Monday, February 3, 2020

[Paleontology • 2020] Laurasichersis relictaSurviving the Cretaceous-Paleogene Mass Extinction Event: A Terrestrial Stem Turtle in the Cenozoic of Laurasia

Laurasichersis relicta Pérez-García, 2020
Illustration: José Antonio Peñas

Findings of terrestrial stem turtles are not uncommon at Mesozoic continental sites in Laurasia, especially during the Upper Cretaceous. Thus, the record of several lineages is known in uppermost Cretaceous ecosystems in North America (Helochelydridae), Europe (Helochelydridae and Kallokibotion) and Asia (Sichuanchelyidae). No terrestrial stem turtle had been described in Laurasia after the Cretaceous-Paleogene mass extinction event. Thus, the only representatives described in the Cenozoic record worldwide corresponded to forms from southern Gondwana, where some of them survived until the Holocene. A bizarre terrestrial stem turtle from the upper Thanetian (upper Paleocene) of Europe is described here: Laurasichersis relicta gen. et sp. nov. Despite its discovery in France, in Mont de Berru (Marne), this Laurasian taxon is not recognized as a member of a European clade that survived the Cretaceous-Paleogene extinction event. It belongs to Sichuanchelyidae, a hitherto exclusively Asian Mesozoic group, known from the Middle Jurassic. Finds at the Belgian site of Hainin (Hainaut) show that this dispersion from Asia and the occupation of some niches previously dominated by European Mesozoic terrestrial stem forms had already taken place a few million years after the mass extinction event, at the end of the lower Paleocene.

Systematic paleontology
Testudinata Klein, 1760
Mesochelydia Joyce, 2017
Perichelydia Joyce, 2017
Sichuanchelyidae Tong et al., 2012

Laurasichersis relicta gen. et sp. nov.

Figure 1: Elements of the carapace of the stem turtle (Sichuanchelyidae) Laurasichersis relicta gen. et sp. nov., from the upper Thanetian (upper Paleocene) of Mont de Berru (Marne, France). (A,B), carapace plates corresponding to several individuals, in dorsal (A) and ventral (B) views: nuchal MNHN.F BR13461; neurals (anterior to posterior) MNHN.F BR13458, MNHN.F BR17487, MNHN.F BR13710, MNHN.F BR13459; right costals (anterior to posterior) MNHN.F BR13001, MNHN.F BR13620, MNHN.F BR2785, MNHN.F BR18001, MNHN.F BR13637, MNHN.F BR13628; left costals (anterior to posterior) MNHN.F BR13624, MNHN.F BR4188; right peripherals (anterior to posterior) MNHN.F BR13604, MNHN.F BR18000, MNHN.F BR13611, MNHN.F BR13468, MNHN.F BR15098, MNHN.F BR13610; left peripherals (anterior to posterior) MNHN.F BR13603, MNHN.F BR18002, MNHN.F BR13595, MNHN.F BR13465, MNHN.F BR13608, MNHN.F BR2758, MNHN.F BR4180, MNHN.F BR13480, MNHN.F BR13467. The anterior or posterior views of the peripherals are also shown in A. Scale bars equal 1 cm. For the identification of each element see Fig. 4A. (C,D), nuchal MNHN.F BR13669, in dorsal (C) and ventral (D) views. (E–H), visceral view of the the third (right) to the sixth (left) bridge peripherals MNHN.F BR13622, MNHN.F BR13623, MNHN.F BR18002 and MNHN.F BR18000. (I,J), details of the outer surface of the neural MNHN.F BR13459 (I) and costal MNHN.F BR13579 (J).
Abbreviations for the plates (in lowercase and normal type): c, costal; n, neural; nu, nuchal; p, peripheral. Abbreviations for the scutes (in uppercase and in bold type): CE, cervical; M, marginal; PL, pleural; V, vertebral.

Figure 4: Reconstruction of the shell of stem turtle (Sichuanchelyidae) Laurasichersis relicta gen. et sp. nov., from the upper Thanetian (upper Paleocene) of Mont de Berru (Marne, France). (A), dorsal view of the carapace. (B), ventral view of the plastron. Abbreviations for the plates (in lowercase and normal type): c, costal; ent, entoplastron; ep, epiplastral element; hp, hypoplastron; hy, hyoplastron; ms, mesoplastron; n, neural; nu, nuchal; p, peripheral; py, pygal; spy, suprapygal; xi, xiphiplastral element.
Abbreviations for the scutes (in uppercase and in bold type): AB, abdominal; AN, anal; AX, axillar; CE, cervical; EG, extragular; FE, femoral; GU, gular; HU, humeral; IG, inguinal; M, marginal; PC, pectoral; PL, pleural; V, vertebral.

Locality and Horizon: Mouras quarry, Mont de Berru, Berru, Marne, France. Sables de Bracheux Formation, Franco-Belgian Basin. MP6a, upper Thanetian, upper Paleocene.

Etymology: The generic name is composed by Laurasi-, referring to Laurasia, where this taxon comes from; and –chersis, a Latinized word of Greek origin (Χέρσος) which means “land or dry land”, following the criteria used to establish the generic names of some other terrestrial stem turtles. The specific name refers to the fact that the new taxon is a vestige of Mesozoic fauna, being the only known Laurasian post-Mesozoic terrestrial stem turtle.

Diagnosis: Sichuanchelyid defined by the following characters exclusive within this clade: dorsally directed distal margin of the anterior and middle peripherals, and antero-lateral margins of the nuchal; slightly wider than long nuchal; high bridge peripherals; second to fourth vertebrals narrower than the first one, and than the second and third pleurals; concave anterior plastral margin; epiplastra divided into four elements, acquiring an exclusive morphology; supernumerary xiphiplastra, divided into two elements; humeral-pectoral sulcus at the level of the axillary notch; absence of a complete inframarginal series, but presence of axillar and inguinal scutes; anterior and posterior pairs of ventral foramina for the carotids closer to each other than between the foramina that form each pair. This sichuanchelyid shows the following unique character combination: shell size greater than 60 cm; second costal as long as the first; absence of contact and long distance between the nuchal postero-lateral end and the second peripherals; first vertebral wider than the nuchal; contact of the first vertebral with the second marginals; pleural-marginal sulci on the proximal region of the peripherals; anterior end of the axillary buttress reaching the anterior half of the third peripherals; absence of plastral fontanelles; absence of strongly interfingered plastral contacts; supernumerary epiplastra; long epiplastral symphysis relative to the entoplastron length; gulars overlapping the antero-medial entoplastral area; wide exposure of the squamosals on the skull table; absence of palatal teeth; absence of ventral exposure of the prootics; narrow and deep depression between the tubercula basioccipitale; short basicranium in relation to its width; absence of cleithrum processes.

Figure 8: Calibrated cladogram corresponding to the cladistic analysis performed here (4500 most parsimonious trees, length of 943 steps, CI = 0.316, RI = 0.778, RC = 0.246), in which the position of all representatives of Perichelydia (sensu3) is shown, as well as that of the crown group Testudines. This distribution is based on the strict consensus tree, except in the case of the clade Meiolaniformes, which is obtained in the majority rule tree (67%). The paleobiogeographic distribution of each taxon is indicated. Bremer support values are shown.

Adán Pérez-García. 2020. Surviving the Cretaceous-Paleogene Mass Extinction Event: A Terrestrial Stem Turtle in the Cenozoic of Laurasia. Scientific Reports. 10: 1489. DOI: 10.1038/s41598-020-58511-8 

One single primitive turtle resisted mass extinction in the northern hemisphere