Thursday 30 July 2020

Vegrandichthys coitecus: A new species of Enchodontid Fish from the Early Cretaceous of Chiapas State, southeastern Mexico

The family Enchodontidae is a monophyletic group of extinct Cretaceous Teleost Fish, which show a great morphological disparity in the shape of the trunk and snout, and range from between a few centimeters to over one and half meters in length. These active and fast predatory Fish inhabited the mid-trophic level and were important components in the trophic chain in the Cretaceous seas, evidenced by their relatively common occurrence as well-preserved gut contents in larger Fish and other marine predators such as Cephalopods and Plesiosaurs.

In a paper published in the journal Palaeontologica Electronica on 4 June 2020, Jesús Alberto Díaz-Cruz of the Unidad de Posgrado at the Universidad Nacional Autónoma de México, Jesús Alvarado-Ortega of the Instituto de Geología at the Universidad Nacional Autónoma de México, and Sam Giles of the Department of Earth Sciences at the University of Oxford, and the School of Geography, Earth and Environmental Sciences at the University of Birmingham, describe a new genus and species of enchodontid based on a single well-preserved specimen recovered from the early Cenomanian strata of the El Chango Quarry, Chiapas, southern Mexico.

The Enchodontids have been known since the first half of the nineteenth century; however, their comprehensive study only began in the second half of the 20th century. Prior the present, seven enchodontid genera have been identified; these were inhabitants of marine shallow environments in the peripheral and epicontinental seas of northern Africa (Egypt and Morocco), America (Argentina, Brazil, Peru, Mexico, and the United States), Asia (Lebanon, Israel, Jordan and Japan), and Europe (Netherlands, Germany, United Kingdom, Greece, Italy).

The fossil record of Enchodontids is noticeably rich and diverse along the Tethys Sea, mainly in its North American and Middle Eastern realms. Two areas of Cenomanian Enchodontid endemism have been identified in this region, one located in North Africa and the other in the Middle East. It has been claimed that the biogeography of Enchodontids was dictated, in part, by vicariant events; the origin of Enchodontids is attributed to the central-western region of the Tethys Sea. However, recent studies have revealed intriguing data that contrasts with such an idea, specifically the presence of early-diverging enchodontid taxa in Mexico.

Although North American enchodontids have been long studied by numerous authors and their diversity from numerous sites in the United States of America and Canada is well known, Mexico has recently become a very important territory to study these Fish because they have been extracted in numerous paleontological sites ranging from Albian to Maastrichtian age. The first report of Enchodontids from Mexico dates back to the 1950s, when palaeontologists found remains of these Fish in Turonian limestones exploited in Xilitla, San Luis Potosí, and inside a rocky core drilled near San José de las Rusias, Tamaulipas. In addition, these Fish have been reported from other sites including the Vallecillo quarry (Nuevo León), the Arroyo Las Bocas (Guerrero), the San Jose de Gracia quarry (Puebla), as well as the Las Boquillas, La Mula, Los Pilotes, and Venustiano Carranza quarries (Coahuila), the El Chango quarry and Tzimol (Chiapas), and the Muhi quarry (Hidalgo). The nominal species of Mexican Enchodontids already described are noticeable, because they represent the most ancient record of this group in America, usually include relatively complete and well-preserved fossils, and are among the oldest Enchodontids so far known from the Middle East and North Africa regions. These Mexican Enchodontids are Enchodus zimapanensis, from the Albian-Cenomanian deposits of the Muhi quarry, Hidalgo; Unicachichthys multidentata, as well as Veridagon avendanoi, from the early Cenomanian deposits of the El Chango Quarry, Chiapas. Today, a great quantity of partially described specimens from Mexico is deposited in the palaeontological collections of this country, waiting to be accurately studied.

Enchodontid species from the El Chango Quarry, Chiapas, Mexico; (A) IHNFG-2987, holotype of Unicachichthys multidentata; (B) IHNFG-5816, holotype and single specimen of Veridagon avendanoi. Scale bar equals 1 cm. Díaz-Cruz et al. (2020).

El Chango Quarry is a small outcrop of laminated limestones and dolomites with sporadic flint nodules located at 16º34’14.9” N and 93º16’12.7” W, within Ocozocoautla de Espinosa Municipality, and about 25 km to the southwestern of Tuxtla Gutiérrez in Chiapas State, Mexico. These strata were originally thought to have accumulated in the bottom of an estuarine or salty lagoon with ephemeral freshwater influx during the late Albian; however, subsequent studies have suggested they are Cenomanian, and probably early Cenomanian in age.

Geological map and geographical localization of the El Chango Quarry, Chiapas, Mexico. Díaz-Cruz et al. (2020).

This quarry was opened in 2004 by members of the scientific staff of the Museo de Paleontología 'Eliseo Palacios Aguilera'. Since then, this institution has been in charge of collecting, studying, and preserving the fossils from this Lagerstätte. Today, the fossil collection from the El Chango housed in this museum contains more than 400 fossils, including Molluscs, Crustaceans, Ammonites, Insects, Plants, and Fish. Fish are the best-preserved fossils at the El Chango; these are tightly laterally compressed, usually complete and articulated, and often show parts of soft tissues (muscles) and phosphatised stomach contents.

The specimen studied by Diaz-Cruz et al. was transferred to polyester resin. The bulk of the rocky matrix was mechanically removed using fine pneumatic air scribe; the fossil bones were then totally released from the remaining matrix, with the limestone dissolved through the application of intermittent baths in an acetic acid aqueous solution at 5% combined with baths in clean water. Pin vises and needles were used to remove the remaining rock residues from the fossil. The fossil was hardened with a light dissolution of plexigum in ethyl acetate applied with a fine brush.

The specimen is placed in the Subfamily Eurypholinae. The species included in this subfamily have slightly to very long bodies as well as slightly to very large snouts; ventral portion of the cleithrum widens anteriorly and posteriorly; cleithrum ornamented with tubercles; ventral border of the mandible straight up to the mandibular symphysis; trunk naked except for the predorsal scute series extended between the dorsal fin base and the occiput and a single lateral scale series that encloses the lateral line. It is named Vegrandichthys coitecus, where 'Vegrandichthys' is a combination of the Latin adjective ‘vegrandis’ that means small, and the Greek suffix ‘ichthys’ meaning Fish, and 'coitecus' in reference to the word ‘coiteco’, which is the local given name to the people from Ocozocoautla de Espinosa Municipality.

IHNFG 5927, the holotype and only known specimen of Vegrandichthys coitecus, is a gracile, small, and almost complete specimen, in which the entire caudal fin and postanal preural vertebrae are missing. The standard length in this fish is unknown; however, this feature can be estimated as the length of the preserved part in this fish is 50 mm and the postanal region of the trunk ranges between a fifth to a seventh of the standard length in other members of Eurypholinae, Eurypholis, and Saurorhamphus. Based on these data, the estimated standard length of IHNFG 5927 is between 59 and 61 mm.

(A) Lateral view of Vegrandichthys coitecus, IHNFG 5927 holotype transferred in plastic resin from Early Cenomanian deposits of the El Chango quarry, Chiapas, Mexico. (B) Artistic reconstruction of Vegrandichthys coitecus. Diaz-Cruz et al. (2020).

The head length of Vegrandichthys coitecus is 19.6 mm and probably represents 32 to 33% of standard length. The orbital and postorbital regions of the skull show the same length and together represent 40% of the skull length; the remaining 60% corresponds to the snout or preorbital region.

The body of IHNFG 5927 is shallow and fusiform. The head height is relatively shallow, representing a little less than 40% of the head length and is slightly less than the maximum body height that is present in the predorsal region of the trunk. The dorsal fin is short and placed in the posterior half of the abdominal cavity; the dorsal fin length and the predorsal length represent 25% and 150% of head length, respectively. The posterior end of the anal fin is unknown; however, it is longer than the dorsal fin and is located behind it. The anal fin base is at least 5.5 mm (28% of head length) and rises at 203% of head length. The pectoral fin is in the lateral surface of the trunk, a little closer to the abdominal edge than to the vertebral column. The small pelvic fin is in ventral position and located in
the predorsal region of the trunk, at 136% of the head length. Behind the dorsal fin, the trunk progressively tapers and probably behind the anal fin forms a shallow caudal peduncle.

The skull is triangular, anteriorly tapered, and about three times longer than high Although the contact of the mesethmoid with the anterior tips of the frontals is covered by the anterior dorsal ends of the premaxilla, the frontals are likely at least three-quarters the length of the skull roof. The left frontal is dorsally exposed showing its flat and triangular shape, anteriorly pointed, slightly wider behind the orbit, and rectangular in the postorbital region. Behind the orbit, each frontal is firmly sutured with four bones; laterally with the sphenotic and pterotic and posteriorly with the supraoccipital and parietal. The supraorbital sensory canal runs along the frontal, near its lateral edge, enclosed above the orbit and exposed in its preorbital region. The frontals are superficially smooth along the preorbital region; in contrast, behind the orbit the frontals and other bones are strongly ornamented with small tubercles. The parietals are short triangular bones at the back of the skull roof, expanded laterally, and medially separated by a small supraoccipital bone.

Vegrandichthys coitecus. (A) A close-up of the lateral view of the head; (B) Drawing of the head and pectoral girdle in lateral view. Abbreviations: aa, anguloarticular; cl, cleithrum; den, dentary; df, dilatator fossa; dpl, dermopalatine, dpl.t, dermopaltine teeth; ect, ectopterygoid; en, endopterygoid; epo, epiotic; fr, frontals; hy, hyomandibula; io, infraorbital; iosc, infraorbital sensory canal; la, lacrimal; le, lateral ethmoid; mpt, metapterygoid; msc, mandibular sensory canal; mx, maxilla; op, opercle; pa, parietals; pmf, premaxillar fenestra; pmx, premaxilla; pop, preopercle; ps, parasphenoid; pte, pterotic; ptt, posttemporal; q, quadrate; rar, retroarticular; sc, sclerotic; scl, supracleithrum; soc, supraoccipital; sop, subopercle; sp, sphenotic. Diaz-Cruz et al. (2020).

The sphenotic is a small and triangular bone bordering the dorsoposterior orbital region; the pterotic is an elongated rectangular bone that reaches the back of the skull. The posttemporal fossa is a small depression in the region where the frontal, pterotic, and sphenotic converge. The dilator fossa is a shallow elongated depression laterally extended along the posterior two thirds of the pterotic and roofed by the sphenotic. The parasphenoid is an elongated laminar and straight bar extended along the orbital and the ethmoid region of the skull, with a flat and smooth lateral surface bearing numerous micro-serrations along the ventral edge of its posterior half.

None of the bones of the posterior region of the skull are exposed except for the epiotic, which seems to be short and wide. Much of the lateral otic region of the skull is covered by the hyomandibula. In the preorbital region, the lateral ethmoid is a thick triangular and somewhat curved bone that separates the nasal capsule from the orbit, it sutures with the ventral inner surface of the frontal, and ventrally projects below the parasphenoid. The anterior edge of the nasal capsule is covered by the lateral process of the mesethmoid bone, which is an expanded triangular and flat structure that meets the lateral anterior end of the frontal and is covered by the dorsal edge of the premaxilla.

The circumorbital bones are poorly preserved. This is an open series, with no dorsal bones. Remains of flat, rectangular, and flimsy infraorbital bones from the left and right sides of the head are preserved below the nasal capsule and the middle orbit. All these bones show the infraorbital sensory canal running near to their respective orbital edges, as well as few ventral branches of this canal. A large semicircular and flat basal sclerotic bone occupies the orbit.

Bones of the suspensorium or hyomandibular series are only partially exposed. The hyomandibula is a large T-shaped bone in which the descending process is curved forward and the head is broad with a sinuous articular surface. The central region of this bone is reinforced with apicobasal ridges.

The quadrate is a laminar triangular bone with a small stout ventral head that is inclined forward. The symplectic and the joint between the ventral tip of the hyomandibular and the quadrate are obscured. The ectopterygoid is a laminar and toothless bone that is only partially exposed; its anterior region lies below the lower jaw while its spatula-like posterior part is tilted backward to meet with the quadrate anterior edge. The metapterygoid is laminar and poorly preserved, contacting the concave anterior margin of the hyomandibula and the dorsal edge of the quadrate. Only fragmentary, laminar, and toothless remains of the endopterygoid are preserved.

Although the dermopalatine is entirely covered by the premaxilla, a large, robust dermopalatine tooth is visible. This tooth seems to be placed at the anterior end of the bone, has a stout base, and a conical crown slightly curved backward and projected downward. The dermopalatine appears to bear multiple teeth, but the smaller posterior teeth are difficult to distinguish with precision.

The upper jaw consists of two bones, the premaxilla and maxilla. There is no indication of a supramaxilla. The premaxilla is a flat triangular bone, pointed posteriorly, and extends for three quarters of the jaw. This bone has a single row of small, conical, sharp teeth, which are of similar size and evenly separated, except for a series of progressively smaller teeth along the posterior quarter of the bone. The anteriormost maxillary tooth is peculiar because it is slightly larger and rather sigmoidal. The dorsal ends of both maxillae are bent inwards to form the roof of the anterior half of the snout. A small midline fenestra, near the tip of the snout, presumably accommodated two teeth from the lower jaws. The outer surface of the premaxilla is intensely ornamented with small tubercles. Fragments of the maxilla reveal that this toothless, smooth, and rod-like bone is extended below the orbit.

Three bones of the lower jaw are exposed in IHNFG 5927: dentary, anguloarticular, and retroarticular In lateral view, the lower jaw is elongate, extending below the orbit and the anterior half of the otic region of skull. The first two fifths of its length are shallow, and it becomes progressively deeper more posteriorly: its deepest point is four times the depth of its shallowest point. Both the alveolar and ventral border are somewhat sinuous, the symphysis is very shallow, and the coronoid process is hardly recognizable. At the base of its posterior edge, the jaw has a small posteriorly-directed postarticular process. The mandibular sensory canal runs close to the ventral edge of the lower jaw. It is enclosed by bone, and opens through six pores scattered along the dentary and the anguloarticular bones. The entire labial surface of this jaw is ornamented with numerous tubercles.

The dentary is a triangular bone, occupies a large part of the lower jaw, and has a posterior deep and acute concavity to suture tightly with the anguloarticular bone. The alveolar border of the dentary occupies about two thirds of the jaw and bears two rows of teeth. The labial tooth row includes conical teeth in two sets: groups of one to three small and gracile conical teeth intercalated with larger isolated teeth with thicker bases. The lingual dentary tooth row consists of at least four sharp laterally compressed teeth exposed in the middle of the alveolar border, which resemble the longer teeth on the labial teeth row but they are thinner. Of the labial dentary teeth, the second isolated tooth is so developed that, in life, its tip must have penetrated the premaxilla through its dental fenestra. Since the specimen has single premaxillary dental fenestra, it is possible that the anterior ends of both dentaries were either very close or met in a long symphysial contact.

Although the ventral anterior end of the dentary does not have osseous beards or dentary prongs (cf those of other Enchodontids: Enchodus, Unicachichthys, and Veridagon), it does display a series of conspicuous bony lumps. At this point, it is not possible to decide whether these osseous lumps are a preservation artifact, an analogy, or a homology of the dentary prongs of other specimens. In the absence of additional fossils, the authors consider that Vegrandichthys coitecus lacks dentary prongs.

The anguloarticular occupies the posterior 40% of the lower jaw. This triangular bone bears the postarticular process in which the small and shallow articular facet for the quadrate is exposed in lateral or labio-lingual view. A tiny wedge-shaped retroarticular covers the posteroventral corner of the postarticular process.

This series consists of three laminar bones, opercle, subopercle, and preopercle; the infraopercle is absent. The opercle is semicircular, about twice higher than long, and anteriorly straight and thick. An articular facet for the hyomandibular is borne near its anterior edge about a third of the way down. Seven thickened bars are present on the opercle, laterally exposed as longitudinal straight ridges; the central ridge is the most conspicuous. As the posterior edge of the opercle is very thin, it is not clear whether the bars form true spines or not. The lower third of the opercle is superficially ornamented with tubercles.

The preopercle is a triangular bone, about 2.5 times higher than long, dorsally pointed, ventrally convex, and expanded anteriorly and posteriorly. The anterior edge is thickened, slightly convex, and its base is expanded anteriorly; in contrast, the posterior edge of the preopercle is rather laminar, convex, bears small serrations, and its base forms a posterior thick and conspicuous spine. The preopercle is smooth except for small tubercles borne on its ventroposterior limb. The preopercular sensory canal is enclosed by bone and opens externally through two or three pores present in the ventroanterior preopercle limb.

Although the subopercle is poorly preserved and largely covered by the opercle, some remains of this laminar bone are preserved behind the preopercle and below the opercle. Apparently, the subopercle is ornamented with small tubercles.

No element of the branchiostegal rays or the gill arches are exposed.

Unfortunately, the specimen does not preserve the posterior part of the body; however, a large part of the vertebral column is exposed and consists of at least 30 total vertebrae, including 21 abdominal and at least nine caudal. All centra are hourglassshaped bones and slightly constrained in the center. The centra in the middle of the body are about two times longer than high, while those placed in front and back are slightly and progressively shorter, and become about 1.5 times longer than high. Laterally, a couple of thick longitudinal ridges reinforce the centra. Along the vertebral column, the centra have no parapophyses or zygapophyses, and each centrum is fused with its respective dorsal and haemal arches and spines. The haemal and neural spines are uniformly thick, straight and inclined backward. In the abdominal centra, ribs are present as bars with a small articular head and a sharp distal tip.

Close-up of different anatomical structures in the holotype of Vegrandichthys coitecus. (A) Pectoral girdle and fin, (B) Pelvic girdle and fin, (C) Dorsal fin and (D) Anal fin. All anterior structures are laterally exposed. Abbreviations: cl, cleithrum; co, coracoid; dpt.a, anal fin distal pterygiophores; dra, dorsal fin rays; lw, lateral wing; pb, pelvic bone; pfr, pectoral fin rays; pp, posterior process; ppt.a, anal fin proximal pterygiophores; ppt, proximal pterygiophores; pr, pelvic rays; ra.a, anal fin rays; scl, supracleithrum; v, vertebra. Diaz-Cruz et al. (2020).

At least two supraneurals are present below the predorsal scutes and between the occiput and the dorsal fin rays. These spatula-like bones have a differentiated dorsal head that is anteriorly and posteriorly expanded, as well as an anteriorly directed elongated and straight ventral limb. Along the abdominal centra there are threadlike epineurals inclined backward that probably extended along three or four centra from the lateral surfaces of the neural arches. Remains of posteriorly directed thread-like epipleurals are present along the abdominal region.

Scutes, scales and associated bones of Vegrandichthys coitecus. (A) Dorsal scutes series, (B) close-up of the dorsal scutes region, (C) close-up of the lateral line of scales, and D) drawing of the probable position and overlapping of the lateral line of scales. Abbreviations. llc, lateral lines canal;, lateral line scales; v, vertebrae. Ordinal numbers denote the number of dorsal scute. Black arrows point out each of the dorsal scutes. Red arrows point out the supraneurals. Diaz-Cruz et al. (2020).

Pectoral girdle bones are relatively poorly preserved or obscured. Only fragments of the left posttemporal bone are present behind the skull of the specimen. Most of the supracleithrum is covered by the opercle; only its dorsal end is exposed. The cleithrum is an inverted T-shaped bone in which the dorsal and anterior limbs are not completely observed. The posterior limb of the cleithrum forms a large spiny ventral process that projects posteriorly, and its longitudinal axis is reinforced by a conspicuous ridge. The cleithrum seems to be a smooth bone except for the posterior spine that is ornamented with small tubercles. No postcleitra are preserved.

The left pectoral fin is located above the ventral posterior spine of the cleithrum. This fin consists of six distally branched and segmented rays. Only fragments of the coracoid are preserved. The length of the pectoral fin equals four and a half abdominal vertebrae.

The pelvic fin is set on the anterior half of the abdomen on its ventral border and rises below the abdominal centrum 10. The length of the pelvic fin hardly equals that of two abdominal centra. This fin consists of five distally segmented and branched rays, which are supported on the posterior edge of the pelvic bone. This triangular bone has a laminated triangular lateral wing and a stout and short posterior process.

The dorsal fin is contained within the posterior half of the abdominal cavity. It consists of 12 distally branched and segmented rays supported by 11 stick-like proximal pterygiophores. This fin is rounded because the length of those rays in the middle of the fin equals the length of the base while those placed on the front and back are progressively shorter. The base of this fin is extended above the last six abdominal centrae (15-21). The dorsal fin rays are located close to the body because of the lack of distal pterygiophores.

The anal fin originates posteriorly to the dorsal fin. The posterior edge is not preserved, but there are 13 longitudinally grooved and distally segmented rays supported by 13 rod-like and straight proximal pterygiophores. In the posterior two thirds of the anal fin, there are noticeably elongated distal pterygiophores causing the separation of the associated anal rays. Although the anterior anal fin rays are fragmented; it seems that the fin was roughly triangular in shape. The anal fin extends below the vertebrae 25 to 31; however, its first three proximal pterygiophores are projected into the space between the haemal spines of the first caudal centra (22 and 23).

The caudal fin is not preserved.

The body is almost entirely naked. However, the predorsal trunk edge bears a row of four predorsal scutes. The lateral line is enclosed by a single row of modified scales exposed along each body flank.

The predorsal scutes are ovoid structures, about two times longer than wide, forming a continuous row where the posterior end of one scute is in contact with the anterior tip of the subsequent scute. These scutes consist of a median longitudinal keel with a pointed posterior margin. Either side of the keel, a semi-ovoid lateral wing expands laterally to cover the predorsal edges of both body flanks. The entire outer surfaces of these scutes are intensely ornamented with numerous tubercles.

The modified flank scales of the lateral line are thin sinuous structures with rounded edges. A prominent keel is present on the midline, and either side of this is a posteriorly projecting subrectangular lobe. In the abdominal region these scales are rectangular, slightly higher than long, around two thirds the length of each vertebral centrum, and are placed slightly above the vertebral column. In contrast, the scales close to the caudal region are progressively smaller, shallower, and located at the level of the centra. The longitudinal axis of these scales encloses a tube that housed the lateral sensory canal. Additionally, above the canal, each scale has a conspicuous, laminar, and triangular keel that projects posteriorly.

The family Enchodontidae has been studied by many authors, who agree on its monophyly but disagree over the inclusion of Rharbichthys and the composition of its subfamilies. Vegrandichthys coitecus exhibits some of the characters already included in the diagnosis of this family, such as the lack of interopercle and supraorbital, the presence of the horizontal strengthening bar on the opercle, tubercles ornamenting the opercle and subopercle, and a series of predorsal scutes between the occiput and the dorsal fin. Therefore, the inclusion of Vegrandichthys into the family Enchodontidae is strongly supported.

Additionally, Vegrandichthys coitecus possesses a peculiar combination of characters that is unique among enchodontid genera. Most enchodontids have preopercles with no posterior spines; however, Vegrandichthys and Unicachichthys share the presence of a posterior spine at the base of the preopercle. Such spine is short in Vegrandichthys compared to that of Unicachichthys.

Among Enchodontids, as previously known, there are five different body patterns defined based on the different shapes of the snout and trunk: (1) a short snout and rather pisciform trunk as in Enchodus, Unicachichthys, and Veridagon;  (2) a short snout and rather rounded, short and high trunk, as in Parenchodus; (3) a short snout and very long trunk as in Palaeolycus; (4) a moderately long snout and perciform trunk as in Eurypholis; (5) a long snout and very long trunk as in Saurorhamphus. The body pattern displayed by Vegrandichthys resembles that of Eurypholis; however, the length of the snout is intermediate between those seen in Eurypholis and Saurorhamphus.

Additionally, among Enchodontids the articulation between the lower jaw and the quadrate shows two conditions. This articulation is laterally covered by a posterior extension of the anguloarticular bone in Eurypholis and Saurorhamphus, but laterally exposed in other Enchodontids. Vegrandichthys shares the second condition with Enchodus, Parenchodus, Unicachichthys, Veridagon, and Palaeolycus.

Finally, among Enchodontids, the opercle also shows two conditions. The posterior border of this bone is rounded and continuous in Enchodus, Parenchodus, Unicachichthys, Veridagon, and Palaeolycus. In contrast, in Eurypholis and Saurorhamphus the longitudinal crest in the middle of this bone is extended, forming a posterior spine. Again, in this case Vegrandichthys shows the first of the conditions; however, this new Fish is unique because it has additional strengthening longitudinal crests in the preopercle.

See also...
Follow Sciency Thoughts on Facebook.