Neogene and Extant Capybara footprints Porcellusignum conculcator revisited

MANUEL ROJAS MANRÍQUEZ; MELCHOR, RICARDO N.; FEOLA, SILVERIO F.

Florianópolis

Congreso; ICHNIA 2024 - The 5th International Congress on Ichnology; 2024

ICHNIA

The vertebrate ichnotaxon Porcellusignum conculcator Angulo and Casamiquela, 1982was defined for the Neogene Río Negro Formation (Río Negro province, Argentina) and has beenrecognized with different criteria by several authors. The purpose of this contribution is to reviewthe ichnotaxon to clarify its identification in the type area and formation, as well as to documentadditional findings in the Late Miocene Las Flores Formation (San Juan Province). In addition,the fossil material was compared with extant capybara footprints. In the original description [1]the ichnotaxon was characterized as isolated footprints, essentially tridactyl (not excluding sometetradactyl impressions), with punctiform digit imprints, connected by an arcuate segment, andapproximately 10.5 cm long. However, subsequent studies [2,3] in the same unit recognized thatthis ichnotaxon shows tridactyl pes and tetradactyl manus prints. The pes is 8.5 cm long and 9 cmwide and the manus is 9.5 cm long and 10 cm wide. Additionally, these studies described themorphology as digitigrade and noted the presence of a rounded palmar impression. These trackswere assigned to Hydrocoeridae and are comparable to extant capybaras. In Argentina, thestratigraphic record of capybaras is documented since the Early Miocene [4].This study was based on observations on Porcellusignum conculcator from the type unit,the Río Negro Formation (Upper Miocene-Lower Pliocene). These trace fossils were documentedin a sedimentary paleoenvironment of interdune lakes. Additionally, an analysis was conductedon footprints from the Museo de Ciencias Naturales, Universidad Nacional de San Juan (specimenPVSJ 855) from the Upper Miocene Las Flores Formation). This unit is essentially lacustrine. Tocompare with the fossil material, extant capybara footprints and trackways produced on sandysubstrates were photographed.The ichnotaxon is represented by abundant topotypic material including a singlequadrupedal trackway composed by seven footprints and several isolated footprints or manus-pessets. All the material was preserved in negative hyporelief. The 1.38 m long trackway was slightlycurved, narrow (trackway breadth = 0.18 m) and composed of alternating tetradactyl manus andtridactyl pes prints, with the former in the left side and the latter on the right side. Manus printswere rotated inward (18°), whereas pes prints were roughly parallel to the midline. The manusare paraxonic and semiplantigrade to plantigrade, with four short digit imprints with blunt tips,and a rounded “heel”. Manus prints are wider than they are long, with an average length of 7.5cm (n=14) and an average width of 9.3 cm. The average total divarication is 88°. The pes isfunctionally tridactyl and mesaxonic, although deeper footprints have a shallow imprint of afourth digit. Pes prints are digitigrade to semiplantigrade with a shallowly imprinted rear end. Pesprints are slightly wider than long with an average length of 7.5 cm (n = 11) and average widthof 7.8 cm (n= 13), and the corresponding footprint length/width ratio is 0.97. Average totaldivarication is 81° (n = 6). Footprints are commonly deeply set (Fig. 1A-B).The specimen from the Las Flores Formation is a 50 cm long, straight, and narrow (15cm wide) trackway composed of three consecutive manus-pes sets and two incomplete prints.The manus print exhibits an outward rotation of 20°, while the pes prints also show an outwardrotation of 10°. Manus are paraxonic, semiplantigrade to plantigrade, with four short digits endingin rounded tips and a subrounded heel. The manus are wider than they are long, with an average length and width of 4.1 and 6.2 cm, respectively. The total divarication is 150°. The pes arefunctionally tridactyl and mesaxonic. The pes are wider than they are long, with an average lengthof 5.4 cm and an average width of 6.1 cm. The average total divarication is 71°. Footprints arecommonly deeply set (Fig. 1C). This material was referred to a mammal of the familyHegetotheridae [5].Extant capybara footprints included up to 300 cm long and 30 cm wide trackways andisolated manus pes sets. The trackway consists of manus-pes pairs, with the manus positionedbehind the pes. The manus impression exhibits an outward rotation of 6°, while the pesimpressions also show an outward rotation of 20°. The manus print is paraxonic and plantigradeand have four short digits with rounded tips and a subrounded heel. In general, the digits areconnected to the palm pad, with the greatest depth of the impressions observed in the palm padand the distal region of digits III, IV, V. In contrast, digit II is disconnected from the palm padand is shallower, sometimes are absent. The manus print is wider than long, with an averagelength and width of 9.5 and 11.8 cm, respectively. The total divarication is 126°. The pes print isfunctionally tridactyl and mesaxonic, with a notch in the posterior margin. In general, the digitsare discontinuously connected to the palm pad. The greatest depth of the footprints is observed inthe palm pad and the distal region of digits II, III, and IV. The pes print is slightly wider thanlong, with an average length of 11.2 cm and an average width of 11.3 cm (Fig. 1D).Comparison of material of P. conculcator from the type area, similar material from theSan Juan Province, and extant capybara footprints, reveals morphological similarities, suggestinga common or related producer. All of them exhibit an organization in manus-pes pairs, composedof tridactyl pes and tetradactyl manus prints. Additionally, the manus shows a rounded heel withshort digits that have rounded distal terminations. Another shared characteristic is the ratio oflength to width of the footprints, always wider than long. In both feet and hands, this ratio isapproximately 0.94 and 0.77, respectively. Notable differences are identified, such asdivarication, foot postures (plantigrade-digitigrade), and posterior foot morphology (rounded inthe fossil record vs. concave in the present). These differences are likely strongly influenced bythe type of substrate in which the organism left its traces. Fossil footprints are typically found inlacustrine mudflats (high water saturation), while fresh footprints are found in sandy substrateswith lower water saturation. These substrate characteristics could have affected the stability ofthe producer, influencing its posture, depth of prints, variation of the interdigital angle, andconsequently, the formation of the footprints. Dimensional differences can possibly be attributedto a change in producer size, either due to different ontogenic stages or generic differences.The combination of morphological features such as a quadrupedal trackway, coexistenceof tridactyl and tetradactyl footprints, rounded palmar pads, and digits with rounded and deepdistal ends in a swampy sedimentary environment are distinctive features of Porcellusignumconculcator. The most likely producer is an organism belonging to Hydrocoeridae. In addition,features such as foot posture, divarication, and the registration of digit II suggest different formsof adaptation and behavior of the producer, depending primarily on the type of substrate itinhabits.