Tiny skull has big im­pli­ca­tions for un­der­stand­ing the largest di­nosaurs

Waikato Times - - World -

Sau­ropods were the big­gest di­nosaurs – and the big­gest land an­i­mals – ever to stomp across the planet. Their long-necked group in­cluded ap­atosaurus, bron­tosaurus, ca­ma­rasaurus and the even more mas­sive ti­tanosaurs, whose leg bones were longer than a per­son is tall.

But each of their first steps on Earth were teensy. These great beasts came from lit­tle pack­ages, hatch­ing out of eggs no big­ger than grape­fruits or soccer balls. They must have had ‘‘a ridicu­lous growth rate,’’ said D. Cary Woodruff, di­rec­tor of pa­le­on­tol­ogy at the Great Plains Di­nosaur Mu­seum in Mon­tana.

Woodruff knows how small these an­i­mals be­gan – along with a team of di­nosaur ex­perts, Woodruff de­scribes the small­est diplodocus skull ever found in a new study in the jour­nal Sci­en­tific Re­ports. The skull, from a diplodocus the sci­en­tists nick­named An­drew, could fit in Woodruff’s cupped palms.

Sau­ro­pod skulls are rare. Im­ma­ture skulls, tiny and frag­ile, are rarer still. Pa­le­on­tol­o­gists can glean lots of in­for­ma­tion from skulls: The ori­en­ta­tion of ear canals tells re­searchers how the an­i­mal held its head. Fos­silised teeth are mark­ers of what it ate. This skull was about 23cm long. An­drew had over­size eyes, a short muz­zle and un­usual teeth.

Skulls are par­tic­u­larly valu­able to ex­perts who study sau­ro­pod growth, too, be­cause other devel­op­men­tal char­ac­ter­is­tics are com­par­a­tively rare, Woodruff said. Di­nos like tricer­atops had frills and horns, which sci­en­tists can track though var­i­ous ages of the an­i­mals’ life. Not so for a sau­ro­pod.

The newly de­scribed skull fills crit­i­cal gaps in the un­der­stand­ing of sau­ro­pod size and devel­op­ment, Woodruff said. Adult diplodocuses had teeth like wooden pegs. They were graz­ers, like cat­tle, nuz­zling up to soft ferns with their long snouts. Other sau­ropods, like ca­ma­rasaurus, had spoon-shape teeth, bet­ter to munch on tougher veg­e­ta­tion.

An­drew, sur­pris­ingly, had both types of teeth: pegs in the front, spoons in the back. This, Woodruff pre­dicts, would have al­lowed An­drew to chow down on all sorts of food, nip­ping at soft ferns but also crunch­ing through more fi­brous stuff.

‘‘It would be tough to imag­ine that sau­ropods ate the same things through­out their lives given the size dis­par­ity as they aged,’’ said Ma­calester Col­lege pa­le­on­tol­o­gist Kristina Curry Rogers, who was not in­volved with this re­search but has stud­ied baby sau­ropods from fos­sils found in Mada­gas­car. ‘‘There is cer­tainly no way that ju­ve­nile sau­ropods could feed at the same browse heights as adults.’’

The skull and two ver­te­brae were col­lected from a quarry in Mon­tana. Woodruff es­ti­mated the an­i­mal would have been about 2 to 4 years old, about 6m long and about ches­theight. That’s tiny for an an­i­mal that, had it sur­vived, would have grown to about 27m long and 12 tonnes in the span of two decades.

The nick­name An­drew came from in­dus­tri­al­ist An­drew Carnegie, who funded ex­ca­va­tions to dig up di­nosaur fos­sils and has a name­sake sau­ro­pod – Diplodocus carnegii. The study au­thors aren’t ex­actly sure what the species is, but they know it is a diplodocid, mean­ing a mem­ber of the same fam­ily as diplodocus. (The pa­le­on­tol­o­gists have no idea whether the an­i­mal was male or fe­male.)

An­drew was found among a jum­ble of other young sau­ropods, Woodruff said. He said that this prob­a­bly rep­re­sented an ‘‘age seg­re­gated herd,’’ young an­i­mals within a sim­i­lar age range that found food and shel­ter in a thick for­est. In this view, diplodocuses were the op­po­site of he­li­copter par­ents. He sus­pects the an­i­mals were like sea tur­tles: A mother’s duty ends at lay­ing eggs, leav­ing the hatch­lings to fend for them­selves.

The Swiss-army teeth, Woodruff said, is a sign that the young sau­ropods did not rely on adults to feed them ferns. ‘‘If that’s the case, why do they have dif­fer­ent kinds of teeth?’’ he said.

Curry Rogers was not sure the teeth were so re­veal­ing. ‘‘I don’t see such an ob­vi­ous ar­gu­ment when it comes to the link be­tween dif­fer­en­tial feed­ing strategies and a lack of parental care,’’ she said. That hy­poth­e­sis needs more data, in­clud­ing anatom­i­cal fea­tures be­yond a skull and a few ver­te­brae, she said.

There’s still plenty more to study. ‘‘I want to find sau­ropods smaller than An­drew,’’ Woodruff said. ‘‘There’s still so much more we can learn.’’

‘‘As adults, sau­ropods are so gi­ant that they can al­most seem like bi­o­log­i­cal im­pos­si­bil­i­ties – it is re­ally chal­leng­ing to un­der­stand how some­thing so weird could work out so well in an evo­lu­tion­ary sense,’’ Curry Rogers said. ‘‘Some­times, study­ing sau­ropods is like study­ing aliens.’’ – Wash­ing­ton Post

WASH­ING­TON POST

The fos­sil skull of the young diplodocus, nick­named ‘‘An­drew’’ and held by pa­le­on­tol­o­gist D. Cary Woodruff.

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