Friday, May 7, 2021

[Paleontology • 2021] Sahonachelys mailakavava • A New Pelomedusoid Turtle (Pleurodira: Sahonachelyidae) from the Late Cretaceous of Madagascar provides Evidence for Convergent Evolution of Specialized Suction Feeding among Pleurodires

 Sahonachelys mailakavava
Joyce, Rollot, Evers, Lyson, Rahantarisoa & Krause, 2021

 Artwork by Andrey Atuchin.

The Maevarano Formation in northwestern Madagascar has yielded a series of exceptional fossils over the course of the last three decades that provide important insights into the evolution of insular ecosystems during the latest Cretaceous (Maastrichtian). We here describe a new genus and species of pelomedusoid turtle from this formation, Sahonachelys mailakavava, based on a nearly complete skeleton. A phylogenetic analysis suggests close affinities of Sahonachelys mailakavava with the coeval Madagascan Sokatra antitra. These two taxa are the only known representatives of the newly recognized clade Sahonachelyidae, which is sister to the speciose clade formed by Bothremydidae and Podocnemidoidae. A close relationship with coeval Indian turtles of the clade Kurmademydini is notably absent. A functional assessment suggests that Sahonachelys mailakavava was a specialized suction feeder that preyed upon small-bodied invertebrates and vertebrates. This is a unique feeding strategy among crown pelomedusoids that is convergent upon that documented in numerous other clades of turtles and that highlights the distinct evolutionary pathways taken by Madagascan vertebrates.

Keywords: Maastrichtian, Maevarano Formation, Sahonachelyidae, Pleurodira, Late Cretaceous, Testudines 

Figure 1. (a) Outcrop map of Late Cretaceous and Palaeocene strata in the Mahajanga Basin of northwestern Madagascar (see inset at lower right). The location of the Berivotra Study Area is highlighted by the rectangle. The holotype specimen of  Sahonachelys mailakavava gen. et sp. nov. (UA 10581) was recovered from locality MAD05-38 in the Anembalemba Member of the Maevarano Formation (see stratigraphic relations in inset at upper left), which is of latest Cretaceous (Maastrichtian) age.
(b) Locality MAD05-38, type locality of  Sahonachelys mailakavava, view looking north-northwest.
(c) Close-up view of quarry face at locality MAD05-38 showing longitudinal cross-section of UA 10581 above (white arrow, caudal end of specimen to the right) and large fragments of specimen in foreground (black arrows).


Systematic palaeontology

Testudines Batsch, 1788 
Pleurodira Cope, 1865 
Pelomedusoides Broin, 1988 

Sahonachelyidae new clade name

Registration number. The clade name Sahonachelyidae is registered at RegNum with the number 570.

Definition. The largest extinct clade containing Sahonachelys mailakavava gen. et sp. nov.

Composition. Sahonachelyidae is currently hypothesized to consist of Sahonachelys mailakavava and Sokatra antitra, two extinct turtles from the Maastrichtian of Madagascar.

Figure 3. Sahonachelys mailakavava gen. et sp. nov., UA 10581, holotype, Late Cretaceous (Maastrichtian), Mahajanga Basin, Maevarano Formation, Madagascar. Photograph of skull with hyoid in (a) dorsal, (b) ventral and (c) left lateral views.

Figure 5. Sahonachelys mailakavava gen. et sp. nov., UA 10581, holotype, Late Cretaceous (Maastrichtian), Mahajanga Basin, Maevarano Formation, Madagascar. Three-dimensionally rendered model and line drawing of skull in (a) right lateral, (b) left lateral, (c) posterior and (d) anterior views.

ap, antrum postoticum; bs, basisphenoid; ex, exoccipital; fnh, foramen nervi hypoglossi; fpo, foramen postoticum; fr, frontal; ica, incisura columella auris; ju, jugal; mx, maxilla; op, opisthotic; pa, parietal; pal, palatine; pcf, precolumellar fossa; pf, prefrontal; pm, premaxilla; po, postorbital; pop, posterior opening of antrum postoticum; pp, posterior process of the maxilla; pt, pterygoid; qj, quadratojugal; qu, quadrate; so, supraoccipital; sq, squamosal.

Figure 9. Sahonachelys mailakavava gen. et sp. nov., UA 10581, holotype, Late Cretaceous (Maastrichtian), Mahajanga Basin, Maevarano Formation, Madagascar. Photograph and line drawing of shell in (a) dorsal and (b) ventral views. The lone Roman numerals indicate the neurals.
 Ab, abdominal scute; An, Anal scute; co, costal; Eg, extragular scute; ent, entoplastron; epi, epiplastron; Fe, femoral scute; Gu, gular scute; Hu, humeral scute; hyo, hyoplastron; hyp, hypoplastron; mdf, musk duct foramen; Ma, marginal scute; mes, mesoplastron; nu, nuchal; Pe, pectoral scute; per, peripheral; Pl, pleural; py, pygal; sp, suprapygal; Ve, vertebral scute; xi, xiphiplastron.

Sahonachelys mailakavava gen. et sp. nov.

Type locality. Locality MAD05-38, Berivotra Study Area, approximately 35 km southeast of the city of Mahajanga, Boeny Region, Madagascar. GPS coordinates are archived at the Denver Museum of Nature & Science and the University of Antananarivo and are available to qualified researchers.

Type stratum. Anembalemba Member, Maevarano Formation, dated to the Late Cretaceous (Maastrichtian, see §2 Geological setting).
Etymology. The name Sahonachelys mailakavava combines the Malagasy words ‘sahona' (pronounced sah-WHO-nah) for frog, ‘mailaka' (pronounced my-LAH-kah) for quick, ‘vava' (pronounced VAH-vah) for mouth, and the Greek word ‘chelys' for turtle to mean ‘quick-mouthed frog turtle’ in allusion to the frog-like appearance of the skull and its inferred quick mode of suction feeding.

Diagnosis. Sahonachelys mailakavava gen. et sp. nov. can be diagnosed as a member of Pan-Pelomedusoides by presence of midline contact of prefrontals; presence of processus trochlearis pterygoidei; contact of postorbital with palatine resulting in formation of expanded septum orbito-temporale; involvement of prootic and/or quadrate in formation of foramen posterius canalis carotici interni; the absence of nasals, vomer and splenial; low-domed, oval shell; sutural contact of pelvis with shell; the absence of cervical scutes; reduction in neural count resulting in midline contact of posterior costals; broad plastron; equidimensional mesoplastra; single, median gular; and cervical column consisting of procoelous vertebrae. The shell of S. mailakavava can be distinguished from those of all other pan-pelomedusoids by the presence of elongate gular that broadly crosses entoplastron and fully hinders midline contact of extragulars and humerals (also present in the kurmademydine Jainemys pisdurensis and the taphrosphyines Taphrosphys sulcatus and Ummulisani rutgersensis) in addition to broad contribution of enlarged extragulars to margin of anterior plastral lobe resulting in reduced contribution from gular. The cranium of S. mailakavava can be distinguished from those of all members of the clade formed by Podocnemidoidae and Bothremydidae by plesiomorphic retention of deep temporal emarginations that results in reduced or absent squamosal-quadratojugal and parietal-quadratojugal contacts, retention of clear exposure of prootic in ventral view, laterally open foramen jugulare posterius, and the absence of cavum pterygoidei or fossa pterygoidea. The cranium of S. mailakavava can be distinguished from those of all remaining, more basal-branching representatives of Pan-Pelomedusoides (i.e. Araripemydidae, Pelomedusidae and Atolchelys lepida), but resembles those of Sokatra antitra, Bothremydidae and Podocnemidoidae in derived presence of posterior enclosure of incisura columella auris by quadrate to exclusion of Eustachian tube; flooring of processus paroccipitalis by expanded sheet of bone formed by basisphenoid, quadrate and basioccipital; quadrate-exoccipital contact; reduced ventral exposure of prootic (the latter two characters also present in Araripemydidae); short basioccipital; and exclusion of basioccipital from occipital condyle (absent in Podocnemidoidae). Among pan-pelomedusoids, S. mailakavava uniquely resembles Sokatra antitra by the presence of an elongate posterior process of the maxilla; formation of deeply interfingered sutural contact between jugal and maxilla; reduced ventral process of quadratojugal; formation of foramen posterius canalis carotici interni by basisphenoid, quadrate and prootic; and strong forward inclination of processus articularis. These characteristics serve to diagnose the clade Sahonachelyidae. Sahonachelys mailakavava differs from Sokatra antitra by having a much flatter and broader cranium with more dorsally oriented orbits, presence of prefrontal-palatine contact, absence of parietal-palatine contact, presence of narrower triturating surfaces that lack lingual ridge, shorter midline contact between palatines, presence of distinct supramaxillary artery sulcus on ventral side of jugal, and exposure of prootic anterior and posterior to foramen posterius canalis carotici interni.

Figure 11. A reconstruction of Sahonachelys mailakavava gen. et sp. nov. preying upon young larvae of the giant Madagascan frog Beelzebufo ampinga using specialized suction feeding.
 Artwork by Andrey Atuchin.

We here describe a new species of pelomedusoid turtle, Sahonachelys mailakavava, based on a near-complete skeleton from the Maevarano Formation of northwestern Madagascar, which has been dated Late Cretaceous (Maastrichtian). The new species can easily be diagnosed relative to all other named pleurodires as it shows a large number of unique characters in both the shell and cranium. A phylogenetic analysis using weighted parsimony indicates that Sahonachelys mailakavava is sister to Sokatra antitra and that these coeval Madagascan turtles are the only known representatives of the newly recognized clade Sahonachelyidae. This clade is sister to the group formed by Bothremydidae and Podocnemidoidae within crown Pelomedusoides. The most conspicuous characteristic of this clade is the presence of an extended posterior process that is formed by the maxilla and protrudes deeply into the lower temporal emargination. A number of highly unusual morphological features suggest that Sahonachelys mailakavava was a specialized, aquatic, suction-feeding species that fed upon moving prey. The specialized feeding strategy further highlights the uniqueness of Late Cretaceous Madagascan faunas, as no other crown pelomedusoid is known to have developed this method of prey capture.

Walter G. Joyce, Yann Rollot, Serjoscha W. Evers, Tyler R. Lyson, Lydia J. Rahantarisoa and David W. Krause. 2021. A New Pelomedusoid Turtle, Sahonachelys mailakavava, from the Late Cretaceous of Madagascar provides Evidence for Convergent Evolution of Specialized Suction Feeding among Pleurodires. Royal Society Open Science. DOI: 10.1098/rsos.210098