Eekaulostomus cuevasae, Cantalice & Alvarado-Ortega, 2016

Cantalice, KM & Alvarado-Ortega, J, 2016, Eekaulostomus cuevasae gen. and sp. nov., an ancient armored trumpetfish (Aulostomoidea) from Danian (Paleocene) marine deposits of Belisario Domínguez, Chiapas, southeastern Mexico, Palaeontologia Electronica 19 (3), pp. 1-24 : 4-10

Eekaulostomus cuevasae sp. nov.

zoobank.org/ FE49A9FD-EE5C-4FCA-9556-04DD13138E79

Holotype. IGM 4716 , almost complete specimen exposing the right lateral side of the body ( Figures 2-5 ).

Derivation of name. The specific epithet of this fish honors our colleague, Martha Cuevas García, whose dedication and newly passion for the fossils led us to find the only specimen of Eekaulostomus cuevasae .

Occurrence. Paleocene (Danian, ≈ 63Ma) marine strata of the Tenejapa-Lacandón geological unit. Belisario Domínguez quarry, Salto de Agua Municipality, State of Chiapas, southeastern Mexico ( Alvarado-Ortega et al., 2015).

Diagnosis. Aulostomoidea fish with rigid star-like scutes covering the whole trunk and part of the snout; pelvic fin placed anteriorly, just behind the postcleithrum; two spines in front of the soft rays of dorsal and anal fins; eight soft rays in both anal and dorsal fins; caudal fin formula iv+I+7—5+I+iii.

Description

Body shape. Table 1 summarizes the body measurements and proportion of IGM 4716. This specimen has 81.23 and 76.64 mm of total length (TL) and standard length (SL), respectively. In this elongated fish, the trunk is somewhat chubby because the maximum height of the body is located in the middle of the abdominal region, where it reaches 17.42% of SL, from this point the body height slightly decreases forward and backward. The caudal peduncle is relatively stout and represents almost 50% of the maximum body height. The head occupies 39.11% of SL; however, its preorbital region forms a tube that is twice as long as the rest of the head, which ends with a small mouth opening.

Both anal and dorsal fins are triangular and located far in the back of the trunk. There is just one small dorsal fin lying along the section between 80.88 and 88.36% of the SL. The anal fin is almost totally opposed to dorsal fin and occupies the section between 78.31 and 85.29% of SL. Although the shape of paired fins is unknown, their rays show their position. The pelvic fin rises just below of middle height of body behind the head. The pelvic fins are ahead in the anterior half of the body, at 46.02% of SL. The caudal fin is a small, short, and semicircular structure, whose height hardly exceeds the height of the caudal peduncle. Robust scutes of serrated margins and radiating ridges cover the whole body and the postorbital and preorbital regions of the head.

The skull. Among the regions of the skull, the preorbital and orbital are equal in length, and each represents less than one quarter of the skull length. Therefore, the preorbital region occupies little more than one half of the skull length. No circumorbitals bones are present around the orbit.

Poor morphological information is rescued from the postorbital skull region of IGM 4716 because this is largely covered by scutes and the sutures between the bones are not well-defined. The preorbital region is partially roofed by the frontals, which are anteriorly extended beyond to joint with the mesehtmoid bones, and does not meet the parasphenoid. The supraorbital canal extends along the frontals and opens in elongated porous. The pterosphenoid is present and occupies the region posterior ventral of the orbit; its orbital region is a toothless straight bar that cross the ventral section of the orbit and is extended beyond probably between the lateral ethmoids. The vomer is jointed to the anterior ventral surface of the mesethmoid.

The mesethmoid is an extremely elongated bone that partially roofs the preorbital region of the skull, reaching the anterior tip of the skull. The lateral ethmoid (= prefrontal of Jungersen, 1910; Gregory, 1959) is a stout rectangular bone between the frontals and parasphenoid forming the anterior limit of the orbit and the posterior limit of the nasal capsule.

Jaws. The mouth opening in IGM 4716 is small and tilted upward. The jaws are small and occupy the anterior third of the preorbital region. In this fish, the premaxilla and maxilla are well-preserved showing its natural inclined position. The premaxilla is an inverted L-shaped bone; in which the vertical limb is a well-developed anterior ascending process while the horizontal limb is an edentulous rod. This maxilla is an expanded, ovoid, and edentulous bone, four times longer than the premaxilla. Its dorsal border is gently curved whereas its ventral border is practically straight. The articular head of the maxilla is tiny. There are no supramaxillae bones.

The dentary is the larger bone in the lower jaw, in which it occupies the anterior 80% of this organ. Dentary is somewhat triangular shaped because its symphysis is shallow in comparison with its rear. The ventral edge of this bone is slightly curved. Although the alveolar border of dentary is totally covered below the maxilla; probably, it is a toothless bone too. The posterior part of the lower jaw is occupied by the anguloarticular bone, which posteriorly forms a stout articular process; it is firmly sutured with the dentary. The mandibular sensory canal is extended within a longitudinal ridge alongside the ventral border of these bones, where it opens in small pores.

Hyopalatine bones. In IGM 4716, the anterior bones of the hyopalatine series are elongated and form part of the pipe-shaped preorbital region of the head. In this fossil, such bones are broken and expose their labial surfaces. The hyomandibular is ax shaped, in which the handle is tilted and extended from the middle part the orbit up to the postorbital region of the skull. This bone has a broad, single, articulatory head. The metapterygoid and entopterygoid are almost totally covered.

The quadrate is the longest bone in the hyopalatine series but it not as long as preopercle; its triangular shape forms a high dorsal flange that occupies the posterior two thirds of the preorbital region of the skull. The condyle of the quadrate is anteriorly tilted and its articulation with the lower jaw lies far from the orbit. The sympletic is elongated spine like bone inserted on the rear of the quadrate. The entopterygoid lies totally in front of the quadrate; its shape is an L upside down. The palatine lies in the dorsal anterior half of the entopterygoid; this is an elongate bone with a stout rounded anterior process. The palatine bones are edentulous.

Opercular series. Undoubtedly, the opercle, subopercle, preopercle, and interopercle are present in IGM 4716. All of them are laminated, thin, and smooth bones. The opercle is kidney shaped, 1.3 times longer than high. In this bone, the hyomandibular facet is located in its anterior dorsal corner. The subopercle is under opercle; it has a concave ventral edge and a short anterior ascending process.

In this series, the preopercle is the most remarkable bone because its horizontal limb is extremely elongated; it is four times longer than high. The preopercular sensory canal opens into four pores and probably one or two located along the preopercular horizontal and vertical limbs, respectively. In IGM 4716, the elongated interopercle is almost totally covered by the preopercle and seems to have no posteroventral expansion.

Vertebral column. Given its preservation mode, the axial skeleton of IGM 4716 is almost totally lost; however, some incomplete and strongly distorted vertebral centra are preserved behind the occiput. These are elongated structures, at least three times longer than high, which seem to be somewhat cylindrical with no process or neural arches preserved.

Paired fins and girdles. Bones of the pectoral girdle are largely obscured by scutes. The cleithrum is an elongated, thin, smooth bone that is tilted down and forward; its height is about two thirds the height of the trunk. On its medial region, this bone has an expansion to suture with the coracoid. A conspicuous single postcleithrum is present; this laterally sigmoid bone is almost as high as the cleithrum. Although, a large part of the ectocoracoid is under the scutes and its joint with the cleithrum is obscured, the posterior part of this elongated bone is exposed showing that it reaches the level of the posterior ventral tip of the postcleithrum ( Figure 3 ).

The pectoral fin is somewhat rectangular because its base seems to be vertical and high. Fourteen thin and branched pectoral rays, extended backward being as long as the postcleithrum, form part of this fin. The pectoral fin is located behind the cleithrum, near to the middle of the height of the trunk.

Although the pelvic bones are totally obscured by scutes; six thin branched and segmented fin rays show the position of the pelvic girdle. These rays rise near to the end of the first half of SL, on the abdominal edge of the trunk, and just behind the posterior ventral end of the postcleithrum ( Figure 2 ).

Unpaired fins. The single dorsal and the anal fins are small and triangular, practically opposed to each other, and located far back in the body. The lengths of the dorsal and anal fins represent 7.48 and 6.88% of SL, respectively. Dorsal fin comprises two short stout spines and eight branched and segmented soft rays. This composition is similar to that of the anal fin; however, the anal rays are more closely located to each other. Bones of the internal support of these fins are not preserved.

Caudal skeleton. Supporting bones of the caudal fin are covered by the scutes. The caudal fin is shallow, rounded, and their elements are ordered according the to formula iv+I+7—5+I+iii. On the one hand, four unbranched and unsegmented procurrent rays precede the upper caudal fin lobe, which also consists of one unbranched plus seven branched and segmented principal rays. On the other hand, there are three procurrent rays in front of the lower caudal fin lobe, which includes one unbranched plus five branched and segmented principal rays. In both caudal fin lobes, the principal rays tend to be longer toward the middle line; hence, the longest rays are in the middle of this fin, where they are in horizontal position and parallel to each other ( Figure 4 ).

Scutes. Thick scutes randomly distributed are covering the whole trunk and part of the postorbital and preorbital regions of the head ( Figures 2-5 ). These are star-like scutes, whose general shape is circular or ovoid with borders entirely occupied with serrations. Such serrations are more conspicuous and less numerous in the small scutes while these tend to be shallower and more abundant in the larger scutes ( Figure 5 ). Although it is hard to recognize a pattern on the distribution of these scutes; along the trunk, the larger are distributed longitudinally on the flanks, behind the pectoral girdle while the small scutes are located on the occiput, head, around the fin bases, and along the dorsal and ventral edges of the trunk.

Some scutes on the preorbital region and middle part of the body expose their external surface. In these scutes, strong radiating ridges are projected from a prominent central tubercle up to the tip of each serration on the edge ( Figures 5.1, 5.2 ). Given the conservation mode of IGM 4716, almost all its scutes show the inner surface, which is entirely smooth and has a central depression that corresponds with the thick central tubercle on the external surface of each scute ( Figures 5.3, 5.4 ). In IGM 4716, some of those scutes that expose their external surfaces have a small central pore on the tip of the tubercle; however, these are so scattered that it is not possible to describe the lateral line way along the trunk.

Remarks. According Nelson et al. (2016, p. 409), the families Aulostomidae and Fistulariidae constitute the superfamily Aulostomoidea , as firstly recognized by Greenwood et al. (1966). In the most recent of these works, this superfamily is supported based on three shared characters: 1) The first four vertebrae elongated; 2) the pelvic fin consists of six (or rarely five) soft rays; and 3) the presence of three nuchal plates (median dorsal bony plates located behind the occiput). It is remarkable that the general details of the body of Eekaulostomus cuevasae gen. and sp. nov. resemble those of the aulostomoids or trumpetfishes; however, the inclusion of this Mexican species as an indubitable member of this superfamily obeys to the presence of the first two of these characters ( Figure 3 ).

Nelson et al. (2016) also noted some distinctive characters of Aulostomidae and Fistulariidae ; the comparison of these characters with those here discovered in Eekaulostomus cuevasae gen. and sp. nov. reveals that this new fish does not belong to any of these families. On the one side, in aulostomids the body is covered with small scales; the dorsal fin consists of eight to nine thin and elongated dorsal fin spines plus 22 to 27 soft fins; the anal fin only includes 23 to 28 soft rays, and the caudal fin is rounded. On the another side, in fistulariids the body is naked or is covered with tiny prickles (spinules sensu Fritzsche, 1981) and linear scutes; both unpaired fins have no spines and are relatively shorter because they only consist of 13 to 20 soft rays; and the caudal fin is forked and has a middle caudal filament formed by two inner elongated caudal rays. Although the caudal fin of Eekaulostomus cuevasae gen. and sp. nov. is rounded as that of the aulostomids; the fins and body coverage of this new Mexican fish display different contrasting conditions; its whole body and part of its head are covered with star-like scutes while its unpaired fins have only two stout spines and less soft rays (only eight in both fins) ( Figures 2 , 4 ). The comparison of Eekaulostomus cuevasae gen. and sp. nov. and other fossil taxa already related with the superfamily Aulostomoidea also shows the singularity of this new species. Members of Urosphenidae , Parasynarcualidae , and Fistularioididae do not have anal fin spines, and their bodies are shallow and extremely elongated, conditions that strongly contrast with the presence of spines in both the unpaired fins and the elongated chubby body noted in E. cuevasae . The body armored coverage of E. cuevasae also differs from other extinct aulostomoids, where it is common to find bodies naked or covered with small scales.

The presence of covering scutes among aulostomoids is not a new finding. Previously, Bonde (1997) reported a Danish unnamed armored aulostomoid and Parin and Micklich (1996) named Aulostomus medius that has a body covered with rows of rectangular and evenly sized scales. Again, a glace to these fossils proves the singularity of E. cuevasae . In the first case, the Bondeʼs fish has not yet been neither accurately described nor formally named, and therefore, it is impossible to carry out a fruitful comparison. In the second case, the scales of A. medius are rectangular, evenly small sized, and ordered in rows, which are deeply different to the unordered star-like scutes of different size found in E. cuevasae .

Considering the context described on the two anterior paragraphs, in this work we named a new family, Eekaulostomidae , to include the species Eekaulostomus cuevasae gen. and sp. nov. The relationships of this Mexican fossil fish and other members of the superfamily Aulostomoidea are explored in the phylogenetic analysis described ahead.

Phylogenetic Analysis

In the present research we performed two phylogenetic analyses following the same protocols. Initially we use the data matrix and procedures considered by Keivany and Nelson (2006, figures 1, 2) trying to recover their result. Although we recovered the same and single most parsimonious tree of these authors; the supporting characters of nodes in our first analysis are different (see the comparison in Table 2).

Subsequently, we added Eekaulostomus cuevasae gen. and sp. nov. and the corrections previously described into an updated data matrix. We use this data to perform the second analysis. Although the single and most parsimonious tree recovered in this second study is practically the same generated by Keivany and Nelson (2006, figures 1, 2) and our first analysis, except for the inclusion of Eekaulostomus as the sister group of the unnamed clade including Aulostomidae and Fistulariidae (node J in Figure 6 ). These resulting trees show differences in the distribution of supporting characters (synapomorphies and homoplasies), as it is summarized in Table 2.

The single parsimonious tree including Eekaulostomus cuevasae gen. and sp. nov. obtained in the present study ( Figure 6 ) has 279 steps of length, consistency index of 0.43; retention index of 0.55, and homoplasy index of 0.57. Results of the bootstrap analysis performed in this study support the topology of our tree ( Figure 6 ), except for node G that is supported in less than 50%. It is remarkable the low number of synapomorphies in our result ( Table 2); nevertheless, Eekaulostomus cuevasae gen. and sp. nov. is located as the sister group of Aulostomidae plus Fistulariidae , and all these together form a monophyletic group, the new sense of the superfamily Aulostomoidea ( Figure 6 , nodes K and J).

The anatomy of Eekaulostomus cuevasae gen. and sp. nov. is relatively poorly known. Therefore, the data matrix of 113 characters built in the present study only includes 26 non-doubtful characters of E. cuevasae . Despite this situation, the present phylogenetic study provides enough information to support the inclusion of E. cuevasae into the suborder Syngnathoidei , infraorder Syngnathoida , and superfamily Aulostomoidea sensu Keivany and Nelson (2006 , p. S90) (nodes D, F, and J, respectively in Figure 6 ). The single synapomorphy of Syngnathoidei , the lack of distinctive cranial condyles in the hyomandibular (35-1), is present in Eekaulostomus that has a hyomandibular with a simple undivided articular head ( Figure 3 ). Eekaulostomus also exhibits three of the seven synapomorphies of the infraorder Syngnathoida (node F in Figure 6 and Table 2); its vomer is located superficially anteriorly (3-1), its quadrate shows a deep dorsal flange (29-1), and its interopercle is elongated (41-1) (see Figure 3 ).

Eekaulostomus cuevasae gen. and sp. nov. is a member of Aulostomoidea because it displays the single synapomorphy of this superfamily (clade J in Figure 6 and Table 2), the ectocoracoid is an elongated bone that reaches the posterior level of the postcleithrum (113-1), as well as three of its nine homoplasies [its maxillar is expanded ventrally (22-0); its quadrate articulates with the lower jaw below (33-0); and has a well-developed ectocoracoid (78-1)]. The other six homoplasies present in this superfamily (i.e. 14-0, 16-1, 22-0, 64-1, 68-1, 77-1) are unknown in Eekaulostomus cuevasae gen. and sp. nov.

Eekaulostomus cuevasae gen. and sp. nov. has two autapomorphies: the presence of stout spines close together to the soft rays in both anal and dorsal fins (111-1) and scutes covering the snout (112-1); as well as three homoplasic characters not present in other aulostomoids. In this new species, the premaxillary ascending process is well developed (19-0), the hyomandibular lacks a lateral lamina (34-1), and the anal fin has more than one spine (91-0). In contrast, Aulostomidae and Fistulariidae do not have scutes or stout anal fin spines, the premaxillary ascending process is comparatively smaller, the hyomandibular shows a lateral lamina, and there are no anal fin spines; additionally, the last family neither has dorsal fin spines while the former has dorsal spines scattered and separated from each other along the dorsal border.

Figures

FIGURE 2. Eekaulostomus cuevasae gen. and sp. nov. from the Danian marine deposits of the Belisario Domínguez quarry, near Palenque, Chiapas, southeastern Mexico. 1, General view of IGM 4716, holotype and single specimen known. 2, Idealized line drawing of the same specimen.

FIGURE 3. Head and anterior part of the trunk of Eekaulostomus cuevasae gen. and sp. nov. from the Belisario Domínguez quarry. 1, General view of the head of specimen IGM 4716. 2, Line drawing of 1. Abbreviations: aa, anguloarticular; br, branchiostegals; cl, cleithrum; den, dentary; eccor, ectocoracoid; ecp, ectopterygoid; fr, frontal; hy, hyomandibula; iop, interopercle; la, lachrymal; le, lateral ethmoid; mes, mesethmoid; mx, maxilla; op, opercle; pa, parasphenoid; pal, palatine; pmx, premaxilla; pop, preopercle; pvfr, pelvic fin rays; q, quadrate; sc, scutes; sop, subopercle; sy, symplectic; v, vertebral centra; vo, vomer (the rays of pelvic fin are seen below the cleithrum).

FIGURE 4. Unpaired and caudal fins of Eekaulostomus cuevasae gen. and sp. nov. from the Belisario Domínguez quarry. 1, General view of the head of specimen IGM 4716. 2, Idealized line drawing of 1. Abbreviations: afs, anal fin spines; afr, anal fin rays; dfr, dorsal fin rays; dfs, dorsal fin spines; pr, procurrent rays; black arrows enclose the principal rays of caudal fin.

FIGURE 5. Scutes of Eekaulostomus cuevasae gen. and sp. nov. from the Belisario Domínguez quarry. 1, Close-up of the head of specimen IGM 4716, showing a scute with the external surface exposed. 2, Idealized line drawing of 1. 3, Close-up of the caudal region of the specimen IGM 4716, showing a scute with the internal surface exposed.

FIGURE 6. Phylogenetic relationships of Eekaulostomus cuevasae gen. and sp. nov. and the superfamily Aulostomoidea within the order Gasterosteiformes (modified from Keivany and Nelson, 2006) (check the characters supporting the nodes in Table 2; number near the nodes show the bootstrap values obtained above 50%).