichollsaura in situ.

From the abstract:
“A new plesiosaur taxon, Nichollsia borealis, gen. et sp. nov., from the Wabiskaw Member of the Clearwater Formation (Lower Cretaceous, Albian) of northeastern Alberta, Canada, is described. The exceptionally well-preserved, almost complete, fully articulated specimen represents the oldest known, and one of the most complete, Cretaceous plesiosaurs from North America. High resolution computed tomographic data of the skull provide detailed information regarding cranial structure. Nichollsia is compared to other Cretaceous genera, including Leptocleidus, with which it shares similar skeletal proportions, size, and the presence of a prominent dorsomedian ridge on the premaxillae.

Figure 2. Nichollssaura skull.

From the abstract:
Nichollsia, however, possesses numerous autapomorphies, including the possession of a gracile, narrowly triangular skull lacking a rostral constriction, a vertically oriented Suspensorium, a squamosal vertex that lacks a prominent crest, the presence of longitudinal grooves on the lateral surface of the dentary, a relatively homodont dentition lacking caniniforms, and other unique features of the axial and appendicular skeleton. Nichollsia inhabited the Boreal Sea, the first major marine incursion into the northern part of the Western Interior Basin in the Early Cretaceous prior to the establishment of the Western Interior Seaway.”

Figure 3. Cladogram of plesiosaurs including Nichollssaura.
Figure 3. Cladogram of plesiosaurs including Nichollssaura.

Added here
to the large reptile tree (LRT, 1907 taxa; Fig. 3) Nichollssaura nests basal to Plesiopleurodon (Fig. 4) and the pliosaurid clade (Fig. 5). At this transition plesiosaurs developed longer jaws as they evolved from eating small prey to large prey.

Figure 4. Plesiopleurodon documents the longer snout and relatively smaller eyes that arise after Nichollsaura in the LRT.

Initially the size did not change,
only the proportions (Fig. 5).

Figure 5. Plesiosaur taxa from the transition to pliosaurs. Plesiosaurus,, Nichollssaura, Plesiopleurodon and Trinacromerum are here shown to scale.

pliosaurs grew to several times their initial length, all the better to tackle larger prey.

Figure 1. Three pliosaurs, Trinacromerum, Brachauchenius and Kronosaurus, to scale, all Late Cretaceous.
Figure 6. Three pliosaurs, Trinacromerum, Brachauchenius and Kronosaurus, to scale, all Late Cretaceous.

Druckenmiller PS and Russell AP 2008. Skeletal anatomy of an exceptionally complete specimen of a new genus of plesiosaur from the Early Cretaceous (Early Albian) of Northeastern Alberta, Canada. Palaeontographica Abteilung a -Stuttgart- 283(1):1-33.
Druckenmiller PS and Russell AP 2009. The new plesiosaurian genus Nichollssaura from Alberta, Canada: replacement name for the preoccupied genus Nichollsia“. Journal of Vertebrate Paleontology. 29 (1): 276.


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