▁of ▁the ▁large ▁thumb ▁cla w ▁on ▁the ▁for efe et . ▁Med ium ▁gauge ▁track ways ▁with ▁cla w ▁impress ions ▁on ▁the ▁for efe et ▁probably ▁belong ▁to ▁bra ch ios aur ids ▁and ▁other ▁primitive ▁tit an osa ur iformes , ▁which ▁were ▁evol ving ▁wider - set ▁lim bs ▁but ▁retained ▁their ▁cla ws . ▁Prim itive ▁true ▁tit an osa urs ▁also ▁retained ▁their ▁fore foot ▁cla w ▁but ▁had ▁evol ved ▁fully ▁wide ▁gauge ▁lim bs . ▁W ide ▁gauge ▁lim bs ▁were ▁retained ▁by ▁advanced ▁tit an osa urs , ▁track ways ▁from ▁which ▁show ▁a ▁wide ▁gauge ▁and ▁lack ▁of ▁any ▁cla ws ▁or ▁digits ▁on ▁the ▁for efe et . ▁ ▁Occ asion ally , ▁only ▁track ways ▁from ▁the ▁for efe et ▁are ▁found . ▁F alk ingham ▁et ▁al . ▁ ▁used ▁computer ▁mod elling ▁to ▁show ▁that ▁this ▁could ▁be ▁due ▁to ▁the ▁properties ▁of ▁the ▁substr ate . ▁These ▁need ▁to ▁be ▁just ▁right ▁to ▁preserve ▁tracks . ▁D iffer ences ▁in ▁h ind ▁lim b ▁and ▁fore ▁lim b ▁surface ▁area , ▁and ▁therefore ▁contact ▁pressure ▁with ▁the ▁substr ate , ▁may ▁sometimes ▁lead ▁to ▁only ▁the ▁for efe et ▁track ways ▁being ▁preserved . ▁ ▁Bi ome chan ics ▁and ▁speed ▁ ▁In ▁a ▁study ▁published ▁in ▁P Lo S ▁ON E ▁on ▁October ▁ 3 0 , ▁ 2 0 1 3 , ▁by ▁Bill ▁S ellers , ▁Rod ol fo ▁Cor ia , ▁Lee ▁Mar get