We have revisited the question of Mesozoic non-avialian dinosaur richness using a new global compilation of dinosaur genera from the second edition of The Dinosauria (in press).  Considering body fossils only, this consists of 556 different genera distributed by stage among the Late Triassic (37), Early Jurassic (33), Middle Jurassic (38); Late Jurassic (64); Early Cretaceous (143); and Late Cretaceous (241).  At this level of resolution, dinosaur generic richness unambiguously and dramatically increases throughout their tenure on Earth.  Within the Late Cretaceous, the number of different dinosaur genera increases, reaching a peak in the Campanian (59), but decreasing (39) in the Maastrichtian. 

Because the database is constructed on published records of dinosaur taxa, 39% of all genera have more than one entry.  This suggests an approach allowing comparison by rarefaction:  the published records are treated as repeats in a counting experiment.  Because rarefaction always adjusts downward to the smallest sample, we used Cenomanian (n=48) and rarified samples from the Turonian, Coniacian, Santonian, Campanian, and Maastrichtian. 

 

	Cenomanian	Turonian	Coniacian	Santonian	Campanian	Maastrichtian
# counts	48	51	62	48	257	292
# genera found	37	43	50	38	106	81
Estimated richness	(37)	41	41	(38)	36	31
Standard deviation	(0)	1.4	2.6	(0)	3.9	3.6

 

When this is done the estimated diversity in the Maastrichtian becomes statistically indistinguishable from estimated diversity in the Campanian.  The counts show no obvious relationship to length of time, suggesting that some aspect of real diversity is reflected in these numbers.