Lacantunia enigmatica, Rocío Rodiles-Hernández, Dean A. Hendrickson, John G. Lundberg & Julian M. Humphries, 2005

Rocío Rodiles-Hernández, Dean A. Hendrickson, John G. Lundberg & Julian M. Humphries, 2005, Lacantunia enigmatica (Teleostei: Siluriformes) a new and phylogenetically puzzling freshwater fish from Mesoamerica., Zootaxa 1000, pp. 1-24 : 10-20

Lacantunia enigmatica n. sp. Rodiles-Hernández, Hendrickson & Lundberg

Table 1

Holotype. ECO-SC 3859 (Fig. 1), male, 427 mm SL, México, Chiapas, Río Usumacinta basin, Río Lacantún, Selva Lacandona, Reserva de la Biosfera Montes Azules, 16°08.083' N, 90°55.317' W (Fig. 3), Rodiles-Hernández, 8 November 2002.

Paratypes. All Chiapas, México, Río Usumacinta basin. Museum code (following Leviton et al. (1985) but adding here ECO-SC [ECOSUR San Cristóbal, address as for first author] and ENCB-IPN [Escuela Nacional de Ciencias Biológicas - Instituto Politéc- nico Nacional - address as for IPN in Leviton et al.]) and number is followed when data are available by sex, SL mm, collection date, Reserve (Reserva de la Biosfera Montes Azules = RIBMA; Selva Lacandona = SLac [ Río Lacantún basin]), Municipio (Benemerito de las Américas [BA], Marqués de Comillas [MdC]), specific locality, Lat. N, Long. W, collector (CCS=Celedonio Chan-Salas; EVV=Ernesto Velázquez-Velázquez; SDC=Sara Domínguez-Cisneros; RRH= Rocío Rodiles-Hernández). * indicates articulated and disarticulated material; ** cleared and stained preparation; *** HRXCT data examined: ANSP 178696**, female, 280 mm, 14 May 1996, RIBMA, Río Lacanjá, 16° 25.92’ N, 90° 51.8’ W, RRH ; ANSP 178697, female, 280 mm, 19 May 1997, RIBMA, Río Lacanjá, 16° 26.75’ N, 90° 51.63’ W, CCS ; ANSP 178698*, male, 318 mm, 20 May 1997, SLac, MdC, Zamora Pico de Oro, 16° 20.10’ N, 90° 50.58’ W, RRH ; ECO-SC 1166, female, 406 mm, 01 Mar. 1997, SLac, MdC, Zamora Pico de Oro, 16° 22.8’ N, 90° 44.99’ W, RRH ; ECO-SC 3856, female, 384 mm, 20 May 2001, SLac, MdC, entre Reforma Agraria y Zamora Pico de Oro, 16° 20.10’ N, 90° 50.58’ W, RRH ; ECO-SC 3858, male, 320 mm, 3 May 2002, SLac, MdC, Reforma Agraria, 16° 15.31’ N, 90° 51.99’ W, RRH ; ENCB-IPN 5787, male, 353 mm, 20 May 1997, SLac, MdC, Zamora Pico de Oro, 16° 20.10’ N, 90° 50.58’ W, RRH ; IBUNAM 12739, female, 282 mm, 20 May 2001, SLac, MdC, entre Reforma Agraria y Zamora Pico de Oro, 16° 15.31’ N, 90° 51.99’ W, RRH ; TNHC 29071***, female, 347 mm, 5 Dec. 1998, SLac, BA, Puente Lacantún, 16° 32.45’ N, 90° 41.7’ W, EVV ; TNHC 29072***, female, 223 mm, 20 May 1997, SLac, MdC, Zamora Pico de Oro, 16° 20.10’ N, 90° 50.58’ W, RRH ; UANL 15259, female, 351 mm, 21 Aug. 1998, SLac, MdC, Río Chajulillo, 16° 5.57’ N, 90° 57.47’ W, EVV ; UMMZ 243699, female, 295 mm, 20 Mar. 1997, RIBMA, Río Lacanjá, 16° 24.45’ N, 90° 49.46’ W, CCS ; USNM 378035, male, 354 mm, 11 Dec. 1997, RIBMA, Río Lacanjá, 16° 25.525’ N, 90° 50.986’ W, EVV .

Non-type specimens of Lacantunia enigmatica . ANSP 178695, female, 330 mm, 21 Aug. 1998, SLac, MdC, Río Chajulillo, 16° 5.57’ N, 90° 57.47’ W, EVV ; CAS 220134, female, 290 mm, 21 Sep. 1997, SLac, MdC, Ejido Reforma Agraria, 16° 15.31’ N, 90° 51.99’ W, CCS ; ECO-SC 1239-2, male, 314 mm, 21 Sep. 1997, SLac, MdC, Ejido Reforma Agraria, 16° 15.31’ N, 90° 51.99’ W, CCS ; ECO-SC 1363-3, female, 308 mm, 20 May 1997, SLac, MdC, Ejido Zamora Pico de Oro, 16° 20.10’ N, 90° 50.58’ W, RRH ; ECO-SC 1368-2, 274 mm, 19 May 1997, RIBMA, Río Lacanjá, 16° 26.75’ N, 90° 51.63’ W, CCS ; ECO-SC 1426, female, 319 mm, 19 May 1997, RIBMA, Río Lacanjá, 16° 26.94’ N, 90° 51.84’ W, EVV ; ECO-SC 1511-2, female, 302 mm, 11 Dec. 1997, RIBMA, Río Lacanjá, 16° 25.525’ N, 90° 50.986’ W, EVV ; ECO-SC 2138, male, 356 mm, 21 Mar. 1998, RIBMA, Río Lacanjá, 16° 24.581’ N, 90° 49.502’ W, EVV ; ECO-SC 2582-1, male, 384 mm, 9 Sep. 1998, SLac, MdC, Arroyo Caribe, 16° 34.577’ N, 90° 42.342’ W, EVV ; ECO-SC 2582-2, female, 344 mm, 9 Sep. 1998, SLac, MdC, Arroyo Caribe, 16° 34.577’ N, 90° 42.342’ W, EVV ; ECO-SC 4005, 360 mm, 7 Feb. 2003, SLac, MdC, Reforma Agraria, 16° 15.31’ N, 90° 51.99’ W, CCS ; ECO-SC 4006**, male, 370 mm, 15 May 2003, SLac, MdC, Reforma Agraria, 16° 15.31’ N, 90° 51.99’ W, CCS ; ECO-SC 4008, female, 340 mm, 15 Jun. 1997, RIBMA, Río Tzendales, CCS ; ECO-SC 4009**, female, 322 mm, 12 Jun. 1977, SLac, MdC, not specified, CCS ; ECO-SC 4125, female, 340 mm, 24 Nov. 2003, SLac, MdC, Reforma Agraria, 16° 15.31’ N, 90° 51.99’ W, CCS ; ECO-SC 4126, male, 330 mm, 30 Jan. 2004, SLac, MdC, Reforma Agraria, 16° 15.31’ N, 90° 51.99’ W, CCS .

Diagnosis: Same as genus.

Description: Maximum size exceeding 400 mm SL. Proportional measurements are given in Table 1. Body moderately elongate, markedly depressed anteriorly, strongly compressed posteriorly. Dorsal profile rises vertically from nearly terminal mouth on snout, then in a sharply convex curve from snout to above orbit, then straight and rising gently to occiput; most specimens with low nuchal hump before dorsal-fin origin; profile continuing horizontally below and behind dorsal fin to adipose-fin origin; falling gently below adipose to caudal-fin base. Ventral profile abruptly convex downward from mandible tip to mental barbels; gently convex or straight along head, pectoral girdle and abdomen to pelvic girdle; straight to anal-fin origin, finally rising in straight line to slightly concave curve to caudal-fin base.

Cross-sectional shape flattened oval at vertical through middle of eye; head depth at mid-eye much less than head width at same level, about equal to distance between posterior nares, and contained 3 times in bony HL. Maximum body width across cleithra at pectoral spine insertions always greater than body depth, about 3.6-4.6 times in SL. Cross sectional shape becoming sub-quadrangular and deeper behind eye to pelvic-fin insertions, body depth about equal to width near posterior insertion of dorsal fin, increasingly compressed posteriorly onto laterally-flattened caudal peduncle and fin.

In dorsal and ventral views, snout very broad and bluntly rounded, projecting slightly beyond mandibular symphysis. Mouth scarcely subterminal, opening anteriorly; margins of closed mouth smoothly curved; no teeth exposed when mouth closed; rictus below eye; gape width about 1.3-1.6 in membranous HL; 1.8-2.1 in predorsal distance. Lips moderately papillate, more so in larger fish; lips set off by grooves from bands of jaw teeth and without accessory folds parallel to tooth rows. Fleshy rictal fold in a shallow pocket behind corner of mouth, protruding a little above general surface of head below maxillary barbel; ventral to rictal fold, a thin submandibular groove extends less than 1/2 distance to symphysis; dorsal to rictal fold a shallow groove extends anteriorly to terminate at insertion of maxillary barbel.

Each premaxillary tooth patch broad, its width about 3 times its symphyseal length; premaxillary teeth very fine, needle-like, tall and gently curved inward, especially the inner teeth; teeth arranged in irregular rows, about18 teeth along transect near symphysis in specimens about 30 cm SL. Dentary teeth like those on premaxillary; tooth band broadest near symphysis, tapering laterally onto coronoid process. No teeth on palate.

Head skin thick, concealing skull roofing bones and narrowly open cranial fontanelles, yet skull roof near midline not buried by masses of jaw musculature dorsolaterally and laterally on head and masses of epaxial muscle on nape. Supraoccipital process not evident on surface behind occiput; process very short and remote from dorsal fin base.

Four pairs of proximally flattened barbels. Maxillary barbels inserted above lip midway between verticals through posterior nostril and eye; reaching to below dorsal fin. Maxillary barbels free from upper lip, laterally compressed basally, lying in shallow groove below eye and on cheek. Nasal barbel arises from anterior rim of posterior nostril; reaching a point 3-4 times eye diameter behind eye and about equal to distance between posterior nares. Inner mental barbels closer to margin of lower jaw than to gular fold apex, not reaching edge of gill membrane and about equal to or a little greater in length than distance between their bases. Outer mental barbels inserted at level of gular fold apex; tips of outer mental barbels reach onto depressed pectoral fin to a point slightly less than length of inner fin ray. Gular fold defining a deep groove in front of hyoid arch, its sides meeting apex at less than right angle. Gill membranes anteriorly united to each other, then diverging without overlap, supported by 11-12 branchiostegal rays (two on posterior ceratohyal, 1 or 2 on joint between ceratohyals, 8 on anterior ceratohyal). Gill rakers stiffly-ossified, sharp and slender; rakers on first branchial arch 14-15, (4 upper, 10-11 lower); first, second and fifth branchial arches with single anterior rows of gill rakers, third and fourth arches with anterior and posterior rows.

Anterior nostril tubular, located dorsally on snout, remote from snout tip by about 3 times its own diameter, anterior to a line between anterior edges of maxillary barbel bases. Posterior nostril as close to anterior nostril as to eye (ca. twice its own diameter in front of eye); medially offset from a line between midpoint of anterior border of eye and anterior nostril; nostril rim a thin, hyaline membrane low or incomplete posteriorly and produced anteriorly as thick nasal barbel; aperture ovoid with its long axis parallel to longitudinal body axis.

Eye dorsolateral, centered on a vertical at about first third of bony HL, without a free orbital rim, clear corneal skin defines ocular surface and shape of eye. Ocular surface ovoid to nearly circular; pupil less than 50% of ocular surface. Eye relatively small: horizontal ocular diameter 11.2-14.2 in HL, 5.6-7.5 in interocular, 4-6.3 in snout.

Cephalic sensory canals thin; supratemporal, supraorbital, infraorbital, and preopercular canals lead to short, multiple-branched cutaneous canals and clusters of small pores on sides and top of head; cutaneous canal branches of mandibular and nasal canals not dendritic on chin and snout, respectively. Lateral line canal straight and superficial with simple, short tubes and pores, ending over hypurals.

Dorsal fin inserted a little anterior of first third of SL; its origin near a vertical at tip of inner pectoral-fin rays. 10-12 dorsal-fin rays: spinelet, spine, and 8-10 soft, branched rays; 9-11 pterygiophores. Dorsal-fin base embedded in thick tissue especially at its anterior insertion. Spinelet small, with a shallow anteromedial cavity and its limbs widely diverging at ca. 60° angle. Dorsal spine short, slender, straight, not denticulate; ossified for about half its length, otherwise segmented. Distal margin of dorsal fin rounded, secondfourth soft rays longest, last dorsal-fin ray more than 2/3 length of second soft ray.

Adipose fin large, thick; its anterior insertion at about 70% of SL, remote from dorsal fin base, by about 1.1-1.8 times dorsal base. Adipose-fin margin gently rounded, its apex at a vertical just behind middle of anal fin, ending without a short, free lobe at a weak notch in front of upper caudal fin lobe. Adipose-fin length 2.7-3.8 in SL, about 0.8-1 in HL, 2.2-2.7 in length of dorsal-fin base and 1.3-1.8 in length of anal-fin base; adipose-fin height 6-12 times in adipose fin length, 1.7-4.5 in least caudal peduncle depth.

Caudal fin slightly rounded to truncate with rounded corners. Principal caudal rays 1,7,9,1; 10-12 upper and 11-13 lower procurrent caudal-fin rays.

Anal fin inserted a little behind a vertical at adipose fin origin; fin margin rounded; middle rays the longest. Anal-fin rays 22-26 (9-10 simple). Last two anal-fin rays closely separate; 22-25 anal-fin pterygiophores. Anal-fin base embedded in thick tissue especially at its anterior insertion.

Pectoral fin with a spine and 10-11 branched rays; its margin gently rounded, third soft ray longest; no fleshy membrane along medial edge of inner ray; depressed pectoral fin reaches to a vertical between second and third dorsal -fin soft rays. Pectoral spine formed from basal half of first lepidotrich; distal half filamentous and segmented. Pectoral spine articulating base with typical siluriform dorsal, anterior and ventral processes. Shaft of spine without dentations or serrae, and terminating bluntly at transition to segmented ray.

Posterior cleithral process narrow, short and moderately sharp; surface buried in skin and weakly ornamented with few subparallel ridges. Axillary pore below postcleithral process.

Pelvic fin inserted a little behind a vertical at middle body and below midpoint of depressed last dorsal-fin ray; containing 6 rays, first simple, third longest; no fleshy membrane along medial edge of inner ray; pelvic-fin margin gently rounded; pelvic-fin length contained 6-8 times in SL and separated from anal-fin origin by half its length. Pelvic splint absent.

Total vertebrae 55-57, 22-25 precaudals and 31-33 caudals. Weberian complex including 6 vertebrae, first rib on sixth vertebra.

Urogenital papilla located in a shallow depression immediately behind anus between pelvic fins about midway along length of inner fin rays. No apparent sexual dimorphism. Distance from anus to anal fin origin about equal to length of inner pelvic ray.

Pigmentation. Back, upper sides, nape, and tympanic area laterally over swim bladder medium to dark brown in life and in alcohol pale to dark gray or grayish blue. Upper sides with small dark and irregularly scattered freckles or spots that fade in larger fish; no stripes. Lower sides and venter much lighter, cream to white anteriorly changing to light to medium gray posterior to anus and urogenital papilla or at anal-fin origin.

Dorsal fin with or without dark margin, and darker pigment concentrated in membranes between fin rays. Caudal and anal fins with broad dark margins. Adipose pigmentation like that of adjacent back and sides, with vague freckles or spots. Pectoral and pelvic fins with dorsal surfaces mostly pigmented brown to gray proximally; anterior and distal margins pallid, often strikingly so; ventral surfaces of both paired fins lighter; some individuals with evenly dark pectoral and pelvic fins.

Dorsum of head and opercles (except for light membranous margin) nearly uniform brown in life or dark gray in alcohol; no light spots over cranial fontanelles. Maxillary barbels dusky gray dorsally; often much lighter anteriorly and ventrally; increasingly lighter distally. Nasal barbels gray proximally grading to lighter tips. Membranous rim of posterior naris and tubular rim of anterior naris white. Lower sides and venter of head and inner mental barbels light cream to white. Outer mental barbels proximally with dark posterior edge, lighter distally.

Additional descriptive information on L. enigmatica , including video animations from high-resolution X-ray computed tomography (HRXCT) is available at http://www.digimorph.org and http://clade.acnatsci.org/catfishbone/.

Distribution and habitat. Lacantunia enigmatica occurs in the Ríos Lacantún and Lacanjá, tributaries of Río Usumacinta basin, Chiapas, México,

inhabiting deep (to 18 m) river channels and pools with rocks and strong eddy currents. Few specimens were taken in stream mouths.

Specimens were collected in both high and low water seasons, and generally during the night. Gut contents include fishes, crabs, prawns and large, tough seeds.

Etymology. The name enigmatica is Latin for baffling or inexplicable in reference to the unexpected discovery, obscure relationships and origin of the new catfish. We suggest the common name “Chiapas Catfish” (= “Bagre de Chiapas”) for L. enigmatica .

Discussion

The major phylogenetic lines of Siluriformes are unevenly resolved. South American Diplomystidae are the sister-group to all other catfishes or Siluroidei (Grande 1987, Arratia1987). The North American fossil Hypsidoridae (Grande 1987) are a deep clade subtending the remaining extant siluroid catfishes. The Neotropical extant Cetopsidae may also be among the most basal catfish lines (de Pinna & Vari 1995). The many remaining catfishes are placed in 31 well-supported, monophyletic families, yet few multi-family groups (Arratia et al. 2003). Phylogenetic resolution is high within many families (de Pinna 1993, Arratia et al. 2003), whereas most family interrelationships remain to be determined.

Lacantunia is not a basal catfish within or below Diplomystidae. Instead, the new catfish shares derived characters uniting all non-diplomystid catfishes into the Siluroidei (Grande 1987). These synapomorphies are: 8 upper principal caudal-fin rays (cf. primitively9 in diplomystids); barbels on the chin (cf. no mental barbels); fifth vertebra sutured to compound second-fourth (Weberian) fused vertebrae and partly covered by lamellar bone (cf. fifth vertebra with non-sutural joints and lacking covering of lamellar bone). In Siluroidei, Lacantunia and non-hypsidorid catfishes share a reduced distal maxillary arm and short medial maxillary process (cf. in diplomystids and hypsidorids maxillary arm primitively expanded and maxillary process elongated), loss of maxillary teeth (cf. teeth present), and sutures uniting anterior and posterior ceratohyals (cf. synchondral joint between ceratohyals (Grande 1987, de Pinna 1993)). Also among Siluroidei, the development of interdigitating coracoid symphyseal sutures places Lacantunia with non-cetopsid as well as non-diplomystid catfishes (cf. primitively in diplomystids and cetopsids coracoid symphyseal sutures not present; although the sutures secondarily lost in Siluridae and some Trichomycteridae) (de Pinna 1993, de Pinna & Vari 1995).

Other Río Usumacinta basin catfishes belong to three families: Ictaluridae, Heptapteridae and Ariidae. Discovery of a new catfish belonging to any of these families would be unsurprising, but such is not the case. Lacantunia resembles ictalurids more than it does heptapterids or ariids, however, Lacantunia lacks the synapomorphies uniting crown group Ictaluridae (Lundberg 1982, Lundberg 1992, Grande & de Pinna 1998): skull roof covered by jaw adductor muscles attached to prominent sagittal crest (cf. primitively, as in Lacantunia , muscles not on skull roof and sagittal crest not developed); infraorbital canal exits frontal bone (cf. canal exits sphenotic); supracleithrum with subpterotic process (cf. process absent). Lacantunia also lacks synapomorphies that place the fossil genus Astephus , an early Tertiary North American catfish, as the sister lineage to crown group Ictaluridae(Lundberg 1992, Grande & de Pinna 1998): loss of bony posterior process on pelvic girdle (cf. primitively ossified posterior process present as in Lacantunia ); continuous cartilage along posterior edge of pelvic girdle (cf. cartilage in two parts interrupted by bone); pelvic-fin rays 7 or more and intraspecifically variable (cf. pelvic-fin rays invariably 6). We found no characters suggesting a deeper sister group relationship between Lacantunia and all Ictaluridae.

Lacantunia also lacks synapomorphies of Heptapteridae and Ariidae. Heptapterids (Bockmann 1998, Bockmann & Guazzelli 2003) are diagnosed by laterally branched fourth transverse vertebral processes (cf. transverse processes primitively unbranched as in Lacantunia ); expanded posterodorsal projections of hyomandibula for insertion of levator operculi muscle (cf. process small); hyomandibula and metapterygoid not meeting dorsal to quadrate (cf. hyomandibula and metapterygoid in contact); recurved processes of ventrolateral corners of mesethmoid (cf. processes absent). Ariidae (Mo 1991, Marceniuk 2003) have a unique otic capsule with a distended bulla involving the prootic, pterotic and exoccipital bones that houses an enlarged utricular otolith (cf. primitively otic bulla absent and otolith small as in Lacantunia ).

Some presumably derived characters shared by Lacantunia and various non-hypsidorid siluroids were found. However, without supporting evidence these are not interpreted as unambiguous synapomorphies for placing the new catfish within, or as the sister lineage to, any recognized siluroid subgroup. In previous studies done without Lacantunia , many of these putatively derived characters were interpreted as homoplasious similarities among different catfish groups. For example, the palatine bones of Cetopsidae (de Pinna & Vari 1995), some Claroteidae (Mo 1991) and Lacantunia are similarly truncated anteriorly with enlarged anterior palatine cartilages extended medially onto the lateral ethmoid condyle (Figs. 4A, 7). Siluroids primitively have a more elongate palatine with a small anterior cartilage separate from the lateral ethmoid condyle. Based on character evidence more strongly favoring alternative relationships, de Pinna and Vari (1995) concluded that the palatine condition of cetopsids and claroteids evolved independently. We tentatively identify the palatine condition of Lacantunia as independently derived relative to these other catfish families.

Among the most evident, widely shared derived feature found in Lacantunia and several siluroid taxa are the nasal barbels on the posterior nares (Fig. 1A, C). Posterior nasal barbels are not simple flaps of skin, but contain supportive elastocartilage cores on a cartilage base (Joyce & Chapman 1978). Absence of nasal barbels in diplomystids and cetopsids indicates that these structures evolved within siluroids, however, no Neotropical catfishes or ariids have posterior nasal barbels, and they are lacking in Old World Amphiliidae, Australoglanididae, Auchenoglanidinae, Chacidae, Malapteruridae, Mochokidae, Pangasiidae and Siluridae. Posterior nasal barbels are present in Lacantunia , Ictaluridae, and Old World Bagridae, Claroteidae, Cranoglanididae, Schilbidae, Plotosidae, Clariidae, Amblycipitidae, Akysidae, Sisoridae and Erethistidae. Our phylogenetic analysis does not identify these taxa as a monophyletic group, but indicates homoplasy of nasal barbel evolution among siluroids.

This first assessment, therefore, places Lacantunia above diplomystids, hypsidorids and cetopsids, but unresolved among the remaining monophyletic subgroups of siluroids. As shown by late Campanian to early Maastrichtian fossils of Diplomystidae, Ariidae and Doradoidei (Lundberg 1998, Gayet & Meunier 2003), diversification of modern catfishes was underway by late Cretaceous. Fossils also demonstrate that by at least Paleocene or Eocene several other catfish families and higher groups had originated: extinct Hypsidoridae, and modern Callichthyidae, Pimelodidae, Clariidae, Claroteidae, Bagridae, and Ictaluridae (Lundberg 1975, Grande 1987, Grande & de Pinna 1998, Lundberg 1998, Gayet & Meunier 2003). Significantly, some of these indicate coexistence of related “ghost” lineages yet unrecorded by fossils. For example, late Paleocene fossils of Corydoras , a living callichthyid genus, imply coeval or prior origins of confamilial genera and of other groups of Loricarioidei (Lundberg 1998, Reis 1998). A comparable early Tertiary age seems possible for distinctive, yet unresolved, siluroid lineages such as Lacantunia if these are basal relatives to any single or multi-family clades. These suggestions are testable as more is learned about the interrelationships of catfishes.

Mesoamerica is famous for its complex yet elusive biogeographic and geologic histories(Raven & Axelrod 1979, Savage 1982). The biota of Central America and México is largely composed of plants and animals with separate extralimital affinities and differing inferred ages of origin (Raven & Axelrod 1979, Savage 1982, Wendt 1998). Biogeographers have drawn on the region's rich biota to illustrate or postulate post Late Cretaceous vicariance, endemic diversification and dispersal from North and South America, Caribbean islands and suspect accretionary terranes (Myers 1966, Savage 1982, Humphries 1982, Rosen 1985, Stehli & Webb 1985, Dickinson & Lawton 2001, Miller et al. in press). Nowhere is this broad biotic mix better exemplified than in the Río Usumacinta, where a highly endemic, apparently long-isolated, aquatic biota consists of species with North American affinity (e.g. ictalurid catfishes, catostomid suckers, dermatemydid turtles) living alongside others with South American relationships (heptapterid catfishes, characins, electric knife fish, cichlids), as well as some with deeper Afro-Indo-southeast Asian affinities(genus Ophisternon of Synbranchidae (Rosen 1975)). The Usumacinta also harbors several resident freshwater representatives of Atlantic coastal groups (ariids and gobioids), and many species belonging to species-rich endemic clades in groups such as poeciliids ( Rodiles-Hernández in press), with deeper and less obvious geographic affinities.

Lacantunia enigmatica thus adds a fourth catfish clade to the Mesoamerican aquatic fauna. This biogeographically puzzling species shows no close relationship to any North or South American or marine taxon, as do the other Usumacinta siluriforms. Without a clearly identified and geographically proximate relative, a hypothesis to consider is that Lacantunia represents an ancient and basal siluroid lineage from which intermediate members have disappeared. The phylogenetic resolution needed for more definitive dating and biogeographic placement of Lacantunia will require additional evidence, including both molecular and additional morphological characters.

As enigmatic as any evolutionary question raised by Lacantunia is its late ichthyological discovery. How was such a large and conspicuous species missed until now? Whatever the reasons, Lacantunia reminds us that the most basic scientific inventory of Earth's biodiversity is woefully incomplete. Unfortunately, this fascinating, enigmatic and geographically restricted taxon is acutely threatened by anthropogenic activities including forest removal, proposed hydroelectric reservoirs, and introductions of non-native taxa. Conservation of this unique organism should be a high priority for regional natural resource biodiversity conservation and management planners.