Phareodus sp.

Alvarado-Ortega, Jesús, Cuevas-García, Martha, Melgarejo-Damián, Pilar, Cantalice, Kleyton Magno, Alaniz-Galvan, Abril, Solano-Templos, Gisel & Than-Marchese, Bruno Andrés, 2015, Paleocene fishes from Palenque, Chiapas, southeastern Mexico, Palaeontologia Electronica 10 (14), pp. 1-22 : 9-10

Phareodus sp.

Referred Material. IGM 4549 (Figure 5), complete specimen from the División del Norte quarry (i.e., Locality-IGM 3869). This specimen lacks the posterior part of the tail and its estimated standard length is 145 mm.

Remarks and comparison. Recent phylogenetic analyses published by Wilson and Murray (2008), Zhang (2006), and Hilton (2003), have proved that Osteoglossomorpha and Osteoglossiformes are natural groups. Osteoglossomorpha is a teleostean group with a diagnostic relatively low number of principal caudal fin rays (18); a trend in the reduction in the number of rays in this group is present even in the most derived groups. The 17 principal fin rays present in the caudal fin of IGM 4549 (ordered in the formula i-8+7-i) support its inclusion within Osteoglossomorpha and Osteoglossiformes (e.g., Nelson, 1969; Li et al., 1997).Other important synapomorphies that characterize these taxa and primarily support their naturalness are mainly related to the caudal skeleton, lower jaw, circumorbital bones, and gill and hyoid arches (also see Wiley and Johnson, 2009, p. 132-134). Unfortunately, IGM 4549 does not appear to present them.

The family Osteoglossidae sensu Wilson and Murray (2008) is partially defined by the position of the lower jaw-quadrate articulation, behind the posterior level of the orbit. Among osteoglossomorphs, this condition is present exclusively in the Chinese genus Xixiaichthys Zhang, 2004 . In other osteoglossomorphs, this articulation is located in a forward position. Although in IGM 4549 the horizontal limb of preopercle is partially covering the articulation of the lower jaw, the latter is clearly placed forward, as in most osteoglossids.

Among osteoglossids, only the genus Phareodus presents the following assortment of characters: an exposed scale area entirely covered with reticulate furrows, an opercle depth/width ratio of approximately two or greater, a greatly enlarged and unusually long first pectoral fin ray, a toothless parasphenoid, and a rounded opercle, narrower than its middle-upper point ventral border ( Wilson and Murray, 2008, characters 72(2), 33(1), 61(1), 8(0), 87(1), respectively). The presence of all the above characters in specimen IGM 4549 suggests its identification as a member of the genus Phareodus .

In a comparative analysis of Phareodus, Li et al. (1997) claim that this genus comprises two North American species from the Green River Formation, P. encaustus ( Cope, 1871) and P. testis ( Cope, 1877) , as well as an Australian one, P. queenslandicus Hills, 1934 , from the Redbank Plains Formation of southern Queensland, Australia. They also suggest that Brychaetus muelleri Woodward, 1901 , from the Eocene London Clay strata is in fact a member of the genus Phareodus . Along with Asian species P. songziensis Zhang, 2003 from the Yangxi Formation of China, all species mentioned above share an Eocene age and a freshwater origin, with the exception of P. muelleri , which probably presented marine habits. Hence, regarding the distribution of these species, two possible biogeographical scenarios can explain their wide distribution in different continents. The first possibility is that this genus comprises a long history, and its distribution during the Eocene was closely linked to the hydrological history of each continent. The second alternative is that marine representatives of Phaerodus reached different continents across the seas.

It is early to say whether IGM 4549 is a marine Phareodus representative or a freshwater one that was dragged and deposited in the sea. However and regardless its environment, this Paleocene Mexican osteoglossomorph is the oldest Phaerodus so far discovered and therefore a specimen to consider in future attempts to understand the osteoglossiform paleobiogeography.

IGM 4549 shows a number of meristic differences with the previously reviewed nominal species of Phaerodus ( Li et al., 1997; Zhang, 2003) and may represent a new species. Nevertheless, a comprehensive descriptive study is necessary to confirm the above and is in fact currently in progress.