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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 calorie obtained.

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 safer manner.

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.


Scott Hartman
Science Director
Wyoming Dinosaur Center
110 Carter Ranch Rd.
Thermopolis, WY 82443
(800) 455-3466 ext. 230
Cell: (307) 921-8333


-----Original Message-----
From: Thomas R. Holtz, Jr. <tholtz@umd.edu>
To: soylentgreenistrex@yahoo.com; dinosaur@usc.edu
Sent: Wed, Jul 8, 2009 2:02 pm
Subject: RE: therizinosaurs - how did it happen?

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
the 'niches.'
Any idears?

As Lindsay Zanno showed at SVP last year, there is a fairly good chance that the shift from a strictly carnivorous diet in coelurosaurs may have been a
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 been in a fairly transitional period of dinosaur history. (Of course, the age of
Eshanosaurus is debatable; however, given the presence of Late Jurassic
eumaniraptorans [and maybe even Middle Jurassic ones], the divergence among
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 occur. For example, why did pandas become herbivores? Or the herbivorous varanids?
Or the herbivorous crocodyliforms?

Thomas R. Holtz, Jr.
Email: tholtz@umd.edu   Phone: 301-405-4084
Office: Centreville 1216
Senior Lecturer, Vertebrate Paleontology
Dept. of Geology, University of Maryland
Fax: 301-314-9661

Faculty Director, Earth, Life & Time Program, College Park Scholars
Faculty Director, Science & Global Change Program, College Park Scholars
Fax: 301-314-9843

Mailing Address:    Thomas R. Holtz, Jr.
           Department of Geology
           Building 237, Room 1117
           University of Maryland
           College Park, MD 20742 USA