Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific

Abstract

New Zealand (NZ) has long been upheld as the archetypical example of a land where the biota evolved without nonvolant terrestrial mammals. Their absence before human arrival is mysterious, because NZ was still attached to East Antarctica in the Early Cretaceous when a variety of terrestrial mammals occupied the adjacent Australian portion of Gondwana. Here we report discovery of a nonvolant mammal from Miocene (19-16 Ma) sediments of the Manuherikia Group near St Bathans (SB) in Central Otago, South Island, NZ. A partial relatively plesiomorphic femur and two autapomorphically specialized partial mandibles represent at least one mouse-sized mammal of unknown relationships. The material implies the existence of one or more ghost lineages, at least one of which (based on the relatively plesiomorphic partial femur) spanned the Middle Miocene to at least the Early Cretaceous, probably before the time of divergence of marsupials and placentals > 125 Ma. Its presence in NZ in the Middle Miocene and apparent absence from Australia and other adjacent landmasses at this time appear to reflect a Gondwanan vicariant event and imply persistence of emergent land during the Oligocene marine transgression of NZ. Nonvolant terrestrial mammals disappeared from NZ some time since the Middle Miocene, possibly because of late Neogene climatic cooling.

Fig. 1.

First pre-Pleistocene mammal from NZ. MNZ S.40958, edentulous mandibular fragment. (A) Left lateral view. (B) Right lateral view. (C) Anterior view. (D) Dorsal view. (E and F) Stereopair occlusal view. fs, fused symphysis; i, c, p1, and p2, alveoli for incisor, canine, and two premolars (the homology of premolars is uncertain); mf, mental foramen; MNZ S, prefix for catalog numbers of the Vertebrate Fossil Collection, Museum of New Zealand Te Papa Tongarewa, Wellington. (Scale bar, 2 mm.)

Fig. 2.

First pre-Pleistocene mammal from NZ. MNZ S.42214, proximal section of a right femur. (A) Dorsal view. (B) Ventral view. (C) Medial view. (D) Proximal view. agt, apex of greater trochanter; fc, fovea capita for acetabular ligament; h, head; itf, intertrochanteric fossa; n, notch between greater trochanter and head; olt, origin of lesser trochanter. (Scale bar, 2 mm.)

Fig. 3.

Position of the SB mammal in the phylogeny of mammaliaforms based on the strict consensus of 192 equally parsimonious trees from a matrix of 75 taxa and 414 morphological characters (ref. ; see also SI supporting information). Treelength, 1,826; consistency index, 0.404; retention index, 0.793. Nonmammalian outgroups are not shown. Morganucodontans, australosphenidans, eutriconodontans, multituberculates, spalacotheroids, eutherians, and metatherians all comprise multiple taxa that have been collapsed to single terminals (see SI supporting information). Trechnotheria (including therians and closely related groups) is indicated.