MONOTREMATA, Bonaparte, 1837

Phillips, MJ, 2015, Four mammal fossil calibrations: balancing competing palaeontological and molecular considerations, Palaeontologia Electronica (Basel, Switzerland) 1 (7), pp. 1-16 : 7-8

CROWN MONOTREMATA

Node Calibrated. Monotremata , the divergence between Tachyglossidae (echidnas) and Ornithorhynchidae (platypus). See Figure 1.2 .

Fossil Taxon. Obdurodon dicksoni ( Archer et al., 1992) .

Specimens. QMF20568 (Queensland Museum), holotype of Obdurodon dicksoni is a near-complete skull with left and right upper pre-molars and is sufficient for phylogenetic placement. A partial, edentulous dentary (QMF18977) and several cheek teeth (QM F18978, QMF30249, QMF30716 and QMF30717) are slightly older, providing the calibration minimum bound. The slightly older cheek teeth closely match the holotype skull for size and “insertion” and are near-identical to other dental material from the same site as the skull, thus justifying conspecificity ( Archer et al., 1993; Musser and Archer, 1998).

Phylogenetic Justification. All formal and informal cladistic analyses of monotremes favour grouping Obdurodon with the modern Ornithorhynchus to the exclusion of tachyglossids (e.g., Musser, 1999; Luo et al., 2007; Rowe et al., 2008). Moreover, Phillips et al. (2009) found high statistical support for an Obdurodon - Ornithorhynchus sister-grouping, for which unambiguous synapomorphies include rostral elements (nasal, maxilla, septomaxilla) forming a board ‘bill’, a robust posterolateral maxillary process and several endocranial characters (see Macrini et al., 2006). The sister relationship between living ornithorhynchids and tachyglossids is uncontroversial in molecular and morphological studies (e.g., van Rheede et al., 2006; Luo et al., 2007; Phillips et al., 2009).

Hard Minimum Age. 15.97 Ma.

Soft Maximum Age. 113.0 Ma.

Age Justification. Obdurodon dicksoni occurs in Faunal Zones B and C of the Riversleigh local faunas (northwestern Queensland). The holotype is known from the early Middle Miocene Faunal Zone C (Ringtail Site). However, slightly older Ob. dicksoni molars and a partial dentary are known from Faunal Zone B sites (Neville’s Garden and Dirk’s Towers). Early Miocene dates have consistently been attributed to Faunal Zone B sites by biocorrelation (e.g., Black, 1997; Travouillon et al., 2006). More recently Black et al. (2012) noted that U/Pb radiometric dating of speleothems now confirms this timing. However, until the new dates are published I consider the top of the Early Miocene to provide a minimum for Riversleigh Faunal Zone B and hence, for the crown Monotremata divergence.

Potential crown monotremes are traceable at least back to the Paleocene (~61 Ma) Monotrematum sudamericanum from Argentina, which is known from several ornithorhynchid-like molars ( Pascual et al., 1992) and distal femora ( Forasiepi and Martinelli, 2003). Earlier (Maastrichtian) well-sampled South American faunas lack any monotremes. However, sparse Australasian fossil records provide no solid evidence for mammal faunas lacking crown monotremes until the Albian Lightning Ridge ( Flannery et al., 1995) and Dinosaur Cove ( Rich and Vickers-Rich, 2003) faunas. I use the base of the Albian as a soft maximum for Monotremata .

Discussion. In light of sparse fossil records and ‘platypus’ morphology being ancestral among crown monotremes (see Gregory, 1947; Musser, 2003; Phillips et al., 2009), molecular timetrees calibrated independently of Monotremata have been particularly important for estimating monotreme crown divergence. Modern relaxed-clock molecular dating estimates concur on a Tertiary divergence between the platypus and echidnas, with most estimates falling between 21-48 Ma (e.g., Janke et al., 2002; Hugall et al., 2007; Warren et al., 2008). One exception ( Rowe et al., 2008) provided mean estimates of 79.5 Ma and 88.9 Ma, although their exclusion of non-mammals prevented accurate estimation of evolutionary rates on either side of the root between monotremes and therians. Rectifying this issue again resulted in mid-Tertiary estimates ( Phillips et al., 2009). Hence, molecular dates for the crown monotreme divergence (including the estimates in Table 1) fall within the younger end of the 15.97-113.0 Ma bounds.

Although the Early Miocene minimum bound for crown monotreme origins is based primarily on the platypus, Obdurodon dicksoni , further support comes from the earliest tachyglossid fossil, the already somewhat derived Gulgong echidna, Megalibgwilia robusta ( Dun, 1895) . The age of the Gulgong deposit is 13-14 Ma, based on estimates from overlying basalt ( Woodburne et al., 1985), although this has been contentious, because the fossil preservation is similar to much younger nearby Pleistocene sites ( Augee et al., 2006).

It does not necessarily follow that the platypus affinity of Obdurodon dicksoni secures crown monotreme placement for the dentally similar Late Oligocene (~25 Ma) Obdurodon insignis or Paleocene Monotrematum . None of the unambiguous synapomorphic characters linking Obdurodon dicksoni with Or. anatinus from Rowe et al. (2008) are preserved in either of these older ‘platypuses’. Determining where Monotrematum and Ob. insignis fall relative to the divergence of the platypus from the edentulous echidnas will require further non-dental material.

The suggested basal Albian soft maximum bound for Monotremata is challenged by the recent proposals of Rowe et al. (2008) that the Albian Kryoryctes cadburyi (an isolated, incomplete humerus) could be a stem tachyglossid and the Aptian Teinolophos trusleri (several partial dentaries) is a stem ornithorhynchid. The former suggestion is based on gross morphology and ignores features such as a shallow ulna trochlea and an olecranon fossa, which place the specimen well outside platypuses and echidnas, for which distal humeri are substantially more specialized ( Pridmore et al., 2005). Furthermore, Rowe et al.’s (2008) placement of T. trusleri depends on numerous redundant characters all based on an enlarged mandibular canal. Without this redundancy, cladistic analyses place T. trusleri outside of crown monotremes with high statistical support ( Luo et al., 2007; Phillips et al., 2009).

The mid-Tertiary molecular dates for the monotreme crown divergence indicate that the 113.0 Ma maximum softbound is conservative, although necessarily so, reflecting the sparse and fragmentary nature of the monotreme fossil record. In the absence of any narrow temporal range within which stem-crown transitional monotremes appear it may be advisable to employ the monotreme bounds as a uniform prior.

Figures

FIGURE 1. The phylogenetic placement of reference fossils (full red arrows) relative to modern groups for 1, Macropodoidea; 2, Monotremata; 3 Caviomorpha-Phiomorpha; and 4, Chiroptera. These clades are shaded grey and black circles identify the calibrated nodes. Relationships among modern taxa are taken from the DNA analysis of Meredith et al. (2011), although each is supported by a broad consensus of molecular phylogenetic studies. Dotted red arrows (1, 3, 4) indicate possible alternative placements for the reference fossil that if they can be confirmed, would offer (tighter) calibration of a slightly younger node.