The two groups of archosaurs, crocodilians and birds, form an extant phylogenetic bracket for understanding the reproductive behaviour of dinosaurs. This behaviour is inferred from preserved nests, eggs, and even gravid individuals 1 . Data indicates many 'avian' traits were already present in Paraves -the clade that includes birds and their close relatives -and the early evolution of the modern avian form of reproduction was already well on its way 2,3 . Like living neornithine birds, non-avian maniraptorans had daily ovideposition and asymmetrical eggs with complex shell microstructure, and were known to protect their clutches 4-6 . However, like crocodilians, non-avian maniraptorans had two active oviducts (one present in living birds), relatively smaller eggs and may not have turned their eggs the way living birds do 1,6 . Here we report on the first discovery of fossilized mature or nearly mature ovarian follicles, revealing a previously undocumented stage in dinosaur reproduction: reproductively active females near ovulation. Preserved in a specimen of the long boney-tailed Jeholornis and two enantiornithine birds from the Early Cretaceous lacustrine Jehol Biota in northeastern China, these discoveries suggest basal birds only had one functional ovary, but retained primitive morphologies as a result of their lower metabolic rate relative to living birds. They also indicate basal birds reached sexual maturity before skeletal maturity as in crocodiles and paravian dinosaurs. Differences in follicular morphology between Jeholornis and the enantiornithines are interpreted as forming an evolutionary gradient from the reproductive condition in paravian dinosaurs towards neornithine birds. Furthermore, differences between the two enantiornithines indicate this lineage might also have evolved advanced reproductive traits in parallel to the neornithine lineage.

This chapter discusses the taxonomy of crocodiles and alligators and their anatomical and physiological differences and the different ways in which crocodiles are living or kept. A great portion of this chapter tackles the anatomy, physiology, biochemistry and behaviour of crocodiles. The discussion on the anatomy of crocodiles includes the skeleton, skin, muscles, respiratory system, digestive system, urinary system, reproductive organs, endocrine organs, circulatory system and blood cells, nervous system and sensory organs, fat storage and eggs. The discussion on crocodilian physiology includes yolk sac resorption, sex differentiation, growth locomotion, digestion, normal oral flora, normal intestinal flora, respiration, excretion, responses to high salinity, reproduction, circulation, nervous activity, thermoregulation, immunity, inflammation, fever and diseases. Blood and urine biochemistry, mineral values, composition of crocodile fat, nutrient composition of crocodile eggs and paracloacal and gular glands are discussed in the section on biochemistry, while the topics covered under behaviour include embryonic learning, parental care, imprinting, dispersal of the young, cannibalism, hunting and feed selection, social behaviour, territoriality, sexual behaviour, nesting, thermoregulation and vocalization.

A crop adapted for an herbivorous diet of seeds has previously been documented in the Early Cretaceous birds Sapeornis and Hongshanornis. Here we report on several specimens of Yanornis that preserve a crop containing fish. One specimen preserves two whole fish in the oesophagus, indicating that Early Cretaceous birds shared trophic specializations with Neornithes for the increased energetic demands of flight – namely the storing of food for later consumption when the stomach is full. Whole fish also indicate that despite their presence, teeth were not used to orally process food, suggesting the hypertrophied dentition in this taxon were utilized in prey capture. The presence of macerated fish bones in the crop of other specimens indicates the highly efficient advanced muscular system of peristalsis responsible for moving ingested items between different segments of the alimentary canal was also in place. Despite the fact many features of the modern avian alimentary canal are inferred to compensate for the absence of teeth in birds (expandable oesophagus, grinding gizzard), the derived alimentary canal was apparently present in toothed Cretaceous birds. Although Yanornis was considered to have switched their diet from piscivorous to herbivorous, based on position and morphology we reinterpret the gastroliths reported in one specimen as sand impacted in the intestines, and reconstruct the taxon as primarily piscivorous. This is a novel interpretation for fossilized gastroliths, and the first documentation of this condition in the fossil record.