496 FOSSIL VERTEBRATES/Dinosaurs Reproduction and Growth Like all other archosaurs, dinosaurs laid calciticshelled eggs Eggs have been found that (owing to the presence of embryos) can be attributed to theropods, neornithischians, titanosaurian sauropods, and prosauropods Many other egg types have been found, and these probably represent all the major dinosaur groups However, matching an egg type to a producer without direct evidence from embryonic remains is fraught with difficulty An intriguing feature of non-avian dinosaur eggs is their relatively small size The largest non-avian dinosaur eggs are about l (laid by gigantic sauropods) and are actually smaller than the largest bird eggs, which are about l (laid by the recently extinct elephant bird) A further difference is the average number of eggs laid in a single clutch Fossilized non-avian dinosaur clutches usually contain tens of eggs (up to 40), but the actual average was probably higher, as fossil clutches are almost always partly destroyed by erosion when discovered Avian clutch sizes can be as low as one and are usually less than ten Further differences are that non-avian dinosaurs partly buried their eggs and apparently did not turn them once laid However, oviraptorosaur skeletons found on top of their nests show that they did assume a bird-like brooding posture over their eggs Furthermore, hadrosaurids show evidence of post-hatching parental care The degree of ossification of the articular ends of the baby limb bones found inside nest structures indicates that they were nest-bound and must have relied on food bought back to them by their parents Owing to the great size difference between adult and hatchling dinosaurs (three, perhaps as much as four, orders of magnitude), young dinosaurs must have grown rapidly, which is borne out by the fastgrowth-style microstructure of their bones Estimates of actual growth rates have been obtained by matching the histological type of fossil taxa to modern taxa of known growth rate or by counting lines of arrested growth (LAGs), which are assumed to be seasonal The range of growth rates that can produce each particular type of bone histology has not been adequately explored, and recent evidence suggests that the range is broader than previously suspected Thus, estimates based on the first method are suspect LAGs have proved even more unreliable, with the number of LAGs varying widely between different bones of a single individual Experimental work has shown that LAG formation is not necessarily correlated with seasonal hardship Until the biological meaning of LAGs is better understood, they should not be used to determine growth rates in dinosaurs See Also Fossil Vertebrates: Reptiles Other Than Dinosaurs; Birds; Flying Reptiles Mesozoic: Triassic; Jurassic; Cretaceous; End Cretaceous Extinctions Further Reading Benton MJ (1983) Dinosaur success in the Triassic: a non competitive ecological model Quarterly Review of Biology 58: 29 55 Farlow JO and Brett Surman MK (1997) The Complete Dinosaur Bloomington: Indiana University Press Farlow JO, Dodson P, and Chinsamy A (1995) Dinosaur biology Annual Reviews of Ecology and Systematics 26: 445 471 Horner JR (2000) Dinosaur reproduction and parenting Annual Reviews of Earth and Planetary Science 28: 19 45 Ji Q, Currie PJ, Norell MA, and Ji S A (1998) Two feathered dinosaurs from northeastern China Nature 393: 753 761 Olsen PE, Kent DV, Sues H D, et al (2002) Ascent of dinosaurs linked to an iridium anomaly at the Triassic Jurassic boundary Science 296: 1305 1307 Padian K, de Ricqle`s AJ, and Horner JR (2001) Dinosaurian growth rates and bird origins Nature 412: 405 408 Sereno PC (1997) The origin and evolution of dinosaurs Annual Reviews of Earth and Planetary Science 25: 435 489 Sereno PC (1999) The evolution of dinosaurs Science 284: 2137 2147 Starck JM and Chinsamy A (2002) Bone microstructure and developmental plasticity in birds and other dinosaurs Journal of Morphology 254: 232 246 Weishampel DB, Dodson P, and Osmolska H (1990) The Dinosauria Berkeley: University of California Press