Urumaco Research Articles

Fossil catfishes of the families Doradidae and Pimelodidae (Teleostei: Siluriformes) from the Miocene Urumaco Formation of Venezuela

ABSTRACT Four taxa are added to the fossil history of catfishes based on new identifications and descriptions of material from the Miocene Urumaco Formation in northwestern Venezuela. Three of the fossil catfishes belong to the family Doradidae (thorny catfishes); one, Doras dioneae, represents a new species, the first fossil doradid to be formally named, and the other two, Doraops cf. zuloagai and Rhinodoras cf. thomersoni, are referred to modern taxa endemic to the Maracaibo basin. The fourth fossil catfish belongs to the family Pimelodidae (long-whiskered catfishes) and is assigned to Platysilurus, a modern genus that occurs in the Maracaibo, Orinoco, Amazon, and large rivers of the Guianas. These fossil taxa inhabited the paleo-Amazon-Orinoco, a large ancient river system that drained the South American continent northward prior to the uplift of the Eastern Andes and Coastal Cordilleras and concomitant isolation of the Maracaibo basin in the Late Miocene (ca. 8 Ma). Species novum: Doras dioneae Sabaj ...

A NESTING SITE AND EGG MORPHOLOGY OF A MIOCENE TURTLE FROM URUMACO, VENEZUELA: EVIDENCE OF MARINE ADAPTATIONS IN PELOMEDUSOIDES

Abstract: Fossil eggshells from the Late Miocene Urumaco Formation of north‐western Venezuela are reported. Stereomicroscopy and scanning electron microscopy were used to examine shell gross morphology and ultrastructure, respectively. Diagnostic turtle features in the eggshells include a distinct pattern of crystal aggregations within the shell units. The eggs had an elliptical shape with a maximum diameter of c. 56·5 mm and width of c. 43·5 mm, as measured in a specimen preserving the egg's outline. Scattered clutches of broken eggshells were found embedded in one horizon of a coarse, sandy sediment, with grains not well sorted containing foraminifera and fragments of bivalve shells. The sediments are rich in ichnofossils and the reddish colour of the sandstone indicates an oxidizing environment. These facts suggest that the eggs were deposited in a beach directly facing the sea or brackish waters, perhaps near a river delta or lagoon, as is typical of the Urumaco sequence. In the same stratigraphical layer and next to one of the egg clusters, a carapace assignable to the pelomedusoid Bairdemys venezuelensis was found, suggesting that this species was a colonial nester that laid its eggs in beaches and lived in a marine or nearshore marine environment.

The fossil record of phoberomys pattersoni Mones 1980 (Mammalia, Rodentia) from Urumaco (Late Miocene, Venezuela), with an analysis of its phylogenetic relationships

Synopsis An almost complete skeleton of Phoberomys pattersoni is reported from the Urumaco Formation in northwestern Venezuela. Phoberomys attained the largest body size among known rodents and it lived during the Late Miocene and/or Early Pliocene of Argentina, Brazil and Venezuela. Phoberomys pattersoni is probably the second largest species within its genus. The lower molar enamel shows little difference in thickness between the trailing and leading edges in the first and second lophs, whereas in the third loph the trailing edge is much thinner than the leading edge. In the leading edge of the distal loph the enamel consists of two layers, each covering approximately 50% of the entire enamel thickness, one consisting of thick H unter‐Schreger bands inclined 15–20? towards the occlusal surface, the other formed by radial enamel with the prisms inclined towards the occlusal surface. The postcranial skeleton shows a mosaic of unusual features among caviomorphs (e.g. absence of a supratrochlear foramen) and shared conditions (e.g. tuber calcis of the calcaneum that is wider than deeper dorsoventrally) that evolved in parallel in different lineages. The phylogenetic position of Phoberomys is determined on the basis of a simultaneous analysis of molecular and morphological data of select rodents. Phoberomys appears as the sister group of the Recent genus Dinomys. Several derived character conditions are shared with Dinomys: presence of a site for attachment for the m. rectus femoris on the lateral side of the innominate in the shape of an elongated crest; trochlear ridges of the femur proximally convergent; medial condyle of the femur wider than the lateral one in posterior view; medial ridge of the astragalar trochlea reaches posteriorly further than the lateral one; proximal portion of the coronoid process extends further anteriorly than the medial (and distal) process (or anconeal process). More than one species of Phoberomys may be present in the Urumaco Formation.

New Mylodontoidea (Xenarthra, Phyllophaga) from the Middle Miocene‐Pliocene of Venezuela

Synopsis Two new genera of Mylodontoidea, Mirandabradys and Bolivartherium, and five new species, M. socorrensis (Socorro Formation, Middle Miocene), M. urumaquensis and B. urumaquensis (Urumaco Formation, Upper Miocene), M. zabasi and B. codorensis (Codore Formation, Pliocene) are described. There is a high diversity of tardigrades from Middle Miocene to Pleistocene sites of Northern Venezuela. Almost all the recorded species are large and close to the size of the late Pleistocene Glossotherium robustum or Lestodon armatus. This implies that either the ‘faunas’ were composed only of large‐sized tardigrades, or that the smaller taxa are missing because of taphonomic bias and that the faunal diversity was actually higher than currently indicated.

Cranial variation in Bairdemys turtles (podocnemididae: Miocene of the Caribbean region) and description of new material from Urumaco, Venezuela

Synopsis Four new skulls referable to Bairdemys, from the Late Miocene Urumaco Formation in Venezuela, are described and compared with other specimens allocated to this genus. Several characters previously used to distinguish B. hartsteini (Cibao Formation, middle Miocene of Bayamon, Puerto Rico) from B. venezuelensis (Urumaco Formation) are present in the new Urumaco specimens, which show a mosaic of features from both species as originally diagnosed. Features of another Bairdemys skull from the Castillo Formation in the early Miocene of Lara State, Venezuela are unique, suggesting this is a new species, but poor preservation of known specimens prevents a formal diagnosis. In Bairdemys from Castillo the ventral surface of the secondary palate is proportionally larger in size, correlated with a smaller width of the internal nares and the distance between the foramina palatinae is relatively shorter. The ventral position of the quadrate in relation to the basisphenoid is far more posterior in the Castillo Bairdemys than in any other known specimen of this genus. The lower jaw of a Bairdemys specimen from Urumaco is described for the first time and compared to that of several Recent and fossil Pelomedusoides taxa. The specimen from Urumaco has a relatively large triturating surface, a large and wide dorsal opening of the fossa meckelii, a curved tomial ridge that slopes dorsally, a curved labial ridge that is less acute than in Podocnemis and no obvious accessory ridges comparable to ridges in Podocnemididae. Several shared features separate Bairdemys and Stereogenys from other fossil and Recent Podocnemididae.

First Megatheriines (Xenarthra, Phyllophaga, Megatheriidae) from the Urumaco (Late Miocene) and Codore (Pliocene) Formations, Estado Falcón, Venezuela

Synopsis Two new genera and species of Megatheriinae are described from the Neogene of Venezuela: Urumaquia robusta gen. et sp. nov. from the Urumaco Formation (Late Miocene) and Proeremotherium eljebe gen. et sp. nov. from the Codore Formation (Pliocene). These represent only the second Tertiary record of the subfamily at low latitudes in South America. The anatomical features of Urumaquia robusta suggest that the levels of the Urumaco Formation bearing the fauna here studied may not be correlated with the Laventan SALMA, but with Chasicoan‐Huayquerian SALMAs.

Fossil snakes from the Neogene of Venezuela (Falcón state)

Synopsis The first fossil snakes from Venezuela are described from three different Formations: Colombophis cf. C. portai and Boinae indet. from the Middle Miocene Socorro Formation, Boinae indet. from the Late Miocene Urumaco Formation and Viperidae indet. from the Pleistocene gravels at Cucuruchu. Although the Socorro and Urumaco Formations Boinae cannot be unambiguously referred to any particular taxa, the Venezuelan record, combined with the fauna from La Venta in Colombia, demonstrate that Boinae had achieved much of its more Northern distributions by the middle‐late Miocene. The occurrence of a viperid in the Pleistocene gravels at Cucuruchu is consistent with a geologically young record of the lineage in South America.

A new giant Purussaurus (Crocodyliformes, Alligatoridae) from the Upper Miocene Urumaco Formation, Venezuela

Synopsis Fossil Crocodylia are taxonomically diverse in South America, particularly in the upper Miocene Urumaco Formation (Venezuela). Among them is the giant alligatoroid Purussaurus, previously known from the upper Miocene Solimões Formation in Brasil (P. brasiliensis) and the middle Miocene of La Venta in Colombia (P. neivensis). New specimens of Crocodylia from the Urumaco Formation are described and a new species, Purussaurus mirandai, erected. This is characterised by a large, elongate and extremely flat skull, a very large narial opening comprising almost 60% of rostral length and a large incisive foramen that extends anteriorly between the fossae for the first mandibulary teeth. Purussaurus mirandai sp. nov. was the largest predator in the swampy environments and represented the top trophic level in the Urumaco assemblage. A phylogenetic analysis of 164 characters in 68 ingroup crocodile taxa supported the association of P. neivensis and P. mirandai with five synapomorphies and confirmed the sister‐group relationship of the genus with the Nettosuchidae.

A new specimen of Toxodontidae (Notoungulata) from the Urumaco formation (Upper Miocene) of Venezuela

Synopsis A new specimen of Toxodontidae, consisting of a partial mandibular ramus with three molars, is described from the Middle Member of the Urumaco Formation and compared with type and referred material from all four taxa previously reported from Venezuela and with several better known taxa of advanced Toxodontidae. Incomplete preservation and postmortem distortion make an assessment of its morphology difficult. Despite these shortcomings, the new material does not appear to represent any of the Venezuelan taxa previously described. The specimen shows a unique combination of characters including a pattern of lingual enamel folds, weakly developed metaentoconid and well‐developed ento‐hypoconid fold (m1 and m2, not in m3), subrounded and lingually expanded trigonid, transversely broad hypoconid and open labial enamel fold. In this combination of features, the new specimen most closely resembles the types of Trigodonops lopesiand Mixotoxodon larensis.

A new Megadolodinae (Mammalia, Litopterna, Protherotheriidae) from the Urumaco Formation (Late Miocene) of Venezuela

Synopsis A new genus and species of Proterotheriidae Megadolodinae, Bounodus enigmaticus, from the Upper Miocene Urumaco Formation of Venezuela, is described on the basis of a poorly preserved right maxillary fragment with brachydont cheek teeth. Teeth and alveoli preserved are interpreted as part of the alveolus of P3 and P4‐M3. The new taxon differs from Megadolodus, the only other known genus of the subfamily, in having a proportionally smaller P4 and M1, the latter elongated antero‐posteriorly and with the protocone root more mesial than that of the hypocone. Bounodus enigmaticus gen. et sp. nov., reinforces the hypothesis that the Megadolodinae represent a distinct radiation within the Proterotheriidae, the two other major clades being the Anisolambdinae and Proterotheriinae.

GOLIATH GROUPER (PISCES: SERRANIDAE) FROM THE UPPER MIOCENE URUMACO FORMATION, VENEZUELA

T he goliath grouper (family Serranidae, subfamily Epinephelinae) inhabits tropical and subtropical waters. The Epinephelinae serranids are comprised of about 159 species in 15 genera (Heemstra and Randall, 1993) and are represented in all oceans. According to Heemstra and Randall (1993) the goliath grouper Epinephelus itajara (Lichtenstein, 1822) occurs in the eastern Atlantic Ocean from Florida to Brazil, throughout the Gulf of Mexico and in the Caribbean Sea, in the western Atlantic Ocean from Senegal to the Congo, and in the eastern Pacific Ocean from the Gulf of California to Peru. The maximum size is about 250 cm total length and they can exceed 320 kg in weight. The grouper Epinephelus lanceolatus (Bloch, 1790) occurs throughout the Indo-Pacific region, from the Red Sea to Algoa Bay, South Africa, and eastward to the Hawaiian and Pitcairn Islands, and in the western Pacific Ocean from southern Japan to Australia in the south. The maximum size is about 231 cm total length (Schultz, 1966) and 400 kg in weight (Fourmanoir and Laboute, 1976). These two species are the largest serranids in the world. Sadovy and Eklund (1999) noted that males reach a maximum age of 26 and females 37 years in a population of E. itajara . Fossil specimens of Epinephelus Bloch, 1793 are known only from Caribbean Neogene otoliths (Nolf, 1976; Nolf and Stringer, 1992; Nolf and Aguilera, 1998; Stringer, 1998; Aguilera and Rodrigues de Aguilera, 1999, 2001) and from Atlantic Neogene skeletal remains (Purdy et al., 2001). In this paper we describe the first large and complete fossil neurocrania of Epinephelus itajara and associated bones obtained from the upper Miocene Urumaco Formation (upper member) of Venezuela. These specimens provide an example of a very long-lived tropical marine species, and the explanation for its present amphitropical American …

The late Miocene Phractocephalus catfish (Siluriformes: Pimelodidae) from Urumaco, Venezuela: additional specimens and reinterpretation as a distinct species

Based on additional specimens the fossil pimelodid catfish from the upper Miocene Urumaco Formation, Falcón State, Venezuela originally assigned to the extant species Phractocephalus hemioliopterus is described as a new, extinct species. †Phractocephalus nassi n. sp. is diagnosed by the following combination of characters: 1) posterior half of frontals and anterior half of supraoccipital with elongate, coarse ridges and sulci in addition to reticulating ridges and subcircular pits; 2) very broad and ornamented mesethmoid bone; 3) lateral ethmoid margin convex and eliminating orbital notch but not projecting far anteriorly over palatine condyle; 4) anterior cranial fontanelle closed or represented by a small pit; 5) supraoccipital process rounded laterally and posterolaterally, concave posteriorly and completely concealing Weberian complex in dorsal view; 6) opercle covered with reticulating ridges and pits; 7) cleithrum coarsely ornamented along ventral edge and bulging outward lateral to spine articulation; 8) pectoral spine mostly ornamented with coarse ridges and sulci. †Phractocephalus nassi is compared to modern P. hemioliopterus and an undescribed extinct species from the upper Miocene Solimões Formation, Acre, Brazil. New diagnostic characters of Phractocephalus are presented that apply to the modern and fossil species, including: 1) ornamentation of skull, pectoral girdle and fin spines comprising a coarse meshwork of reticulating ridges surrounding rounded pits plus some elongate ridges and sulci; 2) supraoccipital posterior process greatly expanded laterally and posteriorly behind occipital wall; 3) lateral ethmoid and sphenotic broadly sutured behind eye; 4) anterior cranial fontanelle reduced or completely closed and posterior cranial fontanelle closed; 5) vomerine tooth plate large, roughly pentagonal to triangular in form, and with fine teeth. Today Phractocephalus ranges widely throughout the lowland Orinoco, Amazon and Essequibo basins. However, the genus does not occur west or north of the Andes or Venezuelan coastal ranges. Recognizing the Urumaco Phractocephalus as a distinct species does not alter the obvious conclusion that this catfish marks a large river connection between the Caribbean coastal region and the Orinoco system during at least part of the Neogene. Other Urumaco fossils show this same biogeographic relationship.

Bairdemys, a New Side-Necked Turtle (Pelomedusoides: Podocnemididae) from the Miocene of the Caribbean

A new genus, Bairdemys, is erected for two species of side-necked turtles. Bairdemys venezuelensis (Wood and Díaz de Gamero, 1971) from the late Miocene Urumaco Formation of Venezuela is represented by four skulls and a number of shells. Bairdemys hartsteini from the Miocene of Puerto Rico is known from a skull. Bairdemys is a member of the Podocnemididae because it possesses a cavum pterygoideus, and is related to the Shweboemys Group because it has a well-developed secondary palate. It differs from all other Podocnemididae in having large ventral convexities on the secondary palate, the eustachian tube separated by bone from the fenestra postotica, and the frontal and prefrontal strongly convex dorsally.

Neotectonic and paleoseismicity studies on the Urumaco Fault, northern Falcón Basin, northwestern Venezuela

The northern Falcón Basin in northwestern Venezuela is affected by several small active faults, subordinated to the major right-lateral east–west-trending Oca–Ancón Fault System. A set of prominent NW–SE right-lateral faults — synthetic shears — such as the Urumaco, Rı́o Seco, Lagarto and La Soledad faults, stands out among those. The Urumaco Fault, located between the Lagarto and Mitare rivers (in the Urumaco Trough, west of Coro), presents a rather complex active fault trace that comprises two NW–SE fault segments linked by an ENE–WSW reverse echelon, all showing a restraining stepover geometry. Its western segment seems to continue to the north at sea. Conversely, the eastern one dies out on land and its northern tip ends in a transtensive horse-tail structure, that disrupts an Early Pleistocene conglomerate. This same unit is flexed and upheaved some 30 m at the restraining overlap. The kinematics and present stress tensor, the latest activity and the seismogenic potential of the eastern segment of the Urumaco Fault, have been assessed at a set of three river cuts of an ephemeral tributary stream of the Urumaco River, 3 km north of the Urumaco village, where the Urumaco Formation is truncated by a Late Pleistocene terrace ( 14C date of 20,700±950 yr BP at the base) of the Urumaco River. On the one hand, one of these outcrops features the Urumaco Fault affecting the Late Miocene Urumaco Formation, which comprises two prominent fault planes disposed as a wedge. The southwestern bounding plane juxtaposes two different sequences whereas the northeastern one does not, implying different slip behavior. In fact, the northeastern plane shows oblique-slip striations (29°N, normal-dextral), whereas the other one shows perfectly horizontal striations (right-lateral). On the other hand, both updip plane prolongations in the overlying alluvial unit are not so sharp, if the 17-cm throw of the erosive bottom of such terrace measured at the lowermost part of the southwestern plane is regarded as an artifact. However, a mudflat deposit within this unit is bent with a 14-cm throw right above the northeastern fault plane clearly affecting the underlying Miocene unit. The estimated total offset per event allows to infer the occurrence of two individual events of magnitude ranging between M s 5.8 and 6.4 on this strand of the eastern segment of the Urumaco Fault in the last 20,000 yr.

The changing course of the Orinoco River during the Neogene: a review

Recent studies have revealed that the rich fresh-water vertebrate fossil fauna of the Urumaco Formation in northwestern Falcón, of Late Miocene age, belongs biogeographically to the Orinoco River system. This finding, together with other evidence, can be used to chart the changing course of the Orinoco River during the Neogene and thus date the uplift of various mountain ranges that affected the change in this course. Presence of the Misoa delta in the Middle Eocene of the Maracaibo Basin has been suggested as evidence for a large river, running in a south-north direction and draining the Central Cordillera of Colombia and the Guayana Highlands. The late Middle Eocene uplifting of Western Venezuela changed the paleogeographic setting, and a new delta-building shifted to the south, represented by the extensive Carbonera Formation of Late Eocene to Oligocene age in the Llanos Basin of Colombia and Southwestern Venezuela. In the earliest Miocene, the Falcón Basin, situated to the east of the Maracaibo Basin, was primarily the site of marine shale sedimentation. Stratigraphic and microfaunal evidence suggests the possibility of the presence of the proto-Orinoco River in the western margin of the basin. In any case, from the mid Early Miocene until the end of the Middle Miocene, a fluvio-deltaic sequence was deposited in the northwestern Falcón, with a total thickness of 4.7 km. The Urumaco Formation overlies this sequence and its vertebrate fauna indicates that the river which built the prior delta was definitely the proto-Orinoco. The deformation and uplift of the Eastern Cordillera of Colombia and of the southwestern end of the Mérida Andes caused the distal course of the river to be deflected to a west-east direction. A late Middle Miocene age for this change of course is precisely dated on marine microfaunal evidence. The Late Miocene sedimentary record of the Orinoco River delta appears east of Maturín, in the Eastern Venezuela Basin, and Pliocene and Pleistocene deltaic sediments are especially evident in Trinidad and the Columbus Basin.

The Changing Course of the Orinoco River During the Neogene

Recent studies have revealed that the varied fresh-water vertebrate fossil fauna of the Urumaco Formation in northwestern Falcon, of Late Miocene age, belongs biogeographically to the Orinoco River system. This makes possible to chart the changing course of the Orinoco River through the Neogene. The Misoa delta of Lake Maracaibo, of middle Eocene age, was probably constructed by a proto-Orinoco river that ran in a south-north direction, draining the Central Cordillera of Colombia and the Guayana Highlands. The late Eocene uplifting of western Venezuela changed the paleogeographic setting, with no evidence of delta-building during the Oligocene. In the earliest Miocene, the Falcon Basin was the site of marine shale sedimentation. In the second half of the Early Miocene, a deltaic sequence was initiated in northwestern Falcon that continued, except for a transgressive episode, until the end of the Middle Miocene, depositing a total thickness of over 2.5 km. The Urumaco overlies this sequence and its vertebrate fauna indicates that the river that built the previous delta was the proto-Orinoco. The initial uplifting of the northern part of the Andes and the western edge of the Caribbean Mountains can thus be dated to occur at the end of the Middle Miocene,more » causing the distal course of the river to move to a west-east direction. The sedimentary record of the Orinoco River delta appears in the Maturin Basin of eastern Venezuela at the end of the Miocene, and is especially evident in Trinidad and the Columbus Basin during the Pliocene and Pleistocene.« less

Phractocephalus hemiliopterus(Pimelodidae, Siluriformes) from the upper Miocene Urumaco Formation, Venezuela: a further case of evolutionary stasis and local extinction among South American fishes

ABSTRACT A fossil catfish skull from the upper Miocene Urumaco Formation of Falcon State, Venezuela is identified as Phractocephalus hemiliopterus (Pimelodidae), a living species of the large, lowland rivers east of the Andes. This fossil provides another example, in a growing list of cases, of a long and conservative evolutionary history for an advanced South American riverine fish. The phylogenetic position of Phractocephalus implies that other higher level lineages of Pimelodidae had originated before six Ma. This discovery corroborates geological and other biogeographic evidence for a connection of the ancient Orinoco system to the Caribbean coast river systems of Venezuela. The fossil catfish and other large aquatic vertebrates from the same area suggest former, freshwater communities on the north coast that were much richer than modern faunas. Increasing aridity and loss of large permanent rivers in post-Miocene times has presumably caused local extinction of these communities.