The most recent issue of Nat Geo included a one page note called, “Sizing Up.”
Nat Geo reporter Gretchen Parker sourced Allistair Evans, of Monash U, Australia, who noted “It takes a minimum of 3 million generations for a dolphin-sized aquatic mammal to increase to the size of a blue whale.” 1000x change in size, graphics impressive.
“It takes 1.6 million generations for a sheep-size land mammal to increase to the size of an elephant.” 100x change in size
“But it takes only a minimum of 0.1 million generations for an elephant-sized land mammals to decrease to the size of a sheep. 100x change in size.
“5 million generations” to go from rabbit-sized to elephant-size. 1000x
“24 million generations” to go from mouse-size to elephant-size. 100,000x
All this is interesting, but more interesting to PterosaurHeresies readers might be some similar hypotheses regarding prehistoric reptiles, particularly pterosaurs.
Chinsamy et al. (2008) noted that in Pterodaustro, the only pterosaur for which we have a complete growth series, half grown specimens appear to be sexually mature. At half size, the pelvis is also half size, able to pass eggs of half size producing hatchlings of half size, more or less. In three generations such a progression could lead to a one-eighth size adult, which would be the size of a hatchling of the original Pterodaustro. Now I’m not saying this is exactly how size reduction happened in pterosaurs. The three generations is just the ‘speed limit’ for getting small, something pterosaurs did over and over again, producing new clades following these many size decreases as size thereafter increased.
Some pterosaurs, like Quetzalcoatlus, became very large and very famous. Other pterosaurs became very small. They’re not famous. They don’t even rate a distinct genus, having been relegated to the trash heap with the label, “juvenile.” They are excluded from phylogenetic analysis and unjustly so. They are important.
Of course getting big again simply depends on creating eggs later in life when the mother is slightly surpassing the 8x growth pattern having a larger pelvis to pass a larger egg. Like elephants, getting bigger probably took more time than getting smaller.
Overall size does affect morphology and evolution. Early and late maturation affects the next generation. Hormones count! Hormones also drive secondary sexual characteristics, like frills and crests. These things add up, or subtract out, over many generations.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Chinsamy A, Codorniú L and Chiappe LM 2008. Developmental growth patterns of the filter-feeder pterosaur, Pterodaustro guinazui. Biology Letters, 4: 282-285.