Editors' choice Evolution and identity of synapsid carpal bones SUSANNA KÜMMELL, FERNANDO ABDALA, JUDYTH SASSOON, and VIRGINIA ABDALA Kümmell, S., Abdala, F., Sassoon, J., and Abdala, V. 2020. Evolution and identity of synapsid carpal bones. Acta Palae­ ontologica Polonica 65 (4): 649–678. To date there is little information on carpal bone homology in late Palaeozoic and Mesozoic Synapsida. Crucial to the understanding of homology in synapsid carpal elements is the fact that different nomenclatures are used for the carpals of non-mammaliamorph Synapsida (Gegenbauer’s canonical nomenclature) and Mammaliaformes (mammalian nomencla- ture). The homologies of the carpals of non-mammaliamorph synapsids and mammals were established early last century and have not been reviewed since then. Here we provide a detailed study of the carpal bones of synapsids ranging in age from the early Permian to Late Cretaceous. The mammaliamorph lunate, previously considered the homologue of the intermedium of non-mammaliamorph synapsids, is interpreted here as homologous to their lateral centrale. We interpret the single mammaliamorph centrale as a homologue of the medial centrale of non-mammaliamorph synapsids. In some synapsid specimens, we found that one or two centralia are fused to the radiale (e.g., the gorgonopsian Arctognathus and tritylodontid Bienotheroides), supporting a digging habit. A third centrale is present in the therocephalian Theriognathus, very likely an abnormal duplication. An additional medial bone in a biarmosuchian was interpreted as a prepollex/ sesamoid. A cartilaginous prepollex/sesamoid may also have been present in several non-mammaliamorph synapsids, which have an open space proximal to distal carpal I. Distal carpal V is completely lost in dicynodonts and it is mainly fused to distal carpal IV in the adult stage of most other therapsid groups, but showed a delayed development in most non-mammaliamorph cynodonts. In mammaliamorphs, distal carpal V is not present. Our observations provide an up- dated revision of synapsid carpal homologies, mainly on the basis of position and anatomical contacts and also taking into account the results of embryological studies. Key words: Synapsida, carpus, intermedium, lunate, manus, homology, Permian, Mesozoic. Susanna Kümmell [susanna.kuemmell@uni­wh.de], Institute of Evolutionary Biology and Morphology, Center for Bio­ medical Education and Research (ZBAF), Faculty of Health, School of Medicine, University Witten/Herdecke, Stock­ umer Straße 10, 58454 Witten, Germany. Fernando Abdala [[email protected]], Unidad Ejecutora Lillo (CONICET­Fundación Miguel Lillo), Tucumán, Argentina and Evolutionary Studies Institute, University of the Witwatersrand, South Africa. Judyth Sassoon [[email protected]], School of Earth Sciences, University of Bristol, Queen’s Road, Bristol, BS8 1RJ, UK. Virginia Abdala [[email protected]], Instituto de Biodiversidad Neotropical, CONICET­Universidad Nacio­ nal de Tucumán, Catedra de Biologia General, Facultad de Ciencias Naturales e IML, Tucumán, Argentina. Received 2 December 2019, accepted 12 August 2020, available online 6 November 2020. Copyright © 2020 S. Kümmell et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License (for details please see http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Figures 14B and 15 excluded, see pertinent figure captions. different phalanges in mammals and non-therapsid synap- Introduction sids (Hopson 1995). Under these circumstances, providing reliable hypotheses of homology is a fundamental palaeon- The homologies of carpal bones in late Palaeozoic and tological task. The synapsid carpus is a complex structure Mesozoic Synapsida have not been extensively researched. consisting of many small bones, and homologizing them in Homology is a key concept for different disciplines such as fossils spanning over 170 million years is challenging. Here taxonomy, systematics, and morphology. In some cases, a we investigate the homology and evolutionary change of deep understanding of homologies for the interpretation of the synapsid carpals from the early Permian to the end of evolutionary changes can only be provided by fossils. An the Cretaceous on the basis of currently accepted synapsid example of this is the proposed homology of the amniote systematics. astragalus with four tarsal bones in anamniote tetrapods The carpus is the proximal part of the manus (see ex- (O’Keefe et al. 2006), or the established homologies of the panded concept in SOM, Supplementary Online Ma te rial Acta Palaeontol. Pol. 65 (4): 649–678, 2020 https://doi.org/10.4202/app.00709.2019 650 ACTA PALAEONTOLOGICA POLONICA 65 (4), 2020 Fig. 1. Schematic diagrams of a non-therapsid synapsid (“pelycosaur”) carpus (A) and a mammaliaform carpus (B), labelled using the canonical and mammalian nomenclatures. Abbreviations: ca, capitate; ce, centrale; di, distal carpal; ha, hamate; int, intermedium; l ce, lateral centrale; lu, lunate; mc, metacarpal; m ce, medial centrale; pis, pisiform; ra, radius; rl, radiale; sc, scaphoid; td, trapezoid; tp, trapezium; tq, triquetrum; ul, ulna; ur, ulnare. available at http://app.pan.pl/SOM/app65-Kuemmell_etal_ and Bardeleben (1889). These used the mammalian nomen- SOM.pdf). Traditionally, the carpals of mammals were des- clature more broadly and also applied it to describe the ignated using a different nomenclature than that of reptiles carpals of the Permo-Triassic fossil synapsids. In contrast, and amphibians. These terms were known as the “mam- Broom (1901) established a homology of the Permo-Triassic malian nomenclature” (sensu Shubin and Alberch 1986) synapsids and the mammalian carpals adopting the canon- and the “canonic nomenclature” (sensu Čihák 1972), re- ical and the mammalian systems to describe the proximal spectively. The canonic nomenclature (henceforth «canon- and central carpals of the dicynodont “Udenodon gracilis” ical nomenclature») is not only used for reptiles and am- (= Dicynodontoides recuvidens; Angielczyk et al. 2009). phibians, but also for the non-mammaliaform members of However, in his later publications, he only used the canon- the clade Synapsida (Fig. 1A; e.g., Broom 1904; Jenkins ical nomenclature to describe the carpus of Permo-Triassic 1971; Liu et al. 2017), whereas the carpal bones of Mesozoic synapsids (Broom 1904, 1907, 1913, 1930). Broom (1901) Mammaliaformes are designated with the mammalian no- homologized the canonical and mammalian nomenclature, menclature (Fig. 1B, Table 1; e.g., Kielan-Jaworowska 1977; following Gegenbaur (1864), but he misinterpreted the me- Ji et al. 2002; Luo et al. 2003). These different naming con- dial centrale as distal carpal I, thus identifying only one ventions can lead to confusion in anatomical descriptions. centrale in “Udenodon”. However, later on, he described The use of different nomenclatures for the carpal bones of two centralia in non-mammaliamorph synapsids (Broom mammaliaforms and the remaining synapsids is an histor- 1904, 1907). The carpus in non-mammaliamorph synapsids ical artefact. It probably arose because of the early classi- has five distal carpals and two centralia, two carpals more fication of non-mammaliaform synapsids within the class than extant, basal mammals, which indicates that two carpal Reptilia (“mammal-like reptiles”), whereas those specimens bones were lost along the evolutionary transition to mam- now termed Mesozoic Mammaliaformes were, as a whole, mals (Fig. 1). Broom (1901, 1904, 1907) identified one of the considered to be members of Mammalia in former times. lost carpals as distal carpal V and proposed the second to The nomenclature of the mammalian carpus was first be a centrale when he homologized the mammalian lunate established for the human carpals in the 17th and 18th centu- (= lunar) with the intermedium. ries (Lyser 1653; Monro 1726; Albinus 1726; see McMurrich 1914) and later extended to describe the mammalian carpus Table 1. Homology of the reptilian and mammalian carpals after Ge- genbaur (1864). in general. A combination of the three slightly differing nomenclature systems of Lyser (1653), Monro (1726), and Canonical nomenclature Mammalian nomenclature Albinus (1726), together with the term central bone (cen- radiale scaphoideum trale) are still in use to designate carpal bones of mod- intermedium lunatum ern mammals and Mesozoic mammaliaforms (Table 1; e.g. ulnare triquetrum Kielan-Jaworowska 1977; Ji et al. 2002; Luo et al. 2003). In pisiforme pisiforme 1864, Gegenbaur introduced the canonical nomenclature centrale centrale for the carpus, which was more generally applicable to all carpale 1 mutungulatum majus/trapezium classes of vertebrates. He established homologies of the dif- ferent carpal bones in all vertebrate clades and homologized carpale 2 mutungulatum minus/trapezoides the reptilian and mammalian carpals (Table 1). carpale 3 capitatum carpale 4 The first descriptions of the carpus in non-mammali- hamatum amorph synapsids were provided by Seeley (1888, 1895) carpale 5 KÜMMELL ET AL.—EVOLUTION AND IDENTITY OF SYNAPSID CARPAL BONES 651 The homology of the canonical and mammalian nomen- ontogenetic fusion (Milaire 1978; Holmgren 1952; Čihák clature erected by Gegenbaur (1864; see Table 1) and adopted 1972; Slabý 1967). Thus, in addition to paleontological in- by Broom (1901, 1904) is followed up