Mike Keesey  

A Farewell to Arms

October 10th, 2006 by Mike Keesey :: see related comic

(Tip of the hat to Tim Williams for the title.)

Carnotaurus arms are ridiculous. They’re tiny, with three stubby clawed fingers and one that isn’t even clawed. And there’s barely any forearm, so the hand sprouts almost directly from the elbow. Short and inflexible.

But Carnotaurus and other abelisaurids aren’t alone in this regard. Other members of the Silly Little Arms Club are spinosauroids, carcharodontosaurids, tyrannosaurids, ratites (ostriches, kiwi birds, etc.), phorusrhacids (”fear cranes”), diatrymatids, etc. Why did this reduction happen so many times across so many different lineages? What’s the common factor?

The most obvious commonality is that these are all theropods. The other major dinosaur groups, the generally herbivorous sauropodomorphs and ornithischians, actually trended in the opposite direction, starting out as facultatively bipedal runners and developing into quadrupeds with bulky forelimbs. This happened at least four times: in sauropods, in coronosaurian ceratopsids, in hadrosauriform ornithopods (which retained some degree of bipedality), and in eurypods (ankylosaurs and stegosaurs).

Why are these theropods different? For some, it may be the predatory lifestyle. In many groups, as body size increases, the heads grow larger as the arms grow smaller. Abelisaurids, spinosauroids, carcharodontosaurids, tyrannosaurids, phorusrhachids, and (possibly) diatrymatids all probably used their massive jaws, lined with sharp teeth (or, in the last two cases, sharp beaks) to catch prey, with assistance from taloned feet. No need for arms.

Interestingly, two other types of large theropod, therizinosaurids and ornithomimosaurs, had the opposite condition: gigantic forelimbs and tiny little heads. Perhaps not coincidentally, these are two of the few Mesozoic theropod groups thought to be potentially herbivorous.

Where does this leave ratites, with tiny heads and tiny arms? Perhaps because they are secondarily flightless, the forelimbs had become too modified for any other use, and so they were reduced when flight was abandoned or completely lost, in the case of dinornithid ratites, a.k.a. moas. Penguins (spheniscids) are the only long-flightless birds to have retained substantial forelimbs, and they arguably still fly, albeit in a different, more viscous medium than most avians.

11 Responses to “A Farewell to Arms”
Ivan wrote:

Spinosauroids have puny arms? Oh, wait. You mean the megalosaurs.

Mike Keesey wrote:

Yes, spinosauroids are a.k.a. megalosauroids or torvosauroids. Spinosaurid arms may not be as puny as those of abelisaurids and tyrannosaurids, but they are still punier than those of similarly-sized therizinosaurids, ornithomimosaurs, and dromaeosaurids (well, presumably—we don’t have complete arms for the largest dromaeosaurid, Utahraptor). (You may occasionally see a restoration of a spinosaurid as quadrupedal, but this is erroneous.)

Debbie wrote:

I don’t really understand all the content in the post (and my heart is still sore from the tragic deaths of Ferd and Dan) but the title is BRILLIANT.

dakiwiboid wrote:

Poor Carny! Not only has Carnita left him, but he looks ridiculous!

ReBecca wrote:

I love this one! To funny! Thanks for the laugh, hope to see you at the meeting next week.

Neutrino Cannon wrote:

Alvarezsaurs. Silliest. Limbs. Ever.

And what’s the final word on advanced phorusrhacids having clawed raptorial forelimbs?

Mike Keesey wrote:

Mononykine alvarezsaurs are definitely in the running.

As far as I know, nobody’s ever found such a claw, just a place where it “might have” attached. Been a while since I read about it, though.

Mike Keesey wrote:

Yeah, see ya in a few days, ReBecca!

Albert (Netherlands) wrote:

I compared the forelimbs of the alvarezsaurs with diatrymid birds … fascinating !

Masahiro wrote:

omething else During 1980 to 1990, the stateattained dobdleuigit growth rate in four years while itexperienced double digit growth rate in six years in subsequent decade. Gujarat experienced fasterpace of growth during 1980s when MadhavsinhSolanki and then Amarsinh Choudhary, both ofCongress, led the state government. In this period,the state had highest growth rate for its SDP. Thestate experienced the highest annual economic growth rate of 19.5 per cent during the six yearperiod of 1988-94. Since then this much of highrate is never attained. This period includes thereign of later years of Amarsinh Choudhary andinitial years of Chimanbhai Patel. As against this, thehighest rate of growth during the Narendra Modi era was only 14.77 per cent in 2003-04. Duringfive decades of its existence, the state of Gujarathad experienced 20 per cent or moregrowth rate only during six years. It had crossedmarvelous 40 per cent in 1988-89 and it was morethan 30 per cent in 1992-93 when Amarsinh Choudhary and Chimanbhai Patel ruled the staterespectively. However, there is not a single yearwhen the growth rate is more than twenty percent since 1993-94. In fact, it reached up to 19.33%in 1996-97. Barring it, the the SDP growth rate hasnever been more than 15 per cent during 1992-93 to 2008-09. In other words, the Narendra Modi erahas never reached the high growth rate of SDP thatwas achieved earlier.

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