[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Fw: Elephants and hyenas

Resending due to the truncation demon.

----- Forwarded Message -----

> From: Jura <pristichampsus@yahoo.com>
> To: "dinosaur@usc.edu" <dinosaur@usc.edu>
> Cc: 
> Sent: Monday, 8 April 2013 12:37 AM
> Subject: Re: Elephants and hyenas
> From: Raptorial Talon <raptorialtalon@gmail.com>
>> To: Jura <pristichampsus@yahoo.com> 
>> Cc: "dinosaur@usc.edu" <dinosaur@usc.edu> 
>> Sent: Sunday, 7 April 2013 9:35 PM
>> Subject: Re: Elephants and hyenas
>> Question from a novice: couldn't convergence to fill comparable niches 
> lead to convergences in behavior? If, for example, large ceratopsians and 
> rhinos 
> have evolved broadly similar biomechanical solutions to quadrupedal 
> locomotion, 
> such that their gaits are more comparable to each other's than either's 
> is to a modern crocodile, couldn't something similar be true of their 
> behaviors? (Obviously physiology is a bigger factor in determining k- versus 
> r-strategists and so on, but even so.)
>> I mean, sure, the value of such
> inferences would be low because they're so broad, but isn't it possible
> that convergence could make a dinosaurian group more similar to a 
> mammalian group than to a croc or lepidosaur, whether we're speaking 
> behaviorally or biomechanically? I guess I'm suggesting that the 
> constraints of specific ecological roles can overwrite the signal from 
> phylogenetic bracketing to some extent, making the latter potentially 
> dubious for highly derived taxa . . . I don't entirely trust volant 
> hyper-endotherms and aquatic ambush predators to elucidate the 
> strategies of relatives which had radically different and 
> non-overlapping niches.
>> Thoughts?
>> ++++++++++++++++++++++++++++++++++++++++++
> Certainly. Extensive convergence is something that we see in many animals 
> groups including extant birds and mammals. Both mammals and birds share a 
> suite 
> of behavioural and (grossly) physiological traits to the exclusion of other 
> amniotes. These gross similarities were enough for Owen (1866) to 
. So extant birds and mammals show that extensive convergence can 
> happen in distantly related groups subjected to the same environmental 
> pressures.
> So mammals and birds show many features in common with one another. 
> Unfortunately, few of these features leave marks on the bones. Physiology and 
> behaviour may be observed today, but can only be inferred for prehistoric 
> animals. This brings us to the whole bugaboo regarding inferring dinosaur 
> behaviour based on mammals. When we use crocs and birds (and even lizards) we 
> are limiting what we can say about dinosaurs based on what we know of their 
> extant relatives. However, when we do infer things like nest making / 
> guarding, 
> or unidirectional flowing respiratory systems in dinosaurs based on what 
> extant 
> archosaurs are doing, we at least have some level of confidence in our 
> inferences. Since birds and crocs both show this particular trait we can 
> hypothesize that said trait was present in their last common ancestor. Should 
> that trait leave a mark on the bone then we can even test our hypothesis by 
> looking for those osteological correlates in the fossils. 
> This then leads us to the problem. If we infer that mammals and dinosaurs 
> had some type of similar physiology / behaviour then we are making what 
> Witmer 
> (1995) called a level 3 prime inference. If mammals and dinosaurs shared a 
> particular trait in common, and this trait was to the exclusion of extant 
> archosaurs, then the trait must have evolved two separate times (and was 
> later 
> lost on the lineage that gave rise to birds). For behavioural and 
> physiological 
> traits that means inferring mammalian qualities on dinosaurs without any way 
> of 
> backing that inference up. It is basically just speculation. Yes it may have 
> happened, but what we see in mammals may also be one of dozens of solutions 
> to 
> the problems posed by that particular environment. We have no way of knowing 
> and 
> no way of testing these level 3' inferences out. 
> So yeah, using extan
dinosaurs is bound to result in the underestimation of certain dinosaur 
> qualities. However by forcing this conservative view on our interpretations, 
> we 
> (theoretically) keep ourselves from having our speculations run beyond the 
> reach 
> of our data.
> All this isn't to say that a strict extant phylogenetic bracket approach 
> is the only way. Mammals can provide insights into how large terrestrial 
> animals 
> cope with things. Traits in dinosaurs may even be hypothesized to be 
> analogous 
> to traits seen in mammals, especially when these traits seem to have no real 
> analogue/homologue among extant diapsids. Though given what little we know of 
> extant diapsids, I'm often left wondering how many of those traits actually 
> exist.
> Jason
> Refs
> Owen, R. 1866. On the Anatomy of Vertebrates, Volume 2. Longmans Green and 
> Co., London.
>Witmer, L. M.  1995.  The Extant Phylogenetic Bracket and the importance of 
>reconstructing soft tissues in fossils.  pp. 19–33 in Functional Morphology in 
>Vertebrate Paleontology, J. J. Thomason (ed.), Cambridge Univ. Press, New York.