1. In Argentina, Largest Titanosaur (Henry Baker) FYI -- Time to bring back my theory that pre-K-T oxygen levels were far in excess of today's oxygen levels. Animals this large wouldn't be able to survive in today's <21% O2 levels; they would feel like we do today at 20,000' altitudes (~50% O2 saturation levels). These excessively high levels of O2 pre-K-T were created by a billion+ years of plant-life CO2 reduction, leading to *huge* amounts of surface carbon in the form of coal/oil/etc.
--actually, today's blue whales are larger than titanosaurs, and probably more active and higher metabolisms too, so animals this large have no trouble surviving our oxygen levels. And I think the limits on large land animals are not oxygen, but rather mechanical strength.
The K-T event 66 million years ago set of a planet-wide fire that burned for years and years, and finally brought atmospheric O2 levels down to nearly today's levels.
--I don't believe you. If oxygen levels were high enough to sustain a many year long planet wide fire, then one would have got started without need of a meteor. Lightning, etc. I also claim oxygen levels are self regulating in the sense that if they get too high, there are more fires. This prevents oxygen levels from ever exceeding some amount, which one author estimated to be 25%.
(Tiny models of such fires burn today in West Virginia, some of which have been burning for >100 years.)
--those are in sheltered conditions inside coal mines. Where, incidentally, the oxygen levels are low. Anyhow, despite criticizing your argument, oxygen levels were indeed higher and lower in the past. https://en.wikipedia.org/wiki/Geological_history_of_oxygen and I do not know how solid their reconstruction of the past is, but they have one. Originally there was no O2. "The maximum of 35% was reached towards the end of the Carboniferous period (about 300 million years ago), a peak which may have contributed to the large size of insects and amphibians at that time." Note insects have no lungs and hence their size might be oxygen-limited, although the wikipedia article claims it isn't. Also note the dinosaur-killing meteor hit was 66 million years ago, much later than the carboniferous which ended 300 Myr ago. "At current rates of primary production, today's concentration of oxygen could be produced by photosynthetic organisms in 2000 years. In the absence of plants, the rate of oxygen production by photosynthesis was slower in the Precambrian, and the concentrations of O2 attained were less than 10% of today's and probably fluctuated greatly" which if true also would seem to kill Baker's coal theory, since a mere 2000 years to generate all oxygen, seems peanuts compared to the timescale of the carboniferous, 60 Myr long.