Editing Megafauna (section)

===In marine mammals===
[[File:Baleen whale sizes.JPG|thumb|upright=1.5|Baleen whale comparative sizes]]
Since tetrapods (first [[Marine reptile|reptiles]], later [[Marine mammal|mammals]]) returned to the sea in the [[Late Permian]], they have dominated the top end of the marine body size range, due to the more efficient intake of oxygen possible using lungs.<ref name = "Webb2015">{{Cite news | last = Webb | first = J. | title = Evolution 'favours bigger sea creatures' | publisher = [[BBC]] | date = 2015-02-19 | url = https://www.bbc.com/news/science-environment-31533744 | access-date = 2015-02-22 | work = BBC News | archive-date = 2015-02-22 | archive-url = https://web.archive.org/web/20150222044708/http://www.bbc.com/news/science-environment-31533744 | url-status = live }}</ref><ref name = "Helm2015">{{cite journal | last1 = Heim | first1 = N. A. | last2 = Knope | first2 = M. L. | last3 = Schaal | first3 = E. K. | last4 = Wang | first4 = S. C. | last5 = Payne | first5 = J. L. | title = Cope's rule in the evolution of marine animals | journal = Science | volume = 347 | issue = 6224 | pages = 867–870 | doi = 10.1126/science.1260065 | date = 2015-02-20 | pmid = 25700517 | bibcode = 2015Sci...347..867H | url = http://www.swarthmore.edu/NatSci/swang1/Publications/ | doi-access = free | access-date = 2019-07-13 | archive-date = 2019-07-05 | archive-url = https://web.archive.org/web/20190705023917/http://www.swarthmore.edu/NatSci/swang1/Publications/ | url-status = live }}</ref> The ancestors of [[cetacea]]ns are believed to have been the semiaquatic [[pakicetid]]s, no larger than dogs, of about 53 million years (Ma) ago.<ref name=poster>{{cite journal|last=Thewissen|first=J. G. M.|author2=Bajpai, S.|title=Whale Origins as a Poster Child for Macroevolution|journal=[[BioScience]]|date=1 January 2001|volume=51|issue=12|pages=1037–1049|doi=10.1641/0006-3568(2001)051[1037:WOAAPC]2.0.CO;2|issn=0006-3568|doi-access=free}}</ref> By 40 Ma ago, cetaceans had attained a length of {{cvt|20|m}} or more in ''[[Basilosaurus]]'', an elongated, serpentine whale that differed from modern whales in many respects and was not ancestral to them. Following this, the evolution of large body size in cetaceans appears to have come to a temporary halt and then to have backtracked, although the available fossil records are limited. However, in the period from 31 Ma ago (in the [[Oligocene]]) to the present, cetaceans underwent a significantly more rapid sustained increase in body mass (a rate of increase in ''body mass''<sup>0.259</sup> of a factor of 3.2 per million years) than achieved by any group of terrestrial mammals.<ref name = "Evans2012"/> This trend led to the largest animal of all time, the modern [[blue whale]]. Several reasons for the more rapid evolution of large body size in cetaceans are possible. Fewer [[Biomechanics|biomechanical]] constraints on increases in body size may be associated with suspension in water as opposed to standing against the force of gravity, and with [[Aquatic locomotion|swimming movements]] as opposed to [[terrestrial locomotion]]. Also, the greater heat capacity and thermal conductivity of water compared to air may increase the [[thermoregulation|thermoregulatory]] advantage of large body size in marine [[endotherm]]s, although diminishing returns apply.<ref name = "Evans2012"/>

Among the toothed whales, maximum body size appears to be limited by food availability. Larger size, as in [[sperm whale|sperm]] and [[beaked whale]]s, facilitates deeper diving to access relatively easily-caught, large cephalopod prey in a less competitive environment. Compared to odontocetes, the efficiency of baleen whales' [[filter feeding]] scales more favorably with increasing size when planktonic food is dense, making larger sizes more advantageous. The [[lunge feeding]] technique of [[rorqual]]s appears to be more energy efficient than the [[ram feeding]] of [[balaenid]] whales; the latter technique is used with less dense and patchy plankton.<ref name="Goldbogen2019">{{cite journal|last1= Goldbogen|first1=J. A.|last2= Cade|first2=D. E.|last3= Wisniewska|first3=D. M.|last4= Potvin|first4= J.|last5= Segre|first5=P. S.|last6= Savoca|first6=M. S.|last7= Hazen|first7=E. L.|last8= Czapanskiy|first8=M. F.|last9= Kahane-Rapport|first9=S. R.|last10= DeRuiter|first10=S. L.|last11= Gero|first11= S.|last12= Tønnesen|first12= P.|last13= Gough|first13=W. T.|last14= Hanson|first14=M. B.|last15= Holt|first15=M. M.|last16= Jensen|first16=F. H.|last17= Simon|first17= M.|last18= Stimpert|first18=A. K.|last19= Arranz|first19= P.|last20= Johnston|first20=D. W.|last21= Nowacek|first21=D. P.|last22= Parks|first22=S. E.|last23= Visser|first23= F.|last24= Friedlaender|first24=A. S.|last25= Tyack|first25=P. L.|last26= Madsen|first26=P. T.|author27-link=Nicholas Pyenson|last27= Pyenson|first27=N. D.|title= Why whales are big but not bigger: Physiological drivers and ecological limits in the age of ocean giants|journal= Science|volume= 366|issue= 6471|year= 2019|pages= 1367–1372|doi= 10.1126/science.aax9044|pmid=31831666|bibcode=2019Sci...366.1367G|hdl=10023/19285|s2cid=209339266|hdl-access= free}}</ref> The cooling trend in Earth's recent history may have generated more localities of high plankton abundance via wind-driven [[upwelling]]s, facilitating the evolution of gigantic whales.<ref name="Goldbogen2019" />

Cetaceans are not the only marine mammals to reach tremendous sizes.<ref>{{Cite journal |last1=Baker |first1=Joanna |last2=Meade |first2=Andrew |last3=Pagel |first3=Mark |last4=Venditti |first4=Chris |date=2015-04-21 |title=Adaptive evolution toward larger size in mammals |journal=Proceedings of the National Academy of Sciences |language=en |volume=112 |issue=16 |pages=5093–5098 |doi=10.1073/pnas.1419823112 |doi-access=free |issn=0027-8424 |pmc=4413265 |pmid=25848031|bibcode=2015PNAS..112.5093B }}</ref> The largest mammal [[carnivora]]ns of all time are marine [[pinniped]]s, the largest of which is the [[southern elephant seal]], which can reach more than {{cvt|6|m}} in length and weigh up to {{cvt|5,000|kg}}. Other large pinnipeds include the [[northern elephant seal]] at {{cvt|4,000|kg}}, [[walrus]] at {{cvt|2,000|kg}}, and [[Steller sea lion]] at {{cvt|1,135|kg}}.<ref>{{Cite journal |last1=Churchill |first1=Morgan |last2=Clementz |first2=Mark T. |last3=Kohno |first3=Naoki |date=2014-12-19 |title=Cope's rule and the evolution of body size in Pinnipedimorpha (Mammalia: Carnivora) |url=|journal=Evolution |volume=69 |issue=1 |pages=201–215 |doi=10.1111/evo.12560 |pmid=25355195 |issn=0014-3820}}</ref><ref>{{Cite journal |last1=Haley |first1=Michael P. |last2=Deutsch |first2=Charles J. |last3=Boeuf |first3=Burney J. Le |date=April 1991 |title=A method for estimating mass of large pinnipeds |url=|journal=Marine Mammal Science |language=en |volume=7 |issue=2 |pages=157–164 |doi=10.1111/j.1748-7692.1991.tb00562.x |bibcode=1991MMamS...7..157H |issn=0824-0469}}</ref> The [[sirenia]]ns are another group of marine mammals which adapted to fully aquatic life around the same time as the cetaceans did. Sirenians are closely related to elephants. The largest sirenian was the [[Steller's sea cow]], which reached up to {{cvt|10|m}} in length and weighed {{cvt|8,000|to|10,000|kg}}, and was hunted to extinction in the 18th century.<ref>{{Cite journal |last=Goldbogen |first=J. A. |date=2018-04-17 |title=Physiological constraints on marine mammal body size |journal=Proceedings of the National Academy of Sciences |language=en |volume=115 |issue=16 |pages=3995–3997 |doi=10.1073/pnas.1804077115 |doi-access=free |issn=0027-8424 |pmc=5910879 |pmid=29618615|bibcode=2018PNAS..115.3995G }}</ref>