Fos­sil clues to early mam­mals as nur­tur­ers

Sunday Tribune - - NEWS -

WE ALL know that mam­mals pro­tect and care for their young. In some cases, they also live within com­plex so­cial groups. Was this al­ways the case?

The skele­tal anatomy of mam­mals’ early ances­tors has been stud­ied for more than 150 years. But un­til re­cently, not much was known about their life­style or re­pro­duc­tive habits.

It wasn’t clear whether these ex­tinct an­i­mals pro­tected and cared for their young in the same way as mod­ern mam­mals do.

But a few decades ago a clue to their be­hav­iour was dis­cov­ered in two re­mark­able fos­sils found in South Africa that date back 251 mil­lion years ago.

The sig­nif­i­cance of these fos­sils of Thri­nax­odon li­orhi­nus and Gale­saurus plan­i­ceps have been largely for­got­ten by the palaeon­to­log­i­cal com­mu­nity and they were left out of re­cent dis­cus­sions about parental care in the fos­sil record.

We re­dis­cov­ered them while do­ing other re­search and de­cided to rein­ves­ti­gate their sig­nif­i­cance.

The fos­sils are even more im­por­tant than we imag­ined. They pro­vide di­rect ev­i­dence of parental care in these ex­tinct an­i­mals. They also re­veal com­plex be­hav­iour in our own dis­tant ances­tors.

The Thri­nax­odon and Gale­saurus fos­sils date back to the Early Tri­as­sic pe­riod, soon af­ter the end-per­mian mass ex­tinc­tion and be­fore the age of di­nosaurs.

Mam­mals hadn’t yet evolved. But their ances­tors, the non-mam­malian cyn­odonts, had a few fea­tures we recog­nise today in mam­mals: teeth dif­fer­en­ti­ated into in­cisors, ca­nines and com­plex post­ca­nines, and the pres­ence of a sec­ondary palate. The fos­sils of Thri­nax­odon and Gale­saurus rep­re­sented the first pos­si­ble cases of parental care re­ported in non-mam­malian cyn­odonts. This is sig­nif­i­cant be­cause mam­mals didn’t evolve for an­other 30 mil­lion years.

The Thri­nax­odon fos­sil was found in 1954 by the renowned Ka­roo fos­sil hunter James Kitch­ing. Dr AS Brink, a palaeon­tol­o­gist at Jo­han­nes­burg’s Univer­sity of the Wit­wa­ter­srand, briefly de­scribed it as con­sist­ing of an adult skull pre­served next to a small ju­ve­nile about a third of its size. Brink hy­poth­e­sised that this rep­re­sented a case of parental care.

In 1965 Brink dis­cov­ered a sec­ond case of parental care in Gale­saurus plan­i­ceps, a larger basal cyn­odont about the size of a fox. The Gale­saurus fos­sil block con­tained the skele­ton of an adult sur­rounded by ju­ve­niles. Brink in­ter­preted this fos­sil as ev­i­dence of a mother car­ing for her young.

These re­mark­able fos­sils be­came part of the col­lec­tion of the Evo­lu­tion­ary Stud­ies In­sti­tute at the Univer­sity of the Wit­wa­ter­srand. We came across them while in­ves­ti­gat­ing on­to­ge­netic growth in Thri­nax­odon and Gale­saurus. We wanted to find out how these cyn­odonts’ skull changed as they grew from a small ju­ve­nile into a large adult. In the course of our work, we re­alised that these fos­sils were also telling a for­got­ten story about nur­tur­ing be­hav­iour in our very dis­tant ances­tors.

Some fur­ther work had been done on the Thri­nax­odon fos­sil since Brink’s 1955 de­scrip­tion. The skulls are now sep­a­rated from each other and acid-prepa­ra­tion led to the discovery of a sec­ond ju­ve­nile in­di­vid­ual. But the Gale­saurus fos­sil was found tucked away in a drawer in col­lec­tions, with no ev­i­dence of fur­ther prepa­ra­tion or study. We were in­trigued, and de­cided to rein­ves­ti­gate what the fos­sils might tell us about parental care among the ances­tors of mam­mals.

Our study con­cluded that in both cases there were two young ju­ve­niles as­so­ci­ated with each adult.

We then in­ves­ti­gated more than 100 fos­sils of Thri­nax­odon and Gale­saurus from South Africa to de­ter­mine how of­ten in­di­vid­u­als of each genus were pre­served to­gether. The bones of in­di­vid­u­als that were found to­gether in the same fos­sil block or in close prox­im­ity were, in many cases, pre­served in “life po­si­tion”.

This sug­gests that these an­i­mals were liv­ing to­gether in a group – what’s known as an ag­gre­ga­tion – be­fore they died and were fos­silised.

The two parental care fos­sils Brink de­scribed showed the largest dis­crep­ancy in size among ag­gre­gat­ing in­di­vid­u­als. This im­plies he was right: these fos­sils do in­deed rep­re­sent cases of parental care.

This in­di­cates that com­plex be­hav­iour gen­er­ally at­trib­uted to liv­ing mam­mals has a long his­tory, stretch­ing back mil­lions of years. – The Con­ver­sa­tion

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