DID DINOSAURS EAT C4 PLANTS DURING THE CRETACEOUS? CARBON ISOTOPE EVIDENCE AND POSSIBLE ECOLOGIC SETTINGS

Sedimentary organic matter and dinosaur tooth enamel has been collected from several Late Cretaceous coastal localities in the Hell Creek Formations of North Dakota. Carbon isotope ratios of organic matter range from –26 to –17 per mil, while ratios of tooth enamel range from –10 to -1 per mil. Ratios higher that –20 and -4 per mil, respectively, cannot be easily accounted for by plants utilizing C3 photosynthetic pathways, and may indicate that plants utilizing C4 mechanisms were a part of certain terrestrial ecosystems in this region during the Cretaceous.

The oldest fossil evidence for C4 plants is from the middle Miocene, and the global expansion of C4 grasslands common at the present time did not occur until later in the Miocene. Leading theories of C4 evolution hold that the C4 pathway is a favorable adaptation to lowering atmospheric CO2 levels, more open/light habitats, and increasing aridity that occurred during the late Cenozoic. These conclusions, however, do not preclude the possibility that C4 plants other than modern grasses were abundant in other terrestrial settings at an earlier time. C4 photosynthetic pathways are polyphyletic traits that may have evolved independently many times in the past. More importantly, environmental conditions along the coasts of the Western Interior Seaway of North America were such that plants utilizing the C4 pathway would have had several advantages over C3 plants. In particular, the high water use efficiency that provides C4 plants with an advantage in arid environments also favors them under saline conditions, as does their rapid growth in periodically disturbed areas such as coastal floodplains.

C4-dominated ecosystems can be found at the present time in saline coastal environments in North America and Europe, and it is suggested that they had antecedents in coastal, mudflat/salt marsh biomes that existed along shallow Cretaceous oceans. Equally exciting, these biomes may have been a major food source for certain dinosaur taxa. If so, the evolution, ecological diversity, and importance of C4 plants may be more complicated and expansive than previously realized.