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Re: Brachiosaur Defense

Hi Larry:

The arctometatarsalian condition is the "pinching out" of the third metatarsal proximally. In other words, all theropods stand on their middle three toes (II, III, and IV). The third or central metatarsal (the portion of the foot above the toes and before the ankle) has a normal articular surface for the phalanx (toe bone) it articulates with, but its portion that articulates with the astragalus (ankle) is very narrow, "squeezed" between metatarsals II and IV. This morphological character is present in tyrannosaurids, troodontids, ornithomimids, and a few others, and this collection of theropods has been referred to as the arctometatarsalians, named for this condition. Of course, many other bony characters unite this group too.

Allosaurus and other theropods that lived alongside Brachiosaurus did not have the arctometatarsalian condition and instead have three approximately equal-sized metatarsals. The arctometatarsalian condition does not appear to grant springy-ness to jumps. In fact most theropods have a relatively simple hinge-like joint with no well-developed lever-arms for jumping, like the calcaneum (heel) bone in your own foot which is attached to the Achilles tendon. When the muscles of this tendon contract, it pulls your calcaneum up and subsequently forces your foot down against the ground. Kangaroos have a very big calcaneum to help them provide more force to jump. Theropods have none of these features. While I would agree that most theropods were capable of occasional speed, most do not appear to have jumping legs. There is always the possibility that most theropods were ambush predators, walking or running fast briefly and catching their prey off guard. Most likely, large adult sauropods were left alone, while the young and sick were preyed on as Thom Holmes has already mentioned.




From: "Larry Febo" <larryf@capital.net>
Reply-To: larryf@capital.net
To: "Thom Holmes" <tholmes@dolphinsoft.com>, <ceevans@home.com>, <dinosaur@usc.edu>
Subject: Re: Brachiosaur Defense
Date: Fri, 28 Jan 2000 05:20:07 -0500


-----Original Message-----
From: Thom Holmes
Date: Thursday, January 27, 2000 8:02 AM



>Size, size, and size.
>
>I would guess that a slap to the side of a carnivore with a 3+ ton sauropod
>tail could do a lot of damage. Since theropods could probably not
side-step
>too effectively, and Farlow and others have noted how disabling or even
>fatal a mere stumble could have been for a large theropod, I would think
>that a predatory dinosaur would pick its sauropod victims wisely. The sick
>and the young would probably have been the most likely targets.
>


Pardon my (probable) ignorance here, but....wouldn`t the arctometatarsaly
condition of some of these theropods allow them a "springier" step. Perhaps
enough to leap clear of a sauropod tail? Perhaps even a T-rex (at the more
massive end of the spectrum) could have had the agility.
Sorry, I missed the discussion of "large theropod tripping", so am not aware
of everyones opinions on their agility.
PS...am I misinterpreting arctometatarsality here (again??)....Dr Holtz?


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