MAMMALIA

Benton, MJ, Donoghue, PCJ, Vinther, J, Asher, RJ, Friedman, M & Near, TJ, 2015, Constraints on the timescale of animal evolutionary history, Palaeontologia Electronica (Florence, Italy) 15 (1), pp. 1-107 : 59-60

CROWN MAMMALIA (57)

Node Calibrated. Divergence of monotremes, marsupials, and placentals ( Figure 10 ).

Fossil Taxon and Specimen. Bathonian australosphenidans such as Ambondro mahabo (UA-10602 University of Antananarivo, Madagascar; Flynn et al., 1999).

Phylogenetic Justification. Ambondro mahabo from Madagascar ( Flynn et al., 1999) is placed within the monotreme clade Australosphenida, according to the most widely accepted cladistic analysis of crown Mammalia (Luo et al., 2002; Meng et al., 2011).

Minimum Age. 164.9 Ma

Soft Maximum Age. 201.5 Ma

Age Justification. At present it cannot be said which of the Bathonian eutriconodonts and dryolestoids (Boneham and Wyatt, 1993) and australosphenidans ( Flynn et al., 1999) is oldest, but we select Ambondro as most likely. It comes from the upper part of the Isalo “Group” (middle Jurassic, Bathonian) of Madagascar ( Flynn et al., 1999), dated as generally Bathonian, so we select the top of the Bathonian as the hard minimum calibration date, 166.1 Ma ± 1.2 Myr = 164.9 Ma ( Gradstein, 2012).

The closest relatives of crown mammals are Hadrocodium and Docodonta (Luo et al., 2002; Meng et al., 2011). Hadrocodium was originally reported to be early Jurassic of Yunnan Province, China (Sinemurian; Luo et al., 2001), and the oldest docodonts are from the Bathonian of Europe, with a possible earlier form from the Kota Formation of India. Further outgroups, Morganucodontidae , Sinoconodon , and Adelobasileus , are known from the late Triassic and early Jurassic. Deposits of Late Triassic and Early Jurassic age have yielded fossil mammaliaforms, but nothing assignable to the Australosphenida or Theriimorpha. We therefore suggest the Triassic-Jurassic boundary at 201.3 Ma ± 0.2 Myr = 201.5 Ma as a soft maximum constraint.

Discussion. Following Luo et al. (2002, 2011), Australosphenida comprises the stem lineage encompassing Monotremata . Its oldest representatives include Ambondro from Madagascar, as well as Amphilestes and Amphitherium from the UK (Boneham and Wyatt, 1993), and Asfaltomylos from the late Middle Jurassic (Callovian) Cañadon Asfalto Formation of Cerro Condor, Argentina (Rauhut et al., 2002) .

The base of the crown clade of modern mammals, marking the split between monotremes and therians, depends on how the increasingly diverse, extinct Mesozoic mammal groups are included in the clade. As noted earlier, the oldest eutherian, Juramaia , takes the age of Theria back to the Late Jurassic. According to a widely accepted cladogram of Mesozoic mammals (Luo et al., 2002; Meng et al., 2011; but see Woodburne et al., 2003 and Rougier et al., 2012), therians are part of a larger clade that also includes Eutriconodonta, Multituberculata, Spalacotheroidea, and Dryolestoidea. Most of these originated in the late Jurassic, but some lineages appear to have originated in the middle Jurassic. The oldest occurrences of the monotreme stem clade (Australosphenida) include somewhat older Jurassic forms dating to the Bathonian, including Ambondro from Madagascar ( Flynn et al., 1999) and Amphilestes and Amphitherium from the UK (Boneham and Wyatt, 1993).

Amphilestes from the Stonesfield Slate is likely a eutriconodont and is referred to the Procerites progracilis Zone of the lower part of the middle Bathonian stage on the basis of ammonites (Boneham and Wyatt, 1993). Tooth-based mammal taxa from the early Jurassic of India ( Kotatherium, Nakundon ) and North America ( Amphidon ) that might be spalacotheroids (Kielan Jaworowska et al., 2004) are not convincingly members of the clade (Averianov, 2002) and are ignored here. The oldest dryolestoid appears to be Amphitherium , also from the Stonesfield Slate. The oldest monotremes are Steropodon and Kollikodon from the Griman Creek Formation, Lightning Ridge, South Australia, and dated as middle to late Albian, 109– 100 Ma. Teinolophos is from the Wonthaggi Formation, Flat Rocks, Victoria, and is dated as early Aptian, 125–121 Ma.

Recently, Zheng et al. (2013) and Bi et al. (2014) have argued that haramiyids are closely related to multituberculates, and that both are in turn related to monotremes and thus part of crown Mammalia. As haramiyids are known from the Triassic ( Jenkins et al. 1997), this hypothesis has major implications for the age of the monotremetherian divergence. Previous analyses of haramiyid fossils (e.g., Jenkins et al. 1997) indicated that they were not closely related to multituberculates or crown mammals, but the most recent phylogenetic studies now question this hypothesis (Yuan et al. 2013; Zheng et al. 2013; Bi et al. 2014). If these recent phylogenetic studies are accurate, the first divergence of crown Mammalia would be constrained by the occurrence of Haramiyavia ( Haramiyidae, Jenkins et al. 1997 ) in the Orsted Dal Member of the Fleming Fjord Formation, slightly over 200 Ma ago in?Norian-Rhaetic stages of the late Triassic. We retain a conservative position here.

Figures

FIGURE 10. Calibration diagram for mammals, excluding Archonta. Euarchontoglires continued into Figure 11.