Paleocharacodon guzmanae, Caballero-Viñas & Alvarado-Ortega & Severiano, 2023

Caballero-Viñas, Carmen, Alvarado-Ortega, Jesús & Severiano, Kleyton Magno Cantalice, 2023, A Pliocene goodeid fish of the Paleolake Amajac, Sanctórum, Hidalgo, Mexico, Palaeontologia Electronica (a 30) 26 (2), pp. 1-39 : 5-21

Paleocharacodon guzmanae sp. nov.

( Figures 3-14 ; Tables 1-2)

zoobank.org/ 8F4FF55A-56B2-44DC-8CFF-A69A04A1C151

Holotype. IGM 13117 , a complete female specimen of 37.29 mm of standard length, preserved in part and counterpart ( Figure 3 , Table 1).

Paratypes. IGM 13118 , IGM 13119 , IGM 13120 ( Figure 4A ) , and IGM 13121 ( Figure 4B ) are almost complete and articulated males. IGM 13122 and IGM 13123 are incomplete and significantly disarticulated males. IGM 13124 and IGM 13125 ( Figure 4C ) are almost entirely well-preserved females. IGM 13126 , IGM 13127 , IGM 13128 , IGM 13129 , and IGM 13130 are incomplete specimens of unknown sex ( Table 1).

Type locality and age horizon. Sanctórum-JAO site (recorded as IGM-loc 3875 in the Colección Nacional de Paleontología, Igl-UNAM), Atotonilco El Grande Municipality, Hidalgo, Mexico. Pliocene lacustrine deposits of the Paleolake Amajac belonging to the Atotonilco el Grande Formation ( Figures 1 and 2 ).

Etymology. The species epithet honors our colleague, Dr. Ana Fabiola Guzmán Camacho, who passed away in 2022, for her contributions to Mexican archaeozoology and paleoichthyology. Together the genus and species names mean “the Guzmán’s ancient Characodon fish.”

Diagnosis. Goodeinae fish with a unique combination of characters and a maximum total length is near to 60 mm, the body is elongated; slightly humped behind the head; fins rounded; pelvic fin rising at the beginning of the posterior half of the body at 57.3% of SL in males and 64.8% of SL in females, and in both cases its rear edge does not reach the anterior end of the dorsal fin base; unpaired fins opposed to each other, located far back of the body, and behind the 69% of SL; total vertebrae 35 in males including 15 abdominals plus 20 caudals while there are 31-32 in females including 14 abdominals and 17-18 caudals; anal fin rays typically 16; in males the most anterior anal rays is rudimentary and this and the following five forms the andropodium; dorsal fin rays 15 in males and 12 in females; pectoral rays 12 in males and 13-14 in females; pelvic rays 6 in both sexes; ribs 15 in males and 12-13 in females; the lateral edges of the anterior end of the parasphenoid are parallel and have no lateral wings; jaws shows a dental battery composed of a labial row of long and robust teeth distally bicuspid and acute with bases co-ossified with the bone plus smaller conic and unicuspid teeth are located lingually; the palatoquadrate arch is anteriorly inclined; the articular facet for the quadrate is donut-like with the central part formed by the retroarticular, and the surrounding region is formed by a couple of curved processes of the anguloarticular; the palatine head has an anterolateral projection; the ventroposterior end of coracoid is convex and has the posterior edge vertically tilted and notched. This species is unique in having the posttemporal with a small anteroventral process, and the openings of the supraorbital canal show the formula 1-2a, 2b-3a, 3b-4a, 4b-5a, 5b-7.

Description

Body shape and general proportions. Table 1 shows the body measurements and proportions of Paleocharacodon guzmanae gen. and sp. nov. The fossil fishes from Sanctórum show different modes of conservation. Occasionally, their bones are totally or partially disarticulated and dispersed. Although it is common to find them relatively complete, well-articulated, and resting, showing one of the body flanks, in general, the bones of the anterior part of the head, the jaws, the nape, and the abdominal region are a little dislocated causing some imprecision in the body measurements. The present description is based on the holotype and added data observed in the paratypes. The larger specimens are the female IGM 13117 (its total and standard lengths are 48.27 and 37.29 mm) and IGM 13130 , a sex-undefined and incomplete individual that may be near 100 mm of SL (not included in Table 1) .

This elongated fish has a triangular head, almost as high as long, in which the snout is terminal. The maximum body height (MBH) is allometrically variable in the predorsal region of the trunk, in which the back becomes a kind of hump in the larger specimens ( Table 1; Figures 3 , 4 ). Compared with females, males' bodies and heads are shorter and lower (the SL range is 21.88-40.32 mm vs. 25.38-37.29 mm). Regarding the SL, the MBH is 16.65-22.79% vs. 28.94-35.66%; the head length is 26.66-28.01% vs. 28.77-36.12%; and the head height is 21.67-27.23% vs. 27.39-30.08%). All fins have round terminal margins. The pectoral fin extends in the first third of the abdominal region and rises between the vertebral column and the ventral edge of the body. The pelvic fin is short, located in the posterior half of the body (57.3% of SL in males and 64.8% of SL in females), and its distal ends extension does not reach the cloaca. The unpaired fins are short (concerning the SL, the dorsal fin length is 69.3% in males and 71.87% in females while the anal fin length is 70.56% in males and 73.26% in females), opposite to each other, placed on the last third of the body, and represent about 10 to 15% of SL. The caudal peduncle is robust. The caudal fin is entirely round, its base is as high as the caudal peduncle, and its shorter rays are about half its longest.

Skull. In most specimens studied here, the skull bones are so crushed or fragmented and isolated that it is difficult to recognize them accurately. Despite this situation, these exhibit the following details ( Figures 5-7 ).

In the dorsal view, the skull is trapezoidal with no medial fontanels, in which the interfrontal suture is a somewhat sinuous interfrontal suture, and the frontals and supraoccipital bones are in contact separating the parietals. Anteriorly, each nasal is a flat oval bone with a large pore near its anterior that may correspond to pore 1 of the supraorbital sensory canal ( Figures 6 , 8A ). Each lateral ethmoid bone is a robust, complex C-shaped bone attached to the anterior ventral region of the respective frontal and extends up to the anterior end of the orbital part of the parasphenoid ( Figures 6 , 7 , 8A ).

Each frontal is laminar dorsally expanded and axe-shaped, which forms 70% of the skull length, and markedly broad in its anterior two-thirds. The surface of each frontal shows three sections or low domes joined by inconspicuous ventral ridges, in which the bone thickens ( Figure 7 ). The medial section of the frontal is the largest, dorsally smooth, and ventrally has a series of anterior parallel longitudinal ridges in which the respective lateral ethmoid bone attaches. The lateral section of this bone is roughly triangular and displays a series of 10 large pores alongside its borders with the medial and posterior sections, which corresponds to the pores 2b, 3a, 3b, 4a, 4b, 5a, 5b, 6, and 7 of the supraorbital sensory canal ( Figures 6 , 7 ). Since pores 1 and 2 of this sensory canal open in the nasal bone, the formula of these openings is 1-2a, 2b-3a, 3b-4a, 4b-5a, and 5b-7. Remarkably, the 2b of these sensory pores is complex; this opens into a broad depression in which at least two other associated pores open ( Figure 7 ). The posterior section of the frontal is the smallest, has a triangular shape, and is behind the lateral section. Near its posteromedial limit, the lateral section has a pair of conspicuous, laminar, and parallel ridges forming a bony canal projected lateromedially, which opens dorsally, encloses pore 6, and ends right where pore 7 opens. In the most posterior part of the frontal, where this bone bends ventrally and exposes more laterally, there is a wide shallow depression that seems to be the dorsal end of the dilator fossa ( Figure 6 ).

Each parietal is a small and smooth rectangular bone that joins the rear of the frontal and the lateral edge of the supraoccipital ( Figure 6 ). The supraoccipital bone is an oblong, laminar, and dorsally convex bone with a large posterior crest that is bifid and expanded posteriorly ( Figure 6 ). The basioccipital bone is triangular, anteriorly tapered, as high as its vertebral articulation, and hardly expanded laterally. In the dorsal view, the sphenotic bone is rectangular with a stout lateral process. The epiotic is a small oblong bone located in the posterior region of the skull that bears a straight posterior thick process ( Figure 6 ).

The parasphenoid is a complex unpaired bone that is expanded laterally in its otic region, whereas its orbital part forms a rod-like structure, evenly straight. The anterior end of parasphenoid becomes dorsoventrally flat and bifid. The middle orbital section of this bone changes allometrically; in small individuals, its transversal section is “+” shaped because it has low wings expanded dorsal, ventral, and laterally whereas, in large specimens, the cross-section becomes T-shaped because the dorsal wing disappears ( Figures 6 , 7 , 8A ). This bone displays a triangular basipterygoid process projected dorsally and laterally at the union of its orbital and otic sections.

Otoliths. Although some specimens studied show remains of the three pairs of otoliths, only the sagittal otolith is well-preserved ( Figures 8A , 9 , Table 2). In life, these inner ear stones are solid crystalline structures of calcium carbonate; however, the otoliths of our fossils have a grainy appearance, probably due to the partial dissolution of their calcium carbonate.

The sagittal otolith is a thick lenticular structure crudely resembling an arrowhead pointing backward, in which the ventral section is a little longer than the dorsal one ( Figure 9 ). Its lateral surface is smooth, has a hardly protruding middle region, and peripherical concentric growth lines. On the contrary, its medial surface is irregular; it has two longitudinal grooves, one corresponding to an expanded deep concave dorsal area, and the other is a slender medial sulcus, dorsally and ventrally bordered by the cristae superior and inferior, respectively.

The contour of the sagittal otolith is slightly sinuous all around except for a conspicuous notch in the middle of the dorsal margin ( Figure 9 ). In the rostral margin, the rostrum and antirostrum are prominent convex projections deeply separated by a very open exisura that rises from a sharp notch. In the posterior margin, the middle part is a noticeable convex process projected beyond the postrostrum and pararostrum, which are inconspicuous convex projections.

Table 2 summarizes the measurements and proportion observed in the left sagittal otolith obtained in specimen IGM 13119. This inner ear stone has 1237.1 μm of total length. Its ratio total length/maximum height is 1.36 (the maximum height is 73.3% of its total length). Its ratio ventral length/maximum heigh is 1.01 (the ventral length is 74.3% of its total length). Its relative dorsal length (DL/VL) is 98.5. Its relative medial length (ML/VL) is 1.08. Its relative antirostrum height (AH/MH) and length (AL/VL) are 0.385 and 0.099, respectively. Its relative rostrum height (RH/MH) and length (RL/ VL) are 0.401 and 0.263, respectively. The exisura opens about 90°, and the posterior and postventral angles are 60 and 128°, respectively.

Circumorbital series. The circumorbital series is highly reduced and only consists of the lacrimal and dermosphenotic bones. The lacrimal bone is somewhat rectangular, about twice higher as long ( Figure 5 ); unfortunately, none of the specimens studied here has a lacrimal that clearly shows the trajectory or the number of pores of the preorbital sensory canal. The dermosphenotic is smooth and thickened tubular bone, ventrodorsally projected, which ends with a blunt tip and borders the dorsoposterior region of the orbit. The dermosphenotic is hollow, carries the postorbital branch of the supraorbital sensory canal, and has at least four pores of that sensory canal ( Figure 6 ).

Upper jaw. Each limb of this jaw consists of the premaxilla and the maxilla. In lateral view, the premaxilla is a sinuous bone divided into two parts. The anterior half of this bone corresponds to its toothed section, a stout bar, homogenously high, and ventrally and medially curved, in which the proximal end becomes thick, dorsally expanded, and forms a straight inter-premaxillary articular surface and a short triangular ascending process. In contrast, the untoothed section of the premaxilla is a rhomboidal-like expanded structure that includes a rectangular and flat ventral process, a short dorsal spine posteriorly projected, and a straight posterior process ( Figure 6 , 8D , 10 ).

The maxilla is a long, curved, untoothed bone ( Figures 6 A-B, 7, 10). In this bone, the anterior third is cylindrical, medially bent, and ends in an expanded rounded structure with two globular dorsal processes and an elongated triangular dorsal process with an acute tip. The posterior half of this bone becomes straight, flat, and broader.

In toothed elements of jaws are the premaxilla in the upper jaw and dentary in the lower jaw; these bones show two kinds of teeth, bicuspid or bifid sharp teeth and unicuspid acute teeth ( Figures 6 , 8 , 10 ). The toothed surface of the premaxilla is broad and shows a labial or principal tooth row that consists of eight or nine bicuspid teeth that are long, thick, and have a stake-like base co-ossified with the bone. In these bifid teeth, the base is ovoid, expanded, and laterally compressed; the middle part is progressively thinner; and the terminal end becomes labio-lingually flat, broader, and divided into two symmetrical acute triangular tips with lateral slightly curved edges ( Figure 11 ). Additionally, numerous secondary teeth are in a lingual position; these are conic, unicuspid, smaller, and shorter. Although many teeth of the principal row have the tips broken, their bases are with the dentary and premaxillary ( Figure 8 ). Secondary teeth appear to be just tightly attached to the surface of the bone.

Lower jaw. Each limb of the lower jaw consists of four bones, the dentary, anguloarticular, retroarticular, and coronomeckelian. The dentary is a trapezoidal bone in which the coronoid process is absent, and the posterior end is expanded and deeply bifid. In this bone, the anterior and ventral edges bend medially, the ventral edge is notched, and the symphysis is straight and high ( Figures 6A , 8 B-D, 10). Here, the toothed border is straight and extended about the anterior two-thirds of this bone. The mandibular sensory canal runs alongside the ventral edge of the dentary and opens through at least four large pores; among these, the second and third pores are the most extensive and form long and wide grooves.

The anguloarticular is a small boomerang-like structure that shows limbs slightly differently shaped, forming an interior angle somewhat obtuse. This bone has a small articular process protruding in the base of its posterior edge and has a trapezoidal ventral process near its rear that sutures with the anterior border of the retroarticular ( Figures 6 A-C, 8B, 10). Here, the anterior and dorsal processes form an interior angle of about 110- 120°; the anterior process has straight edges, shows an acute tip, and is about 1.3 longer, whereas the dorsal process shows somewhat convex edges and a rounded tip. This bone does not carry the mandibular sensory canal. At the base of the retroarticular articular process, the anguloarticular and retroarticular forms part of the articular facet for the quadrate. This facet is a shallow and ovoidal depression with a donut-like configuration, in which the retroarticular's dorsal surface forms the central region of such facet, while the anguloarticular has two curved or C-shaped processes forming the surrounding part of this articular surface. The tips of these surrounding processes extend almost to join at the back of the retroarticular ( Figures 6 A-C, 8B, 10).

The retroarticular is a pyramidal and stout bone ( Figure 6 ). This bone forms the ventroposterior end of the lower jaw because it is attached laterally between the articular and ventral processes of the anguloarticular bone. In most of the specimens studied here, the retroarticular and anguloarticular are disarticulated, suggesting that these bones showed cartilaginous sutures in life. The coronomeckelian is a small, flat, untoothed, and comma-shaped bone medially attached to the anterior process near the interior angle of the anguloarticular ( Figure 6 , 8B , 10 ).

The toothed surface of the dentary shows teeth similarly shaped and ordered as those described in the premaxilla ( Figures 6 A-C, 8D, 10, 11). This bone also has two kinds of teeth; six or seven long bicuspid teeth, thick and stake-like based along the labial or principal row, and an indeterminate number of smaller unicuspid teeth forming a lingual or secondary tooth patch ( Figures 8C , 10 , 11 ).

Suspensorium. IGM 13123 shows the bones of the suspensorium series complete and articulated ( Figure 8D ). This comprises the palatine, quadrate, hyomandibula, symplectic, and endopterygoid. These bones are ordered, forming an U-shaped structure. The metapterygoid and ectopterygoid bones are absent. Here, the palatoquadrate arch, formed on the anterior edge and between the heads of the quadrate and palatine, is straight and strongly tilted vertically. The palatine is a triangular laminar bone, about two times higher than long, smooth, with an imbricated suture for the quadrate and endopterygoid. The palatine articular head is narrow and has a small dorsal process protruding anterolaterally ( Figures 6A , 8D ).

The quadrate bone is a triangular bone. Its posterior edge is somewhat concave, whereas its ventral and anterior edges are relatively straight, form an obtuse angle close to 115°, and join the base of the mandibular articular head. This head is robust, slightly protuberant forward, and bicondylar, in which the medial condyle is larger than the lateral one. The incisura for the symplectic is broad and deep and separates the laminar body of this bone from its posterior process, which is markedly thicker and has a sharp tip.

The symplectic is a long triangular bone, anteriorly acute, posteriorly expanded, and dorsoposteriorly projected ( Figure 6A ). It is almost two times longer than the quadrate and joins at the base of the hyomandibular. The endopterygoid [also identified as mesopterygoid by Kobelkowsky (2005, fig. 2) and ectopterygoid by Smith (1980, fig. 15)] is a weak laminar and oblong bone, ventrally expanded, and dorsally tapered. The posteroventral part of the endopterygoid occupies the quadrate's posterior notch, and its anterodorsal end is attached to the base of the rear edge of the palatine. The hyomandibular is an axe-shaped bone with a short handle, an expanded rectangular body, and two long articular facets ( Figure 8D ). The opercular process barely projects near the middle of the posterior edge of the hyomandibula.

Opercular bones. This bone series includes the opercle, preopercle, subopercle, and interopercle ( Figure 6 ). The opercle is a triangular bone laminar, slightly longer than high, and superficially convex; in which the dorsal edge is slightly concave; the anterior border is straight and thick; and the posterodorsal end becomes narrow, somewhat elongated, and rounded. Anterodorsally, the opercle exhibits a short rectangular articular process (= preopercular arm) that protrudes anterodorsally and forms an obtuse angle of about 130-140° with the anterior edge of the bone. On the contrary, in the medial surface of this bone, near its anterior border and at the base of its articular facet (=opercular facet), is a shallow and semispherical concavity that occupies the center of a superficial and oblong structure that protrudes medially.

The preopercle shows the typical inverted Lshape with the horizontal and vertical limbs similarly shaped, forming an angle of about 90° plus a well-developed medial shelf, uniformly expanded, and a rounded anterior edge ( Figures 6A , 8D ). Alongside the ventral and posterior edge, this bone displays a deep groove roofed by a thin laminar shelf, in which four or five pores of the preopercular sensory canal open, three of them in the horizontal limb and the others in the vertical one (these pores are visible under the microscope with the specimen wet with alcohol).

The subopercle is a sickle-shaped laminar bone with an anterior ascending process that is small, thorn-like, and subterminal ( Figures 6 , 10 ). This bone and the posteroventral edge of the opercle are equally long. In the subopercle, the anterior border is high and convex, the dorsal and ventral edges are harmoniously curved, and the posterior end is acute. The interopercle is a long triangular bone, slightly curved dorsally, rounded posteriorly, and sharp anteriorly ( Figure 6 ). The preopercle covers a large part of the interopercle; however, its anterior tip is so long that it almost reaches the articular head of the quadrate.

Hyoid arch and branchial apparatus. IGM 13127 shows the bones of the hyoid arch and branchial apparatus ( Figure 6A, D ). The ventral hypohyal is a solid pyramidal structure anteriorly pointed. Behind this, the anterior ceratohyal is an unpierced rectangular bone, lateromedially flattened, slightly constrained in its anterior half, and expanded in its anterior and posterior ends. The posterior ceratohyal is a solid triangular bone, probably equally longer than high. The anterior ceratohyal is anteriorly and posteriorly tightly attached to the respective ventral hypohyal and posterior ceratohyal. The urohyal is a long bone with a slight anterior articular head plus two triangular longitudinal wings posteriorly projected; the horizontal one forms the ventral edge of the bone, and the vertical one projects dorsally.

Although sometimes poorly preserved, in this fish, there are always five pairs of branchiostegal rays ( Figure 6A, D ). The shape of these laminar, long, and curved bones varies in anteroposterior order, from thread-like to saber-like structures. Among these bones, the anterior four are proximally joining the anterior ceratohyal, while the posterior one does this with the posterior ceratohyal.

Numerous elements of the branchial apparatus are obscured or poorly preserved in the fossils studied here. In IGM 00024 ( Figure 6A, D ), the basihyal is a long bone, trapezoidal, somewhat slender and expanded anteriorly, and has a small posterior articular head. The same specimen also shows a minute thorn-like interhyal bone; however, this does not preserve the basibranchial bones that once separated the basihyal and interhyal bones.

Other bones of the branchial apparatus, such as the hypobranchials, ceratobranchials, epibranchials, pharyngobranchials, and gill rakers, seem present in different specimens studied herein; however, these are so cracked and incomplete that their identification and description are difficult. These include at least a lower and an upper pair of expanded gracile-toothed pharyngobranchial plates and associated posterior branchial bones. In these plates, the teeth are conic, very robust, and irregularly sized, with the tip curved backward and implanted in shallow sockets ( Figure 8G ). Although these teeth are usually unattached and dispersed between the medial surfaces of the opercular bones, those placed in the middle are so high that they equal or exceed the thickness of such plates.

Axial skeleton. The numbers of centra and ribs vary slightly in males and females ( Table 1). In males, the vertebral column consists of 34-35 total centra, including 15 abdominals and 20 caudals. On the contrary, females have 31-32 total centra, 13-14 abdominals centra, and 17-89 caudals. The rib pairs are 15 in males and 12-13 in females. All the centra are autogenous, but the last three fuse into the caudal complex ( Figure 14 ).

All centra are hollow, deep amphycelic, and hourglass-shaped; these are constrained in the middle, equally expanded anterior and posterior, and show circular intervertebral surfaces. Although it is difficult to determine if most anterior abdominal vertebrae have unfused neural arches, all the other centra fuse with the respective hemal and neural arches. At least four abdominal centra (probably the 2 to 5) show neural crests expanded, rectangular, and laminar. The ribs are long, thin, and slightly curved, enclosing almost the entire abdominal cavity; these join the dorsal surface of the paraphyses (= transverse processes in Kobelkowsky, 2005, figure 4), which are straight, long spiny-like structures projected laterally from the dorsal part of the centra, that show a lateromedial groove in its dorsal surface. At least, the anterior half of the ribs are associated with thin epineural bones, which are about two centra long and lie horizontally ( Figure 8F ).

Pectoral fin and girdle. The pectoral fin is rounded and has 12 rays in males and 13-14 in females ( Table 1). These rays are distally branched and segmented, uniformly thin, and extend, covering the anterior third of the abdominal region. At least four rectangular radials support this fin.

The posttemporal is a long triradial bone ( Figure 6A ). This bone is oblong, thin, short, and posteroventrally projected. In contrast, its anteroventral process is small and triangular, while its anterodorsal limb is a bar-like structure, uniformly wide, curved dorsoventrally, slightly flattened, and about four times longer than the anteroventral process. The supracleithrum is a flat bone, short and drop-like, that rests above the dorsoposterior end of the cleithrum. Since the highly compressed preservation of the specimens studied here, only the lateral surface of the cleithrum is identifiable; this thin bone is crescent- shaped and bears a laminar posterior extension with a slightly curved edge. Two postcleithra are present; the first is an ovoid plate in the rear corner of the cleithrum, and the other is a sinuous rod-like structure projected ventrally. The posterior wing of the cleithrum covers a large part of the scapula; however, its rear surface is exposed, showing its articulation with the pelvic radials. Otherwise, only the lateral surface of the coracoid appears as an elongated laminar and triangular structure that is posteriorly expanded and has a ventroposterior end convex, with a posterior margin vertically tilted and anteriorly limited by a narrow and deep notch ( Figure 6A ).

Pelvic fin and girdle. The pelvic fin is entirely opposed to the predorsal region of the trunk. It rises in the posterior half of the body, at 57.3 SL in males and 64.8% of SL in females. The posterior edge of the fin does not reach the origin of the dorsal fin. This small and rounded fin consists of six pelvic rays that are distally segmented, branched, and have bifid articular heads ( Table 1, Figure 12 ). There are no pelvic radials.

The pelvic bone is triangular, about two times longer than wide, laterally and medially almost straight, and anteriorly acute. The dorsal surface of this bone bears a middle thickening bar or keel. The posterior edge of the pelvic bone is concave and shows small facets to join the pelvic rays. In small specimens, this bone shows a shallow posterior notch and three processes relatively small, including an inconspicuous lateral process, a small and rounded medial process, and a stout triangular ischial process ( Figure 12A ). On the contrary, the pelvic bone of larger fishes has a posterior notch conspicuous, a lateral process wider and rectangular, and a long thick medial process that is somewhat sinuous ( Figure 12B ).

Dorsal fin. The dorsal fin is rounded, long, and about equally high than long ( Figure 13A ). The number of rays from this fin is sexually variable; males have 15 dorsal rays, while females show only 12. The dorsal rays are uniformly thin and distally branched and segmented. Here, the first three and last rays are comparatively smaller than the others.

A series of long and straight dorsal proximal pterygiophores support the dorsal fin rays in a into the vertebrae account. Abbreviatures: caudal peduncle height; MBH, maximum body height.

numerical relation 1:1. The series has no distal pterygiophores but includes small square-like medial dorsal pterygiophores in at least three anterior quarters of the dorsal fin. Along the series, the size of the proximal dorsal pterygiophores is almost the same and are so long that they penetrate the interneural spaces present immediately below.

Anal fin. The anal fin is high, long, and rounded; in general, this is a little smaller and located slightly behind the dorsal fin ( Figure 13B , Table 1). This fin comprises around 16 rays (between 15 and 18) in both sexes; the first ray is rudimentary. In males, the subsequent six rays form the andropodium; these rays are segmented, unbranched, shorter than the posteriors, and crowded together. Due to the difference in size, the andropodial rays are separated from the subsequent by a small notch. Posterior anal rays are distally segmented, branched, and at least 20 % longer than the last andropoidal rays. The rear three anal rays decrease in length progressively.

A series of proximal pterygiophores internally support the anal fin. These rod-like bones are straight, and so elongated that they penetrate the interhemal spaces. In extreme cases, these proximal pterygiophores cover three-quarters of the distance between the vertebral column and the base of the fin ( Figure 13B ). The numerical relation between anal rays and proximal pterygiophores is 1:1, except for the first pterygiophore that joins the first two anal rays. There are no distal pterygiophores. The anterior medial pterygiophores are absent or probably fused with the articular head of the anterior seven proximal pterygiophores, which are close-set, mainly the first five; therefore, these proximal pterygiophores are the only ones supporting the andropodial rays. Beyond, a robust rectangular medial pterygiophore is present between the posterior proximal pterygiophores.

Caudal fin. The caudal skeleton involves the neural and hemal spines of the preural 1-3 plus those bones associated with the ural centra ( Figure 14 ). These spines become thicker, broader, and slightly less tilted than those of previous centra. The parhypural and the hemal spines of preural centra 2 and 3 are so broad that they are in contact. The parhypural has a wide proximal tip, probably fused with preural 1. A single wide epural fills the space between the most posterior neural spine and the ural plate. As in other cyprinodontiforms, the end of the vertebral column, the caudal complex involves the fusion of the preural 1, urals 1, and an undetermined number of hypurals (see Thieme et al., 2021). The hypurals form a fan structure with a long conspicuous interhypural foramen longitudinally bordered by two shallow ridges. Near its base, this caudal complex shows three small processes; the urostyle is a small and rectangular anterordorsal process that is projected upward and seems to embrace the anterior tip of the epural; in the base of the hypural plate, there is a small spine-shaped process and a keel-shaped process laterally expanded in dorsal and ventral position, respectively.

The caudal fin is rounded and posteriorly expanded. Its dorsal and ventral lobes form a harmonious convex profile, while the ventral and dorsal edges of the fin are straight. This fin is relatively short, and its length is about 1.2 times the postanal length of the trunk. Regarding those caudal rays anteriorly associated with the caudal complex, the caudal formula is vi+ I+9—8+I+v. In both lobes, the procurrent rays become longer and thicker in anterior to posterior order.

Scales. The scales cover the whole body, head, jaws, and caudal skeleton 8 ( Figures 3-5 , 8F , 13 , 14 ). These are cycloid scales, rounded to slightly ovoid, in which the center half is smooth. The surrounding scale surface has 4-9 concentric circuli that are sinuous, unbranched, and evenly separated. Additionally, most scales covering the anterior third of the trunk have 3-12 straight, horizontal, and parallel radii. In small specimens, only a few scales of the middle part of the trunk show radii, while in larger fishes, the radii are present in more scales. Regardless of the size of the specimens, the scales on the head and jaw have no radii. On the trunk, scales are evenly sized; however, those on the occiput and the head become smaller. Middle longitudinal scale lines on the skull extend symmetrically on each side up to the frontalmesethmoid contact. Nine to 10 scale rows probably cover the height of the trunk, and about 32 rows extend above the vertebral column between the head and the anterior tips of the caudal fin rays.

Coloration pattern. UV light allows us to recognize some details of the color pattern of Paleocharacodon guzmanae gen. and sp. nov., which are invisible under white light. IGM 13117 shows 14 wide dark color spots covering the posterior surface of the abdominal cavity, four behind the pectoral girdle and over the first vertebrae, and at least another five or six (barely observable in the specimen) forming a longitudinal row above the vertebral column, between the pectoral girdle to the middle part of the anal fin. Additionally, a large dark color spot is present along the basal half of the anal fin ( Figure 3 ).

Such spots of color are not present in all the fishes of Sanctórum, probably because the multiple taphonomic factors that controlled and allowed their conservation did not affect all individuals uniformly. Otherwise, the rare preservation of these color spots could respond to the fact that, when collected, many of these fish open in half and therefore do not show their exterior surface details. Reproduction. Skeletal remains attributable to unborn small fishes preserved in the abdominal cavity of some of the females, as IGM 13117 and IGM 13125 ( Figure 5B ), evidence the viviparity of Paleocharacodon guzmanae gen. and sp. nov. Although it is challenging to identify complete individuals in these remains, here, we rule out the possibility that these remains represent stomach contents because these do not show any signs of decay by digestion.

Figures

FIGURE 1. The Sactórum-JAO2 site (= IGM-loc 3875). A) General view. B-D) Close-up views of the millimetric and laminated fossiliferous siltstone strata details. E) Permineralized wood in situ.

FIGURE 2. Map of the Santa María de Amajac-Sanctórum region at Atotonilco El Grande Municipality, Hidalgo, Mexico, and the lithostratigraphic section of the locality IGM-loc 3875 (Sanctórum-JAO); the yellow areas show the lacustrine deposits of the Paleolake Amajac (modified from Arellano-Gil et al., 2005, figure 2).

FIGURE 3. IGM 13117, holotype of Paleocharacodon guzmanae gen. and sp. nov., a pregnant female specimen from the Pliocene lacustrine sediments of the Paleolake Amajac, Sanctórum, Atotonilco El Grande Municipality, Hidalgo, central Mexico. A) Part of the specimen observed in white light. B) Part of the specimen observed under UV light. C) Counterpart of the specimen observed under UV light.

FIGURE 4. Paratypes of Paleocharacodon guzmanae gen. and sp. nov., from the lacustrine sediments of the Paleolake Amajac, in Sanctórum, Atotonilco El Grande Municipality, Hidalgo, central Mexico, observed under UV light. A) Part of IGM 13120, a male specimen. B) Part of IGM 13121, a male specimen. C) Part of IGM 13125, a female specimen.

FIGURE 5. Details of IGM 13125 (part), paratype of Paleocharacodon guzmanae gen. and sp. nov., from the lacustrine deposits of the Paleolake Amajac, in Sanctórum, Hidalgo, central Mexico. A) Head observed under white light. B) Unborn fishes preserved in the abdominal cavity, observed under UV light. Abbreviatures: fr, frontal; la, lacrimal; let, lateral ethmoid; psp, parasphenoid (note its anterior end, bifid and with no lateral expansion); black arrows show articulated vertebrae of unborn fishes; red arrows show the pectoral fin of an unborn fish.

FIGURE 6. μCT imagens of IGM 13127, paratype of Paleocharacodon guzmanae gen. and sp. nov., from the lacustrine sediments of the Paleolake Amajac, in Sanctórum, Atotonilco El Grande Municipality, Hidalgo, central Mexico. A) Left side of the head and anterior part of the trunk. B) Quadrate and bones of both jaws exposed on the right side. C) Close-up of bones involved in the articulation quadrate-lower jaw showing the bilobed head of the quadrate and the articular facet of the anguloarticular in which there is a central participation of the retroarticular (here disarticulate and displaced from its normal position) and two enveloping projections of the anguloarticular. D) Right view of the hyoid arch. The scale only applies to A, B and D. Abbreviations: 1, 2a, 2b, 3, pores of the supraorbital sensory canal; aa, anguloarticular; bh, basihyal; br, brachiostegal ray; bst, bicuspid sharp tooth; cha, ceratohyal anterior; chp, ceratohyal posterior; cl, cleitrum; co, coronomeckelian; id; cor, coracoid; d, dentary; dsp, dermosphenotic; entp, entopterygoid; epi, epioccipital; fr, frontal; hy, hyomandibula; ih, interhyal; iop, infraopercle; let, lateral ethmoid; mx, maxilla; na, nasal; op, opercle; pa, parietal; pcli, postcleitrum inferior; pmx, premaxilla; pop, preopercle; psp, parasphenoid; ptt, posttemporal; q, quadrate; r, rib; rar, retroarticular; scl, supracleitrum; soc, supraoccipital; sp, sphenotic; sop, subopercle; sy, symplectic; uh, urohyal; ust, unicuspid sharp tooth; utt, unicuspid truncated tooth; vhh, ventral hypohyal; dubious elements; within parenthesis: i, internal view (if it is not indicated, the structures show the right view); l, left; r, right; va, ventroanterior view; vp, ventroposterior view.

FIGURE 7. Close-up of IGM 13130, a specimen of Paleocharacodon guzmanae gen. and sp. nov. of unknown sex. A) Disarticulated bones of the head under white light. B) Line drawing based on A. Abbreviations: fr, frontal (dorsal surface); let, lateral ethmoid (lateral surface); mx, maxilla (dorsal surface of the proximal end); psp, parasphenoid (dorsal surface of the ocular part); 2b, 3a, 3b, 4a,4b, 5a, 5b, 6, and 7 are the openings of the supraorbital canal preserved in the frontal.

FIGURE 8. Osteological details of Paleocharacodon guzmanae gen. and sp. nov., from the Paleolake Amajac deposits, Sanctórum, Mexico. A) Close-up of the right side of the head of IGM 13118 showing the medial surface of the left otoliths. B) Left anguloarticular bone of IGM 13121 showing the articular facet for the quadrate formed by a surrounding C-shaped process (note this in the shadow below), as well as the central participation of the retroarticular bone that here is empty (see the red line) because this bone is not preserved. C) Right dentary bone and associated teeth of the part of IGM 13121. D) Suspensorium of IGM 13123. E) Dorsal surface of the lateral process of right sphenotic, preserved in IGM 13130. F) Abdominal region of IGM 13130. G) Close-up of pharyngeal teeth preserved in the counterpart of IGM 13121. Abbreviations: aa, anguloarticular; aadp, anguloarticular dorsal process; aapp, anguloarticular anterior process;ast, asteriscus otolith; bst, bicuspid sharp tooth; cor, coracoid; d, dentary; d, dentary; entp, entopterygoid; epn, epineural; faq, articular facet for the quadrate; hy, hyomandibula; lap, lapillus otolith; mx, maxilla; pal, palatine (note the anterolateral projection in the head); pmx, premaxilla; pmx, premaxilla; pop, preopercle; q, quadrate; r, rib; sag, sagittal otolith; sc, scale; sop, subopercle; sp, sphenotic; sy, symplectic; ust, unicuspid sharp tooth. The red arrow in B shows the space in the articular facet for the square (which should be occupied by the retroarticular bone) surrounded by the pair of curved processes of the anguloarticular that also forms part of such facet (see also the shadow of the process)

FIGURE 9. Left sagittal otolith of Paleocharacodon guzmanae gen. and sp. nov., IGM 13119, a male specimen from the Pliocene Paleolake Amajac deposits in Sanctórum, Hidalgo, Mexico, observed through SEM. Abbreviations: AH, antirostrum height; AL, antirostrum length; DL, dorsal length; ea, exisura angle; maximum height; ML+RL, total length; ML, medial length; pa, posterior angle; pva, posteroventral angle; RH, rostrum heigh; RL, rostrum length; VL, ventral Length (See Table 3).

FIGURE 10. Bones of the jaws preserved in IGM 13121, a female specimen of Paleocharacodon guzmanae gen. and sp. nov., from the Paleolake Amajac deposits, Sanctórum, Mexico. A) closeup of the anterior part of the head under white light. B) Line drawing based on A. Abbreviations: aa, anguloarticular; bst, sharp bicuspid tooth; co, coronomeckelian; d, dentary; mx, maxilla; pl, palatine; pmx, premaxilla; sop, subopercle; ust, unicuspid sharp tooth. In parenthesis i, internal view; l, left; r, right.

FIGURE 11. Jaw teeth of Paleocharacodon guzmanae gen. and sp. nov., IGM 13130, a male specimen from the Pliocene Paleolake Amajac deposits in Sanctórum, Hidalgo, Mexico, observed under UV light. A) Short acute bifid tooth from a lateral position of the principal row of premaxilla; B) long acute bifid tooth from the anterior position of the principal row of the dentary; C) villiform tooth from posterior rows on the dentary anterior region.

FIGURE 12. Pelvic fin of Paleocharacodon guzmanae gen. and sp. nov., from the Pliocene Paleolake Amajac deposits in Sanctórum, Hidalgo, Mexico. A) Preserved in IGM 13129, observed under UV light. B) Preserved in part of IGM 13121, observed under white light. Abbreviations: afpvr, articular facet for pelvic fin rays; isp, ischial process; lp, lateral process of pelvic fin bone; mp, medial pelvic; pvb, pelvic bone; pvfr, right pelvic fn ray; r, rib; (l), left; (r), right.

FIGURE 13. Impaired fins of Paleocharacodon guzmanae gen. and sp. nov., from the Pliocene Amajac Paleolake deposits in Sanctórum, Hidalgo, Mexico, observed under UV light. A) Dorsal fin of IGM 13129 (sex undefined; the distal ends branched and segmented of the dorsal rays are covered). B) Anal fin of IGM 13121 (male with anal rays 2 and 13 only preserved as impressions; since the bone of tips of these rays is not-preserved, the arrows show the anterior short rays forming the andropodium and the posterior rays forming a rounded fin). Abbreviations: afr, anal fin ray; amp, anal medial pterygiophore; app, anal pterygiophore; dfr, dorsal fin ray; dmp, dorsal medial pterygiophore; dpp, dorsal proximal pterygiophore.

FIGURE 14. Caudal skeleton of Paleocharacodon guzmanae gen. and sp. nov., IGM 13119, from the Pliocene Paleolake Amajac deposits in Sanctórum, Hidalgo, Mexico. A) Specimen observed under UV light. B) Idealized line drawing of A. Abbreviations: cc, caudal complex (= preural centrum 1+ ural 1 and 2); ep, epural; hspu, hemal spine of preural centrum; ihf, interhypural foramen; ksp, keel-shaped process; nspu, neural spine of preural centrum; ph, parhypural; pu, preural centrum; sc, scale; ssp, spine-shaped process; ur, urostyle.