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Re: therizinosaurs - how did it happen?
To delve into the theoretical selective pressures between carnivory,
omnivory (an almost requisite intermediate), and herbivory it's worth
noting that energy acquisition and consumption have several factors:
1) Efficiency: energy efficiency is important, but also comes in many
varieties. For example, meat is famously higher in calories than most
plant types, although this delta becomes a lot smaller when fruits,
nuts, insectivory, shell fish, and other sources of calories are taken
into account (remember, the initial switch is to greater omnivory).
Also, time invested per calorie is important, and hunting and catching
prey (and the many failures, etc) mean that more time can be spent per
2) Resource patchiness: Related to the last point. animals that
specialize in any particular food type are more susceptible to
perterbations in climate, rainfall, behavioral changes (e.g. migration
routes) etc. While shifting towards omnivory and eventually herbivory
will require concurrent sacrifices in the ability to obtain meat, it
also reduces the "putting your eggs in once basket" problem, and in
some environments could be very advantageous.
3) Catastrophic danger: hypercarnivores are always at a greater risk of
injury and death from their food sources than are omnivores and
herbivores. Obviously most carnivores tolerate this risk and evolve
responses to reduce it, but it's not hard to imagine situations where
individuals are more successful by getting some of their calories in a
I doubt we can recover the exact set of reasons that lead to the shift
of eating preferences amongst some coelurosaurs, but some combination
of the above reasons are likely to have played a role.
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333
From: Thomas R. Holtz, Jr. <email@example.com>
To: firstname.lastname@example.org; email@example.com
Sent: Wed, Jul 8, 2009 2:02 pm
Subject: RE: therizinosaurs - how did it happen?
As Lindsay Zanno showed at SVP last year, there is a fairly good chance
the shift from a strictly carnivorous diet in coelurosaurs may have
From: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu]
On Behalf Of B tH
Or perhaps, "why?" would be better ... what forces were to
drive a carnivore to a herbivore state? All through the
evolutionary period this would have happened in there were
plenty of meat eaters and plant eaters that surely filled all
single event shared by the common ancestor of ornithomimosaurs,
therizinosaurs, alvarezsaurs, oviraptorosaurs, and eumaniraptorans (with
hypercarnivorous eumaniraptorans being reversals). If so, and given the
presence of the ?Early Jurassic? Eshanosaurus, this origin would have
in a fairly transitional period of dinosaur history. (Of course, the
Eshanosaurus is debatable; however, given the presence of Late Jurassic
eumaniraptorans [and maybe even Middle Jurassic ones], the divergence
maniraptoriforms was certainly a Jurassic event). So the non-carnivorous
coelurosaur radiation may have occurred with the other major dinosaur
radiations of the Early and Middle Jurassic.
All that being said, it is difficult to ascertain why these shifts
For example, why did pandas become herbivores? Or the herbivorous
Or the herbivorous crocodyliforms?
Thomas R. Holtz, Jr.
Email: firstname.lastname@example.org Phone: 301-405-4084
Office: Centreville 1216
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Faculty Director, Earth, Life & Time Program, College Park Scholars
Faculty Director, Science & Global Change Program, College Park Scholars
Mailing Address: Thomas R. Holtz, Jr.
Department of Geology
Building 237, Room 1117
University of Maryland
College Park, MD 20742 USA