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Elgaria

 node name
Elgaria     Look for this name in NCBI   Wikipedia   Animal Diversity Web
 
  recommended citations
https://doi.org/ 10.26879/837 Scarpetta, 2018
 
  node minimum age
16.7 Ma
A tuff near the base of the Split Rock Formation was dated by 40Ar/39At at 17.4 ± 0.08 Ma (Izett and Obradovich, 2001), but was later corrected to 17.60 ± 0.08 Ma (Lander et al., 2013). UCMP V69190 occurs in normal and reversed magnetozones correlated with chrons C5Dn and C5Cr, 17.5-16.7 Ma (Hilgen et al., 2012; Lander et al., 2013). The lizard fossils were found in conjunction with a middle Hemingfordian mammal fauna, which includes the oreodontids Brachycrus vaughani and Brachycrus sweetwaterensis (Lander et al., 2013). Other researchers assigned a chronostratigraphic age to the mammal fauna in the Split Rock Formation of approximately 17.5-16.7 Ma, although they considered the fauna to be late Hemingfordian in age (Tedford et al., 2004). An alternative interpretation of the mammals of the Split Rock Formation is that the fauna is late Hemingfordian to early Barstovian, suggesting that V69190 occurs in a reversed magnetozone that was instead correlated with chron C5Br, 16.1-15.2 Ma (Liter et al., 2008). Because there appears to be more evidence for a Hemingfordian fauna at V69190 with no Barstovian elements, I accept that UCMP 163737 is assigned a temporal range of 17.5-16.7 Ma, in agreement with a correlation of the magnetozone to chrons C5Dn and C5Cr instead of with chron C5Br. Date constraints using the combined evidence of paleomagnetic, radioisotopic, and biochronologic data provide valuable age information for fossils, but the chronostratigraphic age of V69190 remains open for refinement and revision with future improvements of faunal interpretations and new radioisotopic dates.
 
  node maximum age
0 Ma
None specified.
 
 primary fossil used to date this node 
 
UCMP 163737
Elgaria sp., Gray, 1838
Location relative to the calibrated node: Stem

[show fossil details]
     Locality: Granite Mountains
     Stratum: Split Rock Formation
     Geological age: Miocene, Neogene, Cenozoic


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  phylogenetic justification
Four bilateral tooth positions (not including a midline position) are a synapomorphy of Anguidae (Conrad et al., 2011), but this state is also present in Xenosaurus (Bhullar, 2011). Xenosaurus has heavily sculptured and extensive dermal rugosities on the anterodorsal surface of the premaxilla, which are absent in UCMP 163737. Among anguids, a dorsal ossification on the alveolar plate posterior to the medial ethmoidal foramen is present in Gerrhonotinae and Diploglossinae and is absent in Anguinae. All specimens examined of the anguine genera Ophisaurus and Pseudopus do not possess this ossification. A forked palatal process is present in both Anguinae and Diploglossinae (Evans, 2008; Conrad et al., 2011), and is absent in UCMP 163737. Every specimen of Ophisaurus and Pseudopus examined here has a forked palatal process. Additionally, the anteriormost surface of the premaxilla of Ophisaurus is proportionally taller and flatter compared to that of UCMP 163737 and to all other anguids examined. Lateral flanges on the nasal process of the premaxilla are present in Diploglossinae but are absent in UCMP 163737. The specimen is assigned to Gerrhonotinae based on the presence or absence of these characters. An ossified bridge connects or almost connects the nasal process of the premaxilla with the alveolar plate, partially or completely isolating the medial ethmoidal foramen from the external naris, in the gerrhonotine genera Gerrhonotus, Barisia, Mesaspis, and Abronia (Good, 1987). The bridge is generally absent in Elgaria, but is occasionally present in the extensively studied taxon Elgaria multicarinata (see Bhullar, 2011) and Elgaria kingii. The bridge is absent in Gerrhonotus parvus). UCMP 163737 lacks a bridge, but possesses an anterior premaxillary foramen, which is present in most Elgaria (Ledesma, personal commun., 2017) and is absent in both Gerrhonotus parvus examined. An anterior premaxillary foramen is absent in all other taxa examined besides one specimen of Ophisaurus ventralis (CAS 74296). The presence of that foramen is not plesiomorphic for Gerrhonotinae or Anguinae and is derived in Elgaria. Thus, the character combination of the presence of an anterior premaxillary foramen and the absence of an ossified bridge on the premaxilla is apomorphic of Elgaria among gerrhonotines (see Figure 1). The fossil does not have any apomorphies of the other five extant genera of gerrhonotine lizards. However, a lack of a premaxillary bridge is plesiomorphic for Gerrhonotinae with respect to its sister taxon Anguinae (Pyron et al., 2013; Zheng and Wiens, 2016). That character is also plesiomorphic in morphological analyses in which Gerrhonotinae and Diploglossinae are sister taxa (Gauthier et al., 2012). The presence of an anterior premaxillary foramen is derived in Elgaria with respect to most other anguids and to other gerrhonotines, but possessing this character does not definitively place the fossil within the crown of Elgaria.
 
  phylogenetic reference(s)
Scarpetta, S. 2018. The earliest known occurrence of Elgaria (Squamata: Anguidae) and a minimum age for crown Gerrhonotinae: Fossils from the Split Rock Formation, Wyoming, USA. Palaeontologia Electronica 21.1.1FC 1-9.
[View electronic resource]
 
 tree image (click image for full size) 
tree image
Figure 1 from Scarpetta (2018)
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