| Popis: |
Of all the fauna of the Cretaceous period (144 million to 65 million years ago), dinosaurs get most public attention, with mammals a distant second. Less well known is the spectacular adaptive radiation that occurred among crocodyliforms in the southern continents of Gondwana at this time. The notosuchians (or 'southern crocodiles') evolved all manner of strange forms. Their teeth, in particular, were most uncrocodile-like: rather than a line of undifferentiated conical fangs, the teeth were often specialized for biting and crushing, as seen in mammals. A new small-bodied, fossil crocodyliform discovered from Cretaceous sedimentary deposits in the Rukwa Rift Basin of southwestern Tanzania reveals further evidence of convergent morphological evolution. It has dentition capable of the crown-to-crown contact characteristic of mastication and shares almost all of the dental characteristics considered to be 'mammalian'. And in adding a small-bodied constituent to the terrestrial fauna of the region, this find strongly suggests that the notosuchians occupied ecological niches in Gondwana that were the realm of mammals in the northern continents. A spectacular adaptive radiation among notosuchian crocodyliforms in the southern continents of Gondwana led to all manner of strange forms; in particular, their teeth, rather than being undifferentiated conical fangs, were often differentiated into biting and crushing types, as seen in mammals. These authors describe a new form from the Cretaceous period of Tanzania in which upper and lower dentitions were capable of occlusion, a feature otherwise known only from mammals. Fossil crocodyliforms discovered in recent years1,2,3,4,5 have revealed a level of morphological and ecological diversity not exhibited by extant members of the group. This diversity is particularly notable among taxa of the Cretaceous Period (144–65 million years ago) recovered from former Gondwanan landmasses. Here we report the discovery of a new species of Cretaceous notosuchian crocodyliform from the Rukwa Rift Basin6 of southwestern Tanzania. This small-bodied form deviates significantly from more typical crocodyliform craniodental morphologies, having a short, broad skull, robust lower jaw, and a dentition with relatively few teeth that nonetheless show marked heterodonty. The presence of morphologically complex, complementary upper and lower molariform teeth suggests a degree of crown–crown contact during jaw adduction that is unmatched among known crocodyliforms, paralleling the level of occlusal complexity seen in mammals and their extinct relatives7,8,9,10,11,12. The presence of another small-bodied mammal-like crocodyliform in the Cretaceous of Gondwana indicates that notosuchians probably filled niches and inhabited ecomorphospace that were otherwise occupied by mammals on northern continents. |
