Revisión de Vallecillichthys multivertebratum (Teleostei: Ichthyodectiformes), un "pez Bulldog" del Cretácico Tardío (Turoniano temprano) del noreste de México

Alberto Blanco–Piñón¹* and Jesús Alvarado–Ortega^{2, 3}

¹ Centro de Investigaciones en Ciencias de la Tierra, Universidad Autónoma del Estado de Hidalgo, Apdo. Postal 1–288, Admón. 1, 42001 Pachuca, Hidalgo, México. * albblanc@yahoo.com

² Departamento de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México, Ciudad Universitaria, Coyoacán, 04510, México D.F., México.

³ Departamento de Zoología, Instituto de Biología, Universidade do Estado de Rio de Janeiro, São Francisco Xavier 524, Maracanã, Rio de Janeiro, 20559–900, Brasil.

Manuscript received: March 23, 2007
Corrected manuscript received: June 3, 2007
Manuscript accepted: June 4, 2007

ABSTRACT

A complete description of Vallecillichthys multivertebratum Blanco and Cavin, 2003, is provided. This ichthyodectiform fish was collected in the Vallecillo Member of the Agua Nueva Formation (Upper Cretaceous: lower Turonian) at Vallecillo, Nuevo León State, northeastern Mexico. On the basis of a comparative study, this monospecific genus is recognized as member of the suborder Ichthyodectoidei and placed with Ichthyodectes and Xiphactinus into the family Ichthyodectidae. This work provides data on Vallecillichthys for a future comprehensive phylogenetic analysis including all ichthyodectiform species.

Key words: Vallecillichthys, Ichthyodectiformes, Turonian, Agua Nueva Formation, Vallecillo, Mexico.

RESUMEN

Palabras clave: Vallecillichthys, Ichthyodectiformes, Turoniano, Formación Agua Nueva, Vallecillo, México.

INTRODUCTION

During the 1980s, a large array of well–preserved vertebrate and invertebrate fossils from Vallecillo quarries started to be discovered by inhabitants of the Vallecillo village, Nuevo León, northeastern Mexico (Figure1). Later, in the 1990s, Vallecillo started to be recognized as the most important Late Cretaceous (Turonian) fossil fish locality in this country (Blanco–Piñón, 1998, 2003; Blanco et al., 2001; Alvarado–Ortega et al., 2006c). The fish assemblage from Vallecillo includes ichthyodectiformes that were described as Vallecillichthys multivertebratum Blanco and Cavin, 2003 (Figures 2–5) [Figures 2, 3, 4 y 5]. However, that descriptive work just included the diagnostic characters and a short discussion. The aim of present work is to provide a complete description of this Mexican fossil fish as well as to discuss the possible interrelationships of this species with other ichthyodectiforms.

Ichthyodectiformes, commonly called "bulldog fish" due to the flat form of his head and the fierce aspect of his jaws, are an extinct order of basal teleosts erected by Bardack and Sprinkle (1969). Approximately 19 to 23 nominal genera (including about 42 species) are included in this order. These genera are: Saurocephalus Harlan, 1824; Saurodon Hays, 1830; Thrissops Agassiz, 1833; Cladocyclus Agassiz, 1841; Chirocentrites Hekel, 1849; Ichthyodectes Cope, 1870; Xiphactinus Leidy, 1870; Prymnetes Cope, 1871; Chiromystus Cope, 1885; Gillicus Hay, 1898; Eubiodectes Hay, 1903; Proportheus Jaekel, 1909; Allothrissops Nybelin, 1964; Occithrissops Schaeffer and Patterson, 1984; Cooyoo Lees and Bartholomai, 1987; Prosaurodon Stewart, 1999; Faugichthys Taverne and Chanet, 2000; Vallecillichthys Blanco and Cavin, 2003; Antarctithrissops Arratia et al., 2004; and Unamichthys Alvarado–Ortega, 2004. The validity of Itaparica Silva–Santos, 1986, erected on the basis of Chiromystus woodwardi Silva–Santos, 1949, is under investigation by M.E.C. Leal (personal communication, 2006).

Ichthyodectiforms are known from the Middle Jurassic to the Late Cretaceous in both marine and estuarine sediments of North America, South America, Antarctica, Africa, Middle East, Australia, and Europe (Bardack, 1965; Patterson and Rosen, 1977; Taverne, 1986; Forey et al., 2003; Arratia et al., 2004; among others). Mesoclupea, Ping and Yen, 1933, and Chuhsiungihthys Lew, 1974, probably represent this group in the eastern part of Asia; however, their inclusion has been questioned (see Ping and Yen, 1933; Yabumoto, 1994; Bardack, 1965; Cavin and Forey, in press). An isolated and fragmentary skull from the lower Eocene of New Zealand was erected as Portheus dunedinensis Chapman, 1934 [today Portheus is synonymous with Xiphactinus (Bardack, 1965)]; however, this specimen does not exhibit the diagnostic characters neither for the genus Xiphactinus nor for the order Ichthyodectiformes.

The relationships among ichthyodectiforms had been hypothesized in some pre–Henningnian (Saint Seine, 1949; Bardack 1965) and cladistic studies (Patterson and Rosen, 1977; Taverne, 1986; Maisey, 1991; Stewart, 1999; Taverne and Chanet, 2000; Alvarado–Ortega, 1998, 2004, 2005). Nevertheless, a desirable comprehensive phylogenetic analysis requires the inclusion of poorly studied taxa (i.e., Proportheus, Eubiodectes, and Chirocentrites), as well as some bad preserved and scarcely represented (i.e., Prymnetes, Chiromystus, and Cooyoo), in order to understand in detail the interrelationships of Ichthyodectiformes. In addition, ongoing works include the description of new taxa from Jordan (Mustafa et al., 2003), Mexico (Alvarado–Ortega, 2005, Ovalles–Damián et al., 2006), Italy (Signore et al., 2005), and Brazil (Alvarado–Ortega and Brito, personal observation, 2006).

Geological setting of the Vallecillo quarries

The fossil bearing strata at Vallecillo are known as the Vallecillo Member of the Agua Nueva Formation. It consists of a homogeneous sequence about 4 m thick of finely laminated pink marlstone and 6–3 cm thick red–brown shale interlayed (Blanco, 2003; Blanco et al., 2001; Blanco–Piñón et al., 2002). According to Blanco–Piñón et al. (2005, 2007) and Ifrim (2006), an early Turonian age can be assigned to the fossiliferous beds of Vallecillo by the presence of the ammonite Watinoceras coloradoense. Fossil fishes are the most abundant group found in Vallecillo, however, also invertebrates (ammonites, bivalves) and reptiles (Blanco–Piñón, 1998, 2003; Blanco–Piñón et al., 2002) have been reported. Fishes are represented mainly by sharks and osteichthyans. Sharks are represented by the trunk of a Scyllyorhinus–like specimen (Blanco et al., 2001), teeth of Ptychodus mortoni (Blanco et al., 2001; Blanco–Piñón, et al., 2007) and centra of indeterminated lamniforms (Blanco–Piñón et al., 2005). Osteichthyan from Vallecillo are Tselfatia sp., Nursallia sp., Araripichthys sp., Goulmimichthys roberti Blanco and Cavin, 2003, Robertichthys riograndensis Blanco–Piñón and Alvarado–Ortega, 2005b, and Rhynchodercetis regio Blanco and Alvarado–Ortega, 2006.

Millimeter–scale lamination, micritic matrix bearing rare to abundant bioclasts (planktic forams and calcispheres), the lack of bioturbation, and the occurrence goethite indicate that in Vallecillo the fossil fish strata were deposited under stagnation and deficient oxygen conditions, probably as a consequence of the Oceanic Anoxic Event (OAE) II occurred during the early Turonian (Blanco, 2003; Blanco et al., 2006; Blanco–Piñón et al., 2007).

MATERIAL AND METHODS

Anatomical abbreviations

The abbreviations follow Patterson and Rosen (1977) and Alvarado–Ortega (2004): alb: alveolar border; ang: angular; ao: antorbital; bpp: basipterygoid process on parasphenoid; boc: basioccipital; br: branchiostegal rays; bsc: basal sclerotic; c: ceratobranchial; cl: cleithrum; cor: coracoid; crp: coronoid process; d: dentary; dsp: dermosphenotic; e: epibranchial; epi: epioccipital; epn: epineural; etpa: ethmopalatine; fr: frontal; hym: hyomandibula; ic: intercalar; io: infraorbital; le: lateral ethmoid; mx: maxilla; n: nasal; na: neural arch or spine of preural centra; op: opercle; pa: parietal; pal: palatine; pcl: postcleithrum; pfr: pectoral fin rays; pmx: premaxilla; pop: preopercle; psp: parasphenoid; pto: pterotic; q: quadrate; r: ribs; rad: radials; rar: retroarticular; rode: rostrodermethmoid; s: symphysis of dentary; sbo: suborbital; sca: scapula; sc: sclerotic; scl: supracleithrum; smx: supramaxilla; sn: supraneural; soc: supraoccipital; stt: supratemporal; suo: supraorbital; tf: tooth foramen; vc: vertebral centra (numbered in anterior–posterior direction).

AMNH: American Museum of Natural History, New York, USA. DNPM: Departamento Nacional de Produção Mineral, Brazil. FCT: Facultad de Ciencias de la Tierra, Universidad Autónoma de Nuevo León, campus Linares, Mexico. IGM: Colección Nacional de Paleontología, Instituto de Geología, Universidad Nacional Autónoma de México. UERJ–PMB: Universidade do Estado do Rio de Janeiro. USNM: United States National Museum, Smithsonian Institute, USA.

Comparative material

The following specimens were reviewed for comparative proposes.

Cladocyclus gardneri Agassiz, 1841: AMNH 1841, AMNH 11877, AMNH 11992, UERJ–PMB 510, UERJ–PMB 520, and UERJ–PMB 531; Early Cretaceous (Aptian–Albian), Romualdo Member, Santana Formation, Ceará, Brazil.

Gillicus Hay, 1898: G. arcuatus (Cope, 1875), AMNH 8571; Late Cretaceous, Niobrara Formation, Kansas, USA.

Ichthyodectes ctenodon Cope, 1870: AMNH 2005, AMNH 2005; Late Cretaceous, Niobrara Formation, Kansas, USA.

Proportheus kameruni Jaekel, 1909: AMNH 8394; AMNH 6302, Early Cretaceous [Aptian or Albian (L. Taverne, pers. comm., 2007)], Bituminous shale of Rio San Benito, Spanish Guinea.

Prymnetes longiventer Cope, 1871, USNM 4090 (silicone peel deposited in IGM), Chiapas, Mexico, from unknown locality and age.

Saurodon Hays, 1830: IGM 6762 (Saurodon sp.), Late Cretaceous (Turonian), Eagle Ford Formation, La Mula Quarry, Coahuila, Mexico. AMNH 9907 (S. leanus), Late Cretaceous (Campanian), Niobrara Chalk, Montana, USA.

Unamichthys espinosai Alvarado–Ortega, 2004: IGM 8373 and IGM 8374, Early Cretaceous (Albian), Tlayúa Quarry, Middle Member of Tlayúa Formation, Puebla, Mexico.

Xiphactinus audax Leidy, 1870: AMNH 8547, AMNH 8574, and AMNH 1639; Late Cretaceous Niobrara Formation, Kansas, USA.

SYSTEMATIC PALEONTOLOGY

Subdivision Teleostei Müller, 1845
Order Ichthyodectiformes Bardack and Sprinkle, 1969
Suborder Ichthyodectoidei Romer, 1966
Family Ichthyodectidae Crook, 1892 ]]>

Vallecillichthys multivertebratum Blanco and Cavin, 2003

Holotype. FCT 079, holotype; articulated specimen lacking the posterior part of the body behind the dorsal fin (Figures 2a and 3).

Referred specimens. FCT 331; articulated specimen preserving the head and anterior abdominal area (Figure 2b). FCT 127, head and pectoral part of the trunk disarticulated (Figure 4). FCT 332, head and anterior part of the body (Figure 5a). FCT 196, articulated cranium (Figure 5b–d).

Occurrence. Early Turonian Marlstones of Agua Nueva Formation at Vallecillo quarries, near Vallecillo Village, Nuevo León, Mexico.

Preservation. All Vallecillichthys multivertebratum specimen have bones largely replaced by calcite crystals. Several boundaries between bones are obscured and their surfaces, including ornaments and sensory canal pores, are often total or partially destroyed. However, the following features are preserved.

Description.

General features. Vallecillichthys multivertebratum is a large and fusiform fish. Currently, because the lack of complete specimens, the total and standard lengths of this species remain unknown. However, other measurements are known from FCT 079 (Figure 2a), which represents the holotype and most complete specimen known for this species. The predorsal length is about 91 cm, the preanal and prepelvic length are 84 and 65 cm respectively, the head length including the opercle is 16 cm, and the maximum body depth is 13 cm. A maximum total length of 122 cm was estimated by Blanco and Cavin (2003). An objective estimation of this measurement is based on three observations: 1) The length of the head and all the abdominal vertebrae is 77 cm in FCT 079; 2) in the same specimen, the length known of the 15 most anterior caudal centra preserved is 14.7 cm (the mean centrum length is 0.98 cm); 3) in other ichthyodectiforms there are 20–32 caudal centra (Table 1). It is possible to calculate for this fish a probable length of the caudal section of vertebral column, which could range from 19 to 30 cm. This length of caudal region, added to the length of anterior part of the body shows that the standard length (SL) of this fish could range between 96 and 107 cm and its total length probably was 120–130 cm.

Skull. The supraoccipital is large and it bears a well high triangular crest, which is extended on the skull roof along all the postorbital area and overhangs the occiput. The maximum depth of the supraoccipital crest is contained two times in the neurocranium depth. The frontal is an elongated bone that covers the skull roof from the ethmoid area to back of the orbit. The parietals are located on the skull roof just behind the postorbital level, probably they are fused forming a mesoparietal and they have a small ascending section forming the anterior part of the supraoccipital crest. In lateral view the mesoparietal is extended laterally, reaching the epioccipital near to the middle part of the supraoccipital crest length (Figures 3 and 4).

The orbital section of the parasphenoid is a thin, long, straight bone. In FCT–079 its maximum length is about 6 cm whereas the maximum depth reaches 0.4 cm (Figure 4). The angle between the orbital and otic sections of the parasphenoid is about 150 degrees. At the end of the orbital section of this bone there is a rounded basipterigoid process directed anteroposteriorly.

The rostrodermethmoid is a complex bone having an anterior rounded border and a couple of lateral processes bordering the nasal capsule anteriorly (Figure 3). The joint between the rostrodermethmoid and frontals is not clear. In FCT 079, just behind the nasal capsule this bone seems to have sinuous zig–zag sutures attaching both frontals. In the floor of nasal capsule, sinuous bony outgrowings of the ethmopalatine sutures it with rostrodermethmoid and lateral ethmoid. It seems that fragments of a single nasal bone are preserved in FCT 079.

Circumorbital bones. Thin bones of the circumorbital series are not well preserved in the referred specimens but they probably form a complete and enclosing series around the orbit (Figure 3). An ovoid anterior supraorbital and posterior triangular dermosphenotic bones, which form the ventral part of this series, are almost equal in length. A large part of a nasal capsule ant lateral ethmoid bone is covered by an ovoid–rounded anterorbital. A rectangular infaorbital, slightly longer than deep, forms the posterior margin of the orbit covering large part of the skull base. A couple of well ossified and laminar sclerotic bones unite dorsal and ventrally involving a rounded sclerotic basal bone, which is strongly serrated anteriorly (Figures 3 and 4).

Jaws. The upper jaw includes a premaxilla, a maxilla and a pair of supramaxillae (Figures 3 and 4). The premaxilla is elongated dorsoventrally and irregular in shape, about twice as deep as long. Its straight alveolar border bears a row of at least six stout teeth, which are more or less regular in size. In FCT 079 the crown of the larger of these premaxillar teeth is approximately 0.4 cm long.

The maxilla is triangular, with its anterior border about twice as deep as its posterior border. Its length is about 4 times the depth of its anterior section. A couple of well developed dorsal articular processes are present in the anterior part of this bone. The alveolar border is straight and bears a row of teeth showing regular sizes, in which the heights of crowns are approximately 0.5 cm. A pair of supramaxillae is present above the posterior half of maxilla. The articulation between the maxilla and premaxilla involves their complete heights.

Lower jaw is rectangular, about three times longer than depth. The symphysis and coronoid process are almost equal in depth, its ventral border is slightly convex and its alveolar border is almost straight. The dentary is the largest bone in the lower jaw; it occupies about 80% of its length and bears a single row of teeth in which the crown height varies between 0.02 and 0.43 cm. In this bone, as well as in the maxilla, the alveoli are as deep as the crowns (Figure 4). The dentary, angular, articular, and retroarticular are not fused and the angular and articular participate on the articulation for the quadrate.

Hyomandibular series. Bones of the hyomandibular series are partially exposed. The hyomandibular is T–shaped. The head of the hyomandibular bone is a massive rectangular structure with slightly rounded margins. This bone articules the hyomandibular facet of the skull dorsally, the preopercle and the opercle posteriorly, the metapterygoid anteriorly, and with the quadrate ventrally. The vertical part of this bone is approximately 10 times deeper than long. The entopterygoid is a long rectangular bone that lies in contact with the ventral margin of the parasphenoid. It is approximately 10 times longer than deep. The ectopterygoid is attached to the endopterygoid dorsally, ventrally its shows a concave margin, and anteriorly it sutures with the palatine. The metapterygoid is rectangular and joins the endopterygoid dorsally, ectopterygoid anteriorly, quadrate ventrally, and hyomandibular posteriorly. The quadrate is an equilateral triangle with a well developed and rounded ventral condyle articulating with the lower jaw and a straight dorsal border. In general view, the palatine is triangular with a rectangular or malleolus head articulating with the maxilla ventrally and ethmopalatine dorsally.

Gill arch. In FCT 127 part of the gill arch is present showing some ceratobranchials and epibranchials (Figure 4). However the precise composition of the gill is not well observed.

Opercular series. Bones of the opercular series are also partially known. Opercle is D–shaped element, which seems to have entire borders although the dorsal part is not preserved (Figure 3). There are superficial parallel grooves radiating from the articular facet for the hyomandibula. The preopercle is inverted L–shape with vertical section twice larger than the horizontal one. The subopercle has convex ventral margin and is as large as the opercle.

The pelvic fin is only exposed in FCT–079. It is located below vertebrae 58–61. The distance between pelvic and pectoral fins is three times that one between pelvic and anal fins (Figure 2a). The triangular pelvic bone is directed up and forward. At least 6–8 wide saber–shape rays are preserved in this specimen.

Unpaired fins. Only the anterior part of anal fin and dorsal fin are preserved in FCT 079 (Figure 2a). It includes the anterior 12 pterygiophores and the base of 5 or 6 rays. The base of this fin begins below the vertebra 77, although the first and larger petrygiophore is located between the haemal spine of centrum 74 (which is the most anterior of caudal centra) and ribs on centrum 73.

The dorsal fin is short and its anterior margin lies above vertebrae 86–88. The rays are not preserved but at least 15 pterygiophores are present, the first and larger of them is located between neural spines of centra 82–83.

Vertebral column. The vertebral column forms a straight line from the posterior part of the skull to the anterior part of the dorsal fin (Fig. 1). In FCT–079, 90 vertebrae are preserved, all of them showing a couple of longitudinal pits. There are 73 abdominal centra, 6 of them obscured by the opercle and pectoral girdle. There are 69 pairs of large uniform ribs enclosing the abdominal area articulating to vertebrae trough out small parapophyses. Based on previous accountings of caudal centra on well known ichthyodectiform species (Bardack, 1965, Bardack and Sprinkle, 1969, Taverne, 1986; Silva–Santos, 1986, Maisey, 1991; Alvarado–Ortega, 2004, 2005; among others), in which their range between 22 and 34 centra (including both urals), it is possible estimate that Vallecillichthys multivetebratum could have at least 100 vertebrae. In lateral view, centra are square. Neural and haemal arches are autogenus. In the abdominal area there are small haemal arches. In the first three quarters of the abdomen, long epineurals are associated with the neural arches (which probably are fused each other) and at least three free epineurals join the posterior part of the skull. Additionally, all predorsal centra behind the occiput are associated with large free spatula–like supraneurals.

Scales. In Vallecillichthys multivertebratum the scales are ovoid, about twice as deep as long. Their external surface is ornamented with anterior punctae and posterior radii, as in the most derived ichthyodectiforms. Some scarce concentric circuli are also preserved near their borders.

DISCUSSION

The relationships of the order Ichthyodectiformes had been recently hypothesized by different authors (Taverne, 1986; Maisey, 1991; Stewart, 1999; Taverne and Chanet, 2000; among others). The actual state of the taxonomy of this teleostean group can by traced back to Romer (1966), who erected the suborder Ichthyodectoidei (into Osteoglossomorpha) to include the fossil forms allied to Ichthyodectes [fossil chirocentrids of Woodward (1919), Saint–Seine (1949), and Bardack (1965)] and the family Saurodontidae (Saurodon and Saurocephalus) erected by Cope (1870). Bardack and Sprinkle (1969) erected the Order Ichthyodectiformes including the Romer's Ichthyodectoidei as an incertae sedis group of basal teleosts, considering all non–saurodontid taxa within the family Ichthyodectidae [firstly named by Crook (1892)]. Patterson and Rosen (1977) erected the suborder Allothrissopoidei to include Allothrissops. Recently, Arratia et al. (2004) described Antarctithrissops as an allothrossopoid fish. Schaeffer and Patterson (1984) described Occithrissops but they were unable to resolve its relationships with Allothrissops (Allothrissopoidei) and Thrissops (Ichthyodectoidei). Taverne and Chanet (2000) suggest the suborder Occithrissopoidei to include Occithrissops. Actually the relationships among these three suborders are unresolved (Figure 6). Alvarado–Ortega (2004) reviewed the relationships of basal ichthyodectoids to include Unamichthys, based on the previous hypotheses suggested by Taverne (1986), Maisey (1991), Stewart (1999), and Taverne and Chanet (2000). However, today the relationships among derived ichthyodectoids also are unclear (Figure 6).

Vallecillichthys multivertebratum is well established as a member of the order Ichthyodectiformes. It has three of those five diagnostic characters of the order suggested by Patterson and Rosen (1977:115, also see Figure 6): 1) A pair of ethmopalatine bones in the floor of the nasal capsule (Figure 3); 2) anal and dorsal fin opposite and remote in the posterior part of the body (Figure 2); and 3) teeth in a single series in the jaws (Figures 3, 4). Other diagnostic characters are not observed in the available specimens of Vallecillichthys: [4) Six or seven uroneurals, the first three or four extending anteroventrally to cover the entire lateral surface of the first, second, or third preural centra; and 5) coracoids enlarged ventrally, meeting each other in a midventral coracoid symphysis].

Ichthyodectoid families already recognized are Saurodontidae Cope, 1970, Ichthyodectidae Crook, 1892, Cladocyclidae Maisey, 1991, and Unamichthyidae Alvarado–Ortega, 2004. Stewart (1999) considered that Ichthyodectidae is an unnatural group, as well as Cladocyclidae according to this author and Alvarado–Ortega (1998, 2004). Unamichthyidae, a monogeneric family characterized by the presence of parietals unfused and a deep mandibular symphysis, is the sister group of all other Cretaceous ichthyodectoids, including Vallecillichthys, that have fused parietals (character 22, Figure 6). Against this background, only the monophyletic nature of Saurodontidae has not been refuted.

The phylogenetic hypotheses of the Ichthyodectiformes include a great number of characters from caudal skeleton. Unfortunately the caudal skeleton of Vallecillichthys is not known; nevertheless, its possible phylogenetic position can be drawn up based on data from its head and body. Vallecillichthys presents characters 12, 13, 18, 19, 20, 22, 25–27, 29 (≈ 35), 48 and 49 (Appendix and Figure 6) showing that it is close related to Xiphactinus and Ichthyodectes. Some aspects of these characters are discussed next.

Vallecillichthys and the rest of Cretaceous ichthyodectoids have a relatively deep dentary symphysis (character 12 in Figure 6, Figure 7). This character altogether with the presence of a relatively reduced slightly elevated coronoid process causes a lower jaw with a rectangular aspect, where alveolar and ventral borders are almost parallel. In contrast, in Jurassic ichthyodectiforms the lower jaw is triangular because in these fishes the dentary has a relatively less deep symphysis and a wider and deeper coronoid process (Figure 7). A poorly understood character is the "angled coronoid process", that was suggested by Taverne (1986) and repeated by Maisey (1991) to separate basal ichthyodectoids from Ichthyodectidae plus Saurodontidae (character 27, Appendix and Figure 6). This Taverne´s character is indicating the reduction of the coronoid process described above and it must be located within node A of Figure 6.

Vallecillichthyys as other Cretaceous ichthyodectoids, has a deep maxilla–premaxilla attachment (character 13, Appendix and Figure 6), in where the maxilla depth is uniform and the articular regions of both bones have the same depth. In contrast, in Jurassic ichthyodectiforms the articular region of the maxilla is reduced and its articulation with the premaxilla is weak.

Taverne and Chanet (2000) suggested the occurrence of rays wide or with saber shape in first positions of pectoral and pelvic fin as a distinctive character of Cretaceous ichthyodectiforms (character 18, Appendix and Figure 6). However, Thrissops and Occithrissops, as well as in Unamichthys also have this characteristic wide, strong, and saber rays (see Nybelin, 1964; Schaeffer and Patterson, 1984, fig. 23A; Tischlinger, 1998; and Alvarado–Ortega, 2004). Therefore, it seems that this character has a wider distribution among ichthyodectiforms but it must be analyzed in other Jurassic forms.

In Vallecillichthys as in other Cretaceous ichthyodectiforms where the mandibular facet for quadrate is known, the angular participates in this (character 19, Appendix, Figure 6). The exclusion of the angular from this facet is only documented in Allothrissops, which facet involves the articular and retroarticular (Patterson and Rosen, 1977:101). Considering the condition found in Allothrissops, it is possible to suggest the exclusion of the retroarticular as a derived character (20, Appendix, Figure 6). Unfortunately, the composition of this facet is documented neither in other Jurassic ichthyodectiforms nor in most of the Cretaceous forms. In Unamichthys, the retroarticular is excluded from such facet as it was documented for Ichthyodectes, Gillicus, Xiphactinus, Saurodon and Prosaurodon (Bardack, 1965; Stewart, 1999; among others). In this scenario, the participation of the retroarticular in the lower jaw articulation of Cladocyclus must be considered as an apomorphic condition acquired after the inclusion of the angular in this facet as seen in Unamichthys (Alvarado–Ortega, 2004).

Unamichthys, Thrissops, Allothrissops, and Occithrissops and probably Antarctithrissops have parietals unfused. A monophyletic group is suggested by the fusion of the parietals, a condition considered as derived that is well documented in Eubiodectes, Chirocentrites vexillifer, Cladocyclus, Prosaurodon, Saurodon, Saurocephalus, Gillicus, Ichthyodectes, and Xiphactinus, as well Vallecillichthys (character 22, Appendix, node B in Figure 6).

After the analysis of the position of the pelvic fin among ichthyodectiforms, Alvarado–Ortega (2004) concluded that Cladocyclidae [which includes Cladocyclus, Chirocentrites, Chiromystus, and Proportheus as synonymous with Cladocyclus] is an unnatural group. According to the first author, all Jurassic ichthyodectiforms plus Unamichthys and Eubiodectes, as well the poorly known Chiromystus and Proportheus, have pelvic fins located just in the middle of the standard length. A derived condition is present in Cladocyclus, Chirocentrites vexillifer, Ichthyodectes, Gillicus, and Xiphactinus, Saurodon (documented in S. elongatus by Taverne and Brozni, 1999), as well as Cooyoo and Vallecillichthys (Figure 2), in which the pelvic fin is located behind the middle SL, closer to the anal fin base (character 25, Appendix, Figure 6).

According to Alvarado–Ortega (2004), Cladocyclus, as well as genera before considered as ichthyodectids and saurodontids, has anterior radii and posterior fine pitting covering the external surface of scales (see Bardack, 1965), similar to those found in Vallecillichthys (character 26, Appendix, Figure 6). In contrast, a pattern of numerous concentric circuli on the scales is present in other ichthyodectiforms.

The angle formed between the orbital and otic sections of the parasphenoid ranges between 149° and 160° in Xiphactinus and Ichthyodectes (Bardack, 1965; Taverne, 1986; Maisey, 1991), as well as in Cooyoo and Faugichthys (Lees and Bartholomai, 1987; Taverne and Chanet, 2000). The parasphenoid of Vallecillichthys is also broad and it is about 150° (character 48, Appendix, Figures 4, 6, 9). In contrast, other ichthyodectiforms where this angle is documented, it is about 130° (Figure 9).

Xiphactinus, Ichthyodectes, and Vallecillichthys share the presence of a hyomandibular facet parallel to the orbital part of the parasphenoid (character 49, Appendix, Figures 4, 6, 9). In contrast, the longitudinal axis of this facet is projected downward crossing with the ethmoid area of the skull (Figure 9) in Allothrissops, Occithrissops, Antarctithrissops, Thrissops, Cladocyclus, Chiromystus, Chirocentrites vexillifer, Saurodon, Gillicus, Unamichthys, Cooyoo, and Faugichthys (see descriptions and illustrations in Silva–Santos, 1949; Lees and Bartholomai, 1987; Taverne, 1977; Schaeffer and Patterson, 1984; Taverne and Chanet, 2000; Arratia et al., 2004; Alvarado–Ortega, 2004; among others).

On the basis of the characters discussed in two prior paragraphs, it is clear that Vallecillichthys forms a monophyletic group with Xiphactinus and Ichthyodectes. This group represents a new sense of the family Ichthyodectidae (Figure 6). These characters were excluded from the analysis performed by Stewart (1999), who only argued on the inexistence of those characters that support the inclusion of Gillicus, Ichthyodectes, and Xiphactinus into the family Ichthyodectidae sensu Maisey (1991, characters 41–43, Appendix, Figure 6), disabling therefore the recognition of the monoplyletic group including the latter two genera. Taverne and Chanet (2000) named this same group as Ichthyodectinae suggesting that it is supported additionally by the presence of a very deep dentary symphysis and a preopercle with a wide dorsal section bearing a broad notch (characters 44 and 45 respectively, Appendix, Figure 6). The lower jaw shape of Xiphactinus and Ichthyodectes is similar to that one of Cladocyclus and Unamichthys (see Bardack, 1965; Patterson and Rosen, 1977; and Alvarado–Ortega, 2004). This leaves the dentary symphysis hypertrophed as an exclusive character of Gillicus (Figure 7). The other Tavernes´s character (45) is not well documented and a comparative study of the preopercle on ichthyodectiformes is required to confirm this suggested diagnostic difference.

Notes about the possible relationships of Gillicus

Some things are remarkable in relation to those characters that support the inclusion of Gillicus and saurodontids within an unnamed group suggested by Stewart (1999) (see characters, 50–52, node I, Figure 6, and Appendix). This author suggested that the quadrate head lies ahead of the orbit in these fishes (character 50); this could be true for saurodontids but, following the illustration of Bardack (1965, pl.2, fig. A), it is not for Gillicus. Also, this author noted that in dorsal view the rostrodermethmoid shows a slight constriction behind of its anterior expansion in both, Gillicus and saurodontids (character 51); whereas in other ichthyodectiforms this constriction is conspicuous. The review of the rostrodermethmoid shape in dorsal view shows that in Gillicus it is similar to that star–shape found in Cladocyclus, Xiphactinus and Ichthyodectes, which is different to the triangular shape seen in Saurodon and Prosaurodon (Figure 9). This result suggest that the star–shape of the rostrodermethmoid (in dorsal view) probably is present in basal ichthyodectoids and that the triangular shape does not represent a synapomorphy of Gillicus + Saurodontidae (see characters 51, Figure 6, Appendix).

Additionally, Stewart (1999:357–358, fig. 10) found that the quadrate condyle shows a simple articular surface in Gillicus, Prosaurodon, Saurodon, and Saurocephalus (character 52, Appendix, Figure 6); whereas in Ichthyodectes and Xiphactinus this surface is complex because it shows an anterior lateral projection. Unfortunately, Stewart (1999) did not recognized the derived condition between these two shapes, suggesting that one present in Gillicus and his saurodontids could be a synapomorphy. Acid–prepared Cladocyclus specimens study by Leal (2003) show the presence of a simple articular surface of the quadrate condyle showing that it is the plesiomorphic condition and the complex articular surface is the derived one (53, Appendix, Figure 6). The character is unknown in Vallecillichthys but it could show a complex a articular surface of the quadrate condyle providing additional support to the monophyly of our Ichthyodectidae family.

CONCLUSIONS

The present description of Vallecillichthys multivertebratum, a Mexican ichthyodectiform fish erected by Blanco and Cavin (2003), provides data for its comparative study into the suborder Ichthyodectoidei. A comprehensive phylogenetic study of this order is desirable; however, today there are a large number of ichthyodectiform species that require a re–description [i.e., Chiromystus species, Chirocentrites species, Proporteus kameruni, among others] and ongoing studies deal with recently discovered forms that represent new species [i.e., one basal ichthyodectoid from Brazil (Alvarado–Ortega and Brito, submitted); new Mexican ichthyodectiforms found in Cretaceous sedimentary outcroups in the states of Puebla, Hidalgo, Chiapas, Coahuila, and San Luis Potosí (Alvarado–Ortega, 2004, 2005; Alvarado–Ortega et al., 2006c; Ovalles–Damián et al., 2006)].

Contrary to Blanco and Cavin (2003), Vallecillichthys is placed with Ichthyodectes and Xiphactinus into the family Ichthyodectidae. The emended diagnosis of this family includes the participation of parietals (fused forming a mesoparietal) in the supraoccipital crest (not well documented in Vallecillichthys); quadrate condyle with an articular surface having a lateral anterior projection (illustrated by Stewart, 1999, fig. 10 E–F; unknown in Vallecillichthys); broad parasphenoid angle (ranging between 149–160°); and hyomandibular facet parallel to orbital part of parasphenoid.

ACKNOWLEDGEMENTS

The authors are deeply grateful to Luis Espinosa–Arrubarrena, Louis Taverne, and David R. Schwimmer for the critical revision of the present manuscript. Volker Griener made all the photographs, Louis Taverne for sending his articles. A.B.P. was supported by the project PROMEP/103.5/05/367 (Mexico) and J.A.O. was supported by a Postdoctoral fellowship Processo N° 151115/2005–2 from CNPq (Brazil).

APPENDIX

1. A pair of ethmopalatine bones in the floor of the nasal capsule.

2. Anal and dorsal fin opposite and remote in the posterior part of the body.

3. Teeth in a single series in the jaws.

4. Six or seven uroneurals, the first three or four extending anteroventrally to cover the entire lateral surface of the first, second, or third preural centra.

5. Coracoids enlarged ventrally, meeting each other in a midventral coracoid symphysis.

6. Well ossified ethmopalatine, which has membranous outgrowths separating and suturing the rostrodermethmoid and lateral ethmoid.

7. High and triangular supraoccipital crest.

8. asal sclerotic bone with serrate margin.

9. alatine head modified into a malleolus.

11. Relatively elongate parietal and epioccipital bones with their anterior borders located near to the posterior orbital border.

12. Deep dentary symphysis.

13. Deep premaxilla–maxilla attachment.

14. Five uroneurals.

15. First uroneurals relatively short, reaching preural centra 2 or 3.

16. Absence of urodermals.

17. Sensitive pit openings in parietals are lacking or reduced.

18. Wide first pectoral and pelvic fin rays showing saber shape.

19. Angular participates in the mandibular facet for quadrate.

21. Presence of a notch on the upper posterior border of preopercle (the Taverne's notch) (also see character 45).

22. Parietals fused each other.

23. Two epurals.

24. One epural.

25. Pelvic fin located behind the middle part of the body (the distance pectoral–pelvic fins is higher than two times the distance pelvic–anal fins).

26. External surface of scales having anterior radii and posterior fine pitting.

27. Angled coronoid process.

28. Ball–and–Socket attachment between hypurals 1 and 2 and ural 1.

29. High vertebral number [approximately 70–100 or more (see Taverne and Bronzi, 1999)].

31. Anterior ceratohyal with narrow fenestra.

32. Parietals participates in the supraoccipital crest.

33. Predentary bone.

34. Internal surface of maxilla, premaxilla and dentary bones with a foramen on (or notch) at the base of each tooth.

35. Near to 100 total vertebrae.

36. Sigmoid dorsal margin of quadrate.

37. Teeth with carinae.

38. Long dentary bone.

39. Deep maxilla without a notch at rear of the palatine facet.

41. Anteriorly extended epioccipitals.

42. Subdivided nasal bone.

43. Short remote anal fin.

44. Very deep dentary symphysis.

45. The widened dorsal end of preopercle with a broad notch.

46. Epioccipials and suparoccipital of equal forward extent.

47. Broad parasphenoid angle.

48. Hyomandibular facet parallel to orbital part of parasphenoid.

49. Enlarged premaxillar fangs.

51. Rostrodermethmoid with slight constriction behind of its anterior expansion (visible in dorsal view) (Figure 9).

52. Quadrate condyle with an articular surface simple (see Stewart, 1999: 357–358, fig. 10 A–D).

53. Quadrate condyle with an articular surface having a lateral anterior projection (see Stewart, 1999: 357–358, fig. 10 E–F).

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