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Journal of Paleontology 92(5):920–937
Emended diagnosis (modified from Godinot, 1984).—Presence of a molariform P4 (i.e., metacone approaching the size of para- cone, expanded distolingual basin), not observed in any other paromomyid. Uppermolars with distolingual basins less expanded distally than in Phenacolemur. Incisor with a lingual and a buccal crest, in contrast to Ignacius. The computed two-dimensional (2-D) area (max length × width of the crown in occlusal view) of P4 is similarto thatofM1, in contrast to Ignacius.Lower molar cusps relatively taller relative to the base of the crown than those of Ignacius. Lower molar trigonids not as mesially inclined relative to the level of the base of the talonid basin as in any other paromomyid genus. Large third molar hypoconulid with a simple rounded lobe, in contrast to Phenacolemur, Ignacius, Acidomomys, and some species of Paromomys, which exhibit a central invagination (evident from a distal view) that runs mesio- distally on the hypoconulid lobe.
Occurrence.—Neustrian (early Eocene) of France, United Kingdom, Portugal, and Spain; Grauvian (middle Eocene) of France.
Remarks.—Following the classification of fossils fromCondé-en- Brie to the genus Phenacolemur by Louis (1966), Russell et al. (1967) subsequently described new fossils from Mutigny and Avenay and classified them under that genus based on their resemblance to some of the North American repre- sentatives known at that time, such as Phenacolemur jepseni Simpson, 1955; Phenacolemur praecox Matthew, 1915; and Phenacolemur citatus Matthew, 1915. Russell et al. (1967) referred to the development of the hypoconulid in M3 in the European forms as reminiscent of that in Phenacolemur citatus and Phenacolemur jepseni, and the lack of ectocingulum in upper molars as similar to Phenacolemur jepseni. However, Russell et al. (1967) also mentioned that the P4 in the European specimens is smaller in size than the M1, a trait that is characteristic of Ignacius rather than Phenacolemur. Following this line of reasoning, Godinot (1984), in his
paper naming the genus Arcius, suggested that it was closely related to Ignacius instead, specifically to Ignacius graybullianus Bown and Rose, 1976 from the early Eocene of Wyoming. One of the characters thatGodinot (1984) highlighted as diagnostic of the genus was that the mandible (or, more appropriately, the alveolar ridge) reaches its highest point in the area of P4 andM1 alveoli, but at that time, the only mandible known was that of Arcius rougieri (Fig. 1). Still today, Arcius gnathic fossils are extremely rare, with the only ones reported being the mandible and maxillary fragments of A. rougieri (PAT1, PAT1 bis, and PAT2), two mandibles of A. lapparenti from Condé-en-Brie (CBX1-ph and an uncatalogued specimen [for the uncatalogued specimen, see Aumont, 2003, appendix G, pl. 3, fig. 3]), a maxilla and a mandible of Arcius sp. from Fournes (FNR43 and FNR54), an uncatalogued mandible of Arcius sp. from Le Quesnoy (Aumont, 2003, appendix G, pl. 3, fig. 4), and a mandible with M2-3 from Abbey Wood. The only lower jaw specimen known to exhibit the character described by Godinot (1984) is PAT2 (Fig. 1; Arcius rougieri), and it is not present in other species (Aumont, 2003, appendix G, pl. 3, figs. 3 and 4). Therefore, this character is an autapomorphy of A. rougieri,and
Figure 1. Micro-CT scan images of a cast of the mandible of Arcius rougieri Godinot, 1984 (PAT2) (1) in occlusal, (2) buccal, and (3) lingual views. Note that the strong line cutting through the mandible is a feature of the cast, not on the original specimen. Scale bar=0.5 cm.
not monophyletic and classified this genus in the subfamily ‘Phenacolemurinae.’ According to Robinson and Ivy (1994), this subfamily contained the genera Phenacolemur, Ignacius, and Elwynella Rose and Bown, 1982, along with their proposed genera ‘Simpsonlemur,’‘Pulverflumen,’ and ‘Dillerlemur,’ whose validity has been a source of debate (Silcox and Gunnell, 2008). By contrast, Aumont (2003) recovered a monophyletic Arcius in her cladistic analysis, with the North American Acidomomys Bloch et al., 2002 as its sister taxon. Diagnostic characters for Arcius included here that have not
should not be considered diagnostic of the genus, but rather solely diagnostic of A. rougieri (see the following). Later, Robinson and Ivy (1994) suggested that Arcius was
been discussed previously in the literature include the simple enlarged hypoconulid lobe, the molariform P4, and the weak mesial inflection of the molar trigonids. With the exception of primitive representatives of the paromomyid clade (i.e., Paromomys farrandi), a markedly enlargedM3 hypoconulid lobe is typical of paromomyids. There are, however, two types of morphology of these lobes: simple and divided. Simple lobes, as observed in all species of Arcius, have a rounded shape in distal view, with a concave outline (Fig. 2.6–2.9). Divided lobes, as observed in Phenacolemur, Ignacius, Edworthia Fox et al., 2010, some species of Paromomys Gidley, 1923, and to a lesser extent, Acidomomys, exhibit an invagination of the occlusal surface that runs mesiodistally through the center of the lobe (Fig. 2.1–2.5). This gives the lobe a heart-shaped appearance in distal view. Arcius is also characterized by having a P4 that resembles a molar, more so than in other genera (Figs. 3.13, 3.17, 4.1). Although the P4 metacone is smaller than the paracone, it approaches the size of the paracone. This, combined with the similarity in size of P4 toM1 makes the premolar verymolar-like. The similarity between the adult P4 to theM1 in Arcius parallels the resemblance in morphologies between the deciduous P4 and the M1 observed in other paromomyids, such as Phenacolemur
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