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i would think that it is the genes....back then they had genes that allowed them to be big...when they died off some of the smaller animals survived which they passed on their "small genes." That is just me guessing tho..
The answer is quite simple really.... in the time of the dinosaurs the levels of oxygen were much higher than they are now. 250 million years ago there was about 35% oxygen
There were more forest fires, at the time! The higher levels of oxygen aren't really in dispute, but how they arose are not quite agreed on. I'll look for a good reference and post it!
Theoretical calculations, based on both the chemical and isotopic composition of sedimentary rocks, indicate that atmospheric O2 has varied appreciably over Phanerozoic time, with a notable excursion during the Permo-Carboniferous reaching levels as high as 35% O2. This agrees with measurements of the carbon isotopic composition of fossil plants together with experiments and calculations on the effect of O2 on photosynthetic carbon isotope fractionation. The principal cause of the excursion was the rise of large vascular land plants and the consequent increased global burial of organic matter. Higher levels of O2 are consistent with the presence of Permo-Carboniferous giant insects, and preliminary experiments indicate that insect body size can increase with elevated O2. Higher O2 also may have caused more extensive, possibly catastrophic, wildfires. To check this, realistic burning experiments are needed to examine the effects of elevated O2 on fire behavior.
Theoretical models predict a marked increase in atmospheric O to ∼35% during the Permo-Carboniferous (∼300 Ma) occurring against a low (∼0,03%) CO level. An upper O value of 35%, however, remains disputed because ignition data indicate that excessive global forest fires would have ensued. This uncertainty limits interpretation of the role played by atmospheric oxygen in Late Paleozoic biotic evolution. Here, we describe new results from laboratory experiments with vascular land plants that establish that a rise in O to 35% increases isotopic fractionation (ΔC) during growth relative to control plants grown at 21% O. Despite some effect of the background atmospheric CO level on the magnitude of the increase, we hypothesize that a substantial Permo-Carboniferous rise in O could have imprinted a detectable geochemical signature in the plant fossil record. Over 50 carbon isotope measurements on intact carbon from four fossil plant clades with differing physiological ecologies and ranging in age from Devonian to Cretaceous reveal a substantial ΔC anomaly (5‰) occurring between 300 and 250 Ma. The timing and direction of the ΔC excursion is consistent with the effects of a high O atmosphere on plants, as predicted from photosynthetic theory and observed in our experiments. Preliminary calibration of the fossil ΔC record against experimental data yields a predicted O/CO mixing ratio of the ancient atmosphere consistent with that calculated from long-term models of the global carbon and oxygen cycles. We conclude that further work on the effects of O in the combustion of plant materials and the spread of wildfire is necessary before existing data can be used to reliably set the upper limit for paleo-O levels.
65 million years ago (MyBP) mass disappearances of some 70% of all biotic species, delineated the Cretaceous-Tertiary (K/T) boundary (Hallam and Wignall, 1997Go). Alvarez et al. proposed that the causal agent was a climatic cataclysm, resulting from the impact of a bolide (Alvarez et al., 1980Go). This has now been widely accepted (Macleod and Keller, 1996Go). However, one conundrum remains: the decline of certain flora and fauna species which commenced some ten million years before the K/T event (Sloan et al., 1986Go; Johnson and Hickey, 1990Go; Sweet et al., 1990Go; Johnson, 1993Go; Macleod et al., 1997Go). These and other findings have suggested more gradually developing biosphere degrading factors, such as extreme volcanism (Hallam, 1987Go; Venkatesen et al., 1993Go; Courtillot, 1999Go).Models of the paleo-atmosphere indicate that there have been periods of comparatively high oxygen such as the 40 kPa event in the Carboniferous (Berner and Canfield, 1989Go; Berner et al., 2000Go). High levels of CO2, such as were prevalent in most of Phanerozoic eon, tend to counteract the photorespiration-inducing, photosynthesis- reducing effect of high O2, although decreased activation of other stromal enzymes has also been reported (Leegood and Walker, 1982Go). Oxygen reduced net photosynthesis is significant even at the present atmospheric level of 21 kPa O2 (von Caemmerer and Farquhar, 1981Go). At the end-Cretaceous, oxygen is estimated to have reached ~28 kPa, while CO2 was comparatively low, with estimates ranging from 23 Pa (Lasaga et al., 1985Go) to the more recent 30–90 Pa (Berner, 1997Go). It is suggested that the high oxygen atmosphere at the K/T boundary may also have been involved in the pre-K/T boundary decline of both flora and dependent fauna.Beerling (Beerling, 1994Go) calculated rates of leaf photosynthesis for the O2/CO2 atmospheric composition and temperatures of the late Cretaceous to the present time, using the Farquhar et al. and von Caemmerer and Farquhar leaf biochemistry models (Farquhar et al., 1980Go; von Caemmerer and Farquhar, 1981Go). His calculations showed a strong effect of high O2. Interpolating in Beerling's Fig. 6Go, for the late Cretaceous, the calculated rate of photosynthesis is seen to decline from 100 to 60 MyBP by 40–60%. However, these models do not take into account possible changes in leaf area (see below) and gas conductance (Rachmilevitch et al., 1999Go) in response to elevated O2. The effect on plants of the very high (35 kPa) O2 event calculated for the Carboniferous period (345–280 MyBP) has been studied (Beerling et al., 1998Go). After 6 weeks growth under 35 Pa CO2 and either high or present-day O2, photosynthesis per unit leaf area was reduced 29% by the high O2.Crucial factors of this study are the actual partial pressures of carbon dioxide and oxygen prevalent at the end-Cretaceous. All estimates have wide confidence limits (cf. Berner and Canfield, 1989Go; Berner et al., 2000Go). Just one high-end estimate for O2, namely 28 kPa, has been used, and a range of carbon dioxide values between 24 and 60 Pa (Lasaga et al., 1985Go; Berner, 1997Go).Care is needed when projecting the characteristics of ancient plants from the response of their extant representatives. There is an inherent assumption that physiological responses to environmental parameters have not changed significantly with time, in this case the last 150 million years. Moreover, modern, highly bred plants, such as the Z. mays studied here, have almost certainly had their photosynthesis/growth characteristics modified. However, Rubisco, the main enzyme involved in the response of C3 plants to ambient CO2 and O2 has been highly conserved. Such changes as have been found, such as subunit composition or O2/CO2 specificity, are for species originating in periods separated by hundreds of millions of years (Jordan and Ogren, 1981Go; Badger and Andrews, 1987Go). Theoretical models of the response of leaf photosynthesis to ambient O2 and CO2 are based on known biochemical properties of Rubisco (Farquhar et al., 1980Go; Beerling, 1994Go). Responses found here were generally as could be expected from these models. However, hitherto unreported effects have also been found of high O2 on leaf area, leaf diffusion–conductivity, and the respiration/photosynthesis (R/P) ratio.The paleo-atmosphere and the K/T boundary extinctionsThe C3 species studied here are considered to be representative of important angiosperm forage plants of the late Cretaceous period. However only a few species were tested and the degree of their reactions varied. Even so the results obtained strongly suggest that for some millions of years prior to the bolide impact of 65 MyBP, primary production of many plant species would have been severely diminished. Reduced plant production would have adversely affected the sustenance of dependent herbivores (insect and animal) with the effect amplified in carnivores and along the ecological food chain. This, together with other factors of biosphere degradation at the end-Cretaceous period (see Introduction) may explain the decline in many biota species which occurred prior to the 65 MyBP K/T boundary catastrophe. As ventured by Hallam and Wignall, the bolide impact may have been ‘only the final coup de grace’ (Hallam and Wignall, 1997Go). It is intriguing to note that according to the models of Berner and Canfield and Berner et al. (Berner and Canfield, 1989Go; Berner et al., 2000Go), the end-Permian, the greatest of all the mass extinctions (Courtillot, 1999Go) was preceded (albeit by some tens of millions of years) by the highest ever O2 event (~40 kPa), with concomitant low, ~25 Pa, CO2 (Berner, 1994Go).The paleo-atmosphere and the emergence of C4 plantsContrary to the response of the C3 species, the primary productivity of the C4 species was unaffected by the (assumed) 28 kPa O2 of the end-Cretaceous, even at the lowest estimate for CO2 (24 Pa) for that period, which was tested. This suggests that high O2 may have constituted a strong driver for the evolution of C4 from C3 plants, at least as far back as about 100 MyBP, the earliest date for which evidence has been found suggesting their presence (Kuypers et al., 1999Go). This is in addition to other C4 evolution-inducing environmental factors, such as low CO2, high temperatures and water stress (Ehleringer and Monson, 1993Go; Cerling et al., 1994Go, 1998).