Gyrodus sp.

Alvarado-Ortega, Jesús, Barrientos-Lara, Jair Israel, Espinosa-Arrubarrena, Luis & Melgarejo-Damián, Pilar, 2013, Late Jurassic marine vertebrates from Tlaxiaco, Oaxaca State, southern Mexico, Palaeontologia Electronica (24 A) 17 (2), pp. 1-25 : 10-12

Gyrodus sp.

Figure 7

Type species. Gyrodus hexagonus ( Blainville, 1818) , originally described as Stromateus hexagonus .

Referred material. IGM 9318 ( Figure 7 ), anterior part of the body showing part of the head bones disarticulated. Collected in shale strata between levels L1 and L2 ( Figure 4 ). IGM 9319, a set of scales and disarticulated bones found within a node of 10 cm diameter from shale strata between levels L4 and L5.

Occurrence. Yosobé, Tlaxiaco, Oaxaca. Kimmeridgian-Tithonian shales of Sabinal formation.

Description. The fragmented head preserved in IGM 9318 is approximately 170 mm long. Regarding this measurement and the body proportions of Gyrodus published by Lambers (1992, p. 51, 52), where the head length is about 33% of the standard length, it is possible to suggest that IGM 9318 could reach a standard length close to 500 mm.

The noteworthy characteristic of IGM 9318 is the presence of two isolated oval teeth (probably vomerine teeth), in which the occlusal surface has central papilla surrounded by a mammillated ring, which is surrounded by a strong crenulated ridge; additionally, the largest of these teeth shows a visible groove between its mammilated ring and crenulated ridge ( Figure 7 ). Part of the lateral surface of the skull is visible, showing numerous tubercles ornamenting the external surface of frontals, parietals, and dermosupraoccipital. Numerous tubercles also ornament the surface of the opercle and preopercle. Two well-developed sclerotic bones frame the orbit.

The scales in IGM 9318 and IGM 9319 are rhomboidal; they are well articulated with each other because their anterior border is thick and extends into a ventral spine-like projection that corresponds with a cavity on the dorsal border of the following scale. The surface of scales is ornamented with tubercles that can be randomly distributed or clustered in groups that frequently produce reticular ridges.

Remarks. Gutiérrez-Zamora (2011) considered that the first pycnodont specimens recovered in Yosobé belonged in the genus Mesturus Wagner, 1862 . However, the present revision of the scale rows of these specimens shows that these pycnodonts do not exhibit "serrated or jagged suture between the scales", which is a diagnostic characteristic of Mesturus ( Poyato-Ariza and Wenz, 2002, p. 194) .

Poyato-Ariza and Wenz (2002, p. 218) and Kriwet and Schmitz (2005) placed Gyrodus in its own family and regarded it as one of the most basal members of the order Pycnodontiformes . According to Poyato-Ariza and Wenz (2002), the presence of central papilla in the vomerine and prearticular teeth is a unique characteristic (autapomorphy) of Gyrodus , and the fact that IGM 9318 displays this dental characteristic ( Figure 7.2 ) supports the idea of its appurtenance in this genus.

Gyrodus is one of the most commonly cited pycnodont from the marine deposits of Late Jurassic in Europe ( Kriwet and Schmitz, 2005). It contains about 32 nominal species, including a large number of probably invalid or synonymous species known just by fragmentary material. At present, only two unquestionable valid species described based on complete and articulated skeletons are known from German localities, including the type species G. hexagonus ( Blainville, 1818) from the Kimmeridgian of Solnhofen, and G. circularis Agassiz, 1833 , from the Tithonian of Nusplingen (e.g., Lambers, 1991; Poyato-Ariza and Wenz, 2002). The fossils of Gyrodus from Yosobé are important since they include the best preserved fossils external to the eastern Tethys Sea domain, and they potentially represent one of the richest records in America. Therefore, future studies of these fossils will provide new data about the taxonomic diversity and biogeographical history of this genus.

The finding of Gyrodus in Yosobé is in accord and complements the paleobiogeographic pattern of this pycnodont described by Kriwet and Schmitz (2005), which include well documented Middle and Late Jurassic records and discards the questionable Cretaceous records. This geographic-temporal pattern began in the Middle Jurassic and comprises records from deposits within the Northeastern Tethys Sea domain (in Scotland, England, and northern France), records from the beginning of the Late Jurassic, when this genus was more abundant in Europe and then experienced a westward migration beyond the Western Tethys Sea domain as evidenced by the Caribbean remains of Gyrodus found Cuba and Mexico ( Gregory, 1923; Dunkle and Maldonado-Koerdell, 1953) and the marine deposits within Southeastern Pacific domains in the western coast of South America ( Chile), which finishes at the end of Late Jurassic, when this fish reached the northern Pacific (Siberia and Japan).

Division Ginglymondi sensu Grande, 2010

Order Lepisosteiformes sensu López-Arbarello, 2012

Family unnamed

Genus Scheenstia López-Arbarello and Sferco, 2011

Figure 8.1-4

Referred material. IGM 9320, specimen divided in 22 fragments that include teeth and scales of a single specimen preserved in association with several small oysters ( Figure 8.1-4 ).

Occurrence. La Lobera, Tlaxiaco, Oaxaca. Oxfordian-Early Kimmeridgian marl limestone strata known as the “Caliza con Cidaris ” geological unit ( Figure 3 ).

Description. The fragments of IGM 9320 have an uncertain number of scales, many of which are strongly fragmented, partially preserved or complete, as well as a possible fragment of the vomer with some complete teeth preserved. The scales are thick and rhomboidal; some scales range between 40 and 30 mm in length and height, although some scales are larger. The thick ganoine expands in a continuous and smooth layer with rounded corners. The ganoine is thicker along the borders, so the scales have a shallow central depression. These scales have a wide rounded vertical articulation, their dorsal anterior process is stout, prominent, and sharp, and the ventral anterior process is relatively small, thin, and gently convex.

In the teeth bearing bone fragment recovered, which probably represents part of the vomer, the tooth bases are fused with the bone. These teeth show the tritoral morphology described by Jain (1985), in which a short root supports a crown that is slightly elevated, exceptionally broad and rounded, and practically smooth. Two not emerged and unused teeth preserved in the specimen show a central small sharp protuberance whereas those worn are completely smooth.

Remarks. The IGM 9320 specimen does not show enough characteristics to support its inclusion in the division Ginglymondi sensu Grande (2010). However, it is possible to reach its accurate taxonomical identification within the order Lepisosteiformes , as it was recently defined by López-Arbarello (2012). If characteristics 84, 85, 86, and 55 are considered, this specimen shows one of the unambiguous synapomorphies of the order Lepisosteiformes , its scales have two processes forming their rostro-caudal articulation (state 2 of character 86) (also see Cavin et al., 2009). Additionally, the scales of this specimen show a wide rounded vertical articulation; these conditions differ from those present in the lepisosteiformes crown taxa, whose scales have the peg-and-socket vertical articulation reduced or absent in the superfamily Lepisosteoidea (state 1 of character 85,) and a strong posteriorly directed spine in the suborder Lepisosteoidei (state 1 of character 84).

According to López-Arbarello (2012, figure 25) and Deesri et al. (in press), there are three genera of lepisosteiformes not in the suborder Lepisosteiodei , Lepidotes Agassiz, 1832 , Isanichthys Cavin and Suteethorn, 2006, and Scheenstia . The latter is well diagnosed by the presence of strong tritoral teeth (state 2 of character 55), remarkably similar to the few teeth preserved in the specimen from Yosobé. The characteristics described of IGM 9320 support its inclusion in the genus Scheenstia .

There are not enough characteristics in IGM 9320 to determine its specific identity. As a remark, López-Arbarello (2012, p. 27) noted that the morphological limits and taxonomical relationships between Lepidotes and Scheenstia are not completely resolved, partly because not all the known Lepidotes species were considered in her study.

Figures

FIGURE 3. Geologic map of Tlaxiaco showing the position and shape Yosobé and La Lobera localities (modified from SGM 2000a, 2000b, 2009).

FIGURE 4. Geologic map and lithostratigraphical column of Yosobé. M, Marl stratum; L1-L8, limestone strata; the intermediate strata (L1-L2, L2-L3, ... L7-L8) are the bituminous shales layers that carry the vertebrate assemblage of Yosobé.

FIGURE 7. Gyrodus sp., specimen IGM 9318 from Yosobé. 1, general view. 2, close up of possible vomerine teeth. Abbreviations: dsoc, dermosupraoccipital; fr, frontal; hm, hyomandibular; op, opercle; pa, parietal; pop, preopercle; s, scale; sc, sclerotic.

FIGURE 8. Jurassic remains of Ginglymondi fishes from localities near Tlaxiaco. 1-4, Scheenstia sp., specimen IGM 9320 from Tithonian strata of La Lobera, including a group of disarticulated scales (1), isolated scale (2), and a possible fragment of a vomer with teeth (in ventral view and lateral view, 3 and 4, respectively). 5, isolated scale of an undetermined Lepisosteiform described here as Morphotype 1, specimen IGM 9321 from Kimmeridgian strata of Yosobé. 6-7, scales of both sides of the body of an undetermined Lepisosteiform described here as Morphotype 2, specimen IGM 9322 from Kimmeridgian strata of Yosobé. Abbreviations: adp, anterior dorsal process; avp, anterior ventral process; dp, dorsal process; arrows indicate the anterior edge of the scales.