A juvenile subfossil crocodylian from Anjohibe Cave, Northwestern Madagascar

Abstract

Madagascar's subfossil record preserves a diverse community of animals including elephant birds, pygmy hippopotamus, giant lemurs, turtles, crocodiles, bats, rodents, and carnivorans. These fossil accumulations give us a window into the island's past from 80,000 years ago to a mere few hundred years ago, recording the extinction of some groups and the persistence of others. The crocodylian subfossil record is limited to two taxa, Voay robustus and Crocodylus niloticus, found at sites distributed throughout the island. V. robustus is extinct while C. niloticus is still found on the island today, but whether these two species overlapped temporally, or if Voay was driven to extinction by competing with Crocodylus remains unknown. While their size and presumed behavior was similar to each other, nearly nothing is known about the growth and development of Voay, as the overwhelming majority of fossil specimens represent mature adult individuals. Here we describe a nearly complete juvenile crocodylian specimen from Anjohibe Cave, northwestern Madagascar. The specimen is referred to Crocodylus based on the presence of caviconchal recesses on the medial wall of the maxillae, and to C. niloticus based on the presence of an oval shaped internal choana, lack of rostral ornamentation and a long narrow snout. However, as there are currently no described juvenile specimens of Voay robustus, it is important to recognize that some of the defining characteristics of that genus may have changed through ontogeny. Elements include a nearly complete skull and many postcranial elements (cervical, thoracic, sacral, and caudal vertebrae, pectoral elements, pelvic elements, forelimb and hindlimb elements, osteoderms). Crocodylus niloticus currently inhabits Madagascar but is locally extinct from this particular region; radiometric dating indicates an age of ∼460-310 years before present (BP). This specimen clearly represents a juvenile based on the extremely small size and open sutures/detached neural arches; total body length is estimated to be ∼1.1 m (modern adults of this species range from ∼4-6 m). This fossil represents the only juvenile subfossil crocodylian specimen reported from Madagascar.

Keywords: Anjohibe Cave; Crocodylus niloticus; Holocene; Madagascar; Pleistocene; Subfossil; Voay robustus.

Figure 1. Locality Map.

Map showing location of the Anjohibe Cave system in Northwest Madagascar.

Figure 2. UAP-03.791, crocodylian specimen recovered from Anjohibe Cave, Northwestern Madagascar.

Scale bar = 10 cm.

Figure 3. Skull of UAP-03.791.

(A) dorsal view; (B) right lateral view;. (C) ventral view; (D) occipital view. Top scale bar = 10 cm; Bottom scale bar = 5 cm. Abbreviations: bo, basioccipital; ect, ectopterygoid; eo, exoccipital; eoa, external otic aperture; f, frontal; fm, foramen magnum; ic, internal choana; j, jugal; lac, lacrimal; lcf, lateral carotid foramen; mx, maxilla; m5, maxillary alveolus 5; ns, nasal; oc, occipital condyle; orb, orbit; pa, parietal; pmx, premaxilla; pm 4, premaxillary alveolus 4; po, postorbital; prf, prefrontal; pt, pterygoid; q, quadrate; qj, quadratojugal; s.mx, sutural surface on palatine for maxilla; soc, supraoccipital; sq, squamosal; vf, vagus foramen.

Figure 4. Postcranial remains of UAP-03.791.

(A) Partial left humerus, dorsal view; (B) left coracoid, dorsal view; (C) partial right scapula, ventral view; (D) left radiale, dorsal view; (E) right radiale, dorsal view; (F) metacarpal, dorsal view; (G) partial left ilium, lateral view; (H) partial left ischium, medial view; (I) partial left pubis, medial view; (J) partial left tibia, dorsal view; (K) partial left fibula, dorsal view; (L) left calcaneum, lateral view; (M–O) left metatarsals. Scale = 5 cm

Figure 5. Right maxilla.

Medial view of the right maxilla showing the wall of the caviconchal recess. (A) UAP-03.791 Anjohibe Cave specimen. (B) Uncatalogued UA specimen, Voay robustus. Abbreviations: ccr, medial wall of the caviconchal recess; cr, caecal recesses on the wall of the caviconchal recess; s.n, sutural surface with the nasal; s.pmx, sutural surface with the premaxilla. Scale = 1 cm.

Figure 6. Mandible of UAP-03.791.

(A) Doral view; (B) Right lateral view. Abbreviations: d4, alveolus for dentary tooth number 4; emf, external mandibular fenestra; faa, articular foramen aereum; gf, glenoid fossa of articular. Scale = 10 cm.

Figure 7. Teeth found in association with UAP-03.791.

(A) anterior tooth in (l-r) lingual view, labial view, and lateral view; (B) posterior tooth in (l-r) lingual view and labial view. Abbreviations: ca, carinae; str, striations. Scale = 1 cm.

Figure 8. Neurocentral suture closure.

Elements of the vertebral column showing degree of disarticulation and neurocentral suture closure. (A) atlas elements; (B) axis; (C) cervical vertebra 6; (D) posterior thoracic vertebra; (E) anterior thoracic vertebra; (F) first sacral vertebra. Abbreviations: hyp, hypapophysis; ic, intercentrum; na, neural arch; ns, neurocentral suture; od, odontoid process; pro, proatlas. Scale = 1 cm.

Figure 9. Skull shape comparison.

Dorsal view. (A) FMNH 37216, Crocodylus niloticus. (B) UAP-03.791, Anjohibe Cave specimen. (C) UCMP 140796, C. niloticus. Scale = 5 cm.

Figure 10. Ventral view of the internal choana of Crocodylus niloticus and Voay robustus.

(A) uncatalogued UA specimen, Voay robustus (left) and uncatalogued UA specimen, Crocodylus niloticus (right), Scale = 2 cm. (B) Subfossil specimen UAP-03.791, Scale = 10 cm; (C) computed tomography image of Crocodylus niloticus, FMNH 37216, Scale = 5 cm. Abbreviations: cn, choanal neck; ic, internal choana; oc, occipital condyle.

Figure 11. Squamosal horns. Left lateral view and posterior view of skulls showing degree of squamosal horn development.

(A) uncatalogued UA specimen, Crocodylus niloticus. (B) uncatalogued UA specimen, Voay robustus. Skulls have been scaled to the same size for comparison. Abbreviation: sh, squamosal horn. Scale = 10 cm.

Figure 12. Posterior view of skulls showing degree of squamosal horn development in young individuals.

(A) UAP-03.791, Anjohibe Cave specimen. (B) FMNH 37216, Crocodylus niloticus. Abbreviation: sh, squamosal horns. Scale = 5 cm.