Eocyzicus Daday, 1914: 193 sensu Rogers et al. 2017

Schwentner, Martin, Rabet, Nicolas, Richter, Stefan, Giribet, Gonzalo, Padhye, Sameer, Cart, Jean- François, Bonillo, Céline & Rogers, D. Christopher, 2020, Pseudosesarma crassimanum, Zoological Studies (Zool. Stud.) 59 (44), pp. 1-23 : 15-18

Eocyzicus Daday, 1914: 193 sensu Rogers et al. 2017

= Caenestheria Daday, 1914 (pro partim)

Diagnosis: As for the family.

Comments: The type species for the genus is Eocyzicus orientalis Daday, 1914 , fixed here by designation. The synomyzation of Cyzicus with Eocyzicus is not supported. Species following Daday’s (1913a b) description of Caenestheria are now summarized under Cyzicus (see above). Relationships within Eocyzicus remain unclear, and further sampling across many taxa is required before any meaningful relationships can be determined.

Biogeographic History

To date, little is known about the biogeographic history of extant Spinicaudata . The spinicaudatan fossil record is rich and diverse, but limited almost exclusively to carapaces. Thus the fossil record contributes little information for the evolutionary history of extant taxa as the fossil relationships are not well resolved and still debated ( Astrop and Hegna 2015). It is worth noting that all four spinicaudatan families have a nearly global distribution (the Americas, Eurasia, Africa and Australia) and even genera such as Leptestheria , Eocyzicus , Cyzicus and Eulimnadia occur on all or most of these landmasses. Within continents, Spinicaudata have revealed remarkable dispersal potential with species being distributed over> 1000 km and relatively low levels of population differentiation ( Cesari et al. 2007; Schwentner et al. 2012 a 2014 2015a). Such longdistance dispersal can be mediated via birds or other animal vectors or wind ( Bilton et al. 2001) and probably follow the same model as described for the related Anostraca ( Rogers 2015 and references cited within).

Our molecular clock analyses suggest that the four extant spinicaudatan families diverged prior to the break-up of Gondwana and Laurasia and possibly of Pangea. Also the divergence between species and species groups inhabiting different continents mostly predate the separation of the respective continental plates. Historic vicariance appears to be the main factor explaining the transcontinental distribution of extant spinicaudatan taxa, though in a few instances more recent transoceanic dispersal events have occurred as well. For other large Branchiopoda, like Anostraca , Laevicaudata , and Cyclestherida, vicariance apparently also played a major role in shaping today’s distribution of taxa ( Schwentner et al. 2013; Rogers 2015; Sigvardt et al. in press), in particular for southern hemisphere taxa. For Notostraca , previous molecular clock analyses obtained different estimates of their divergence times. While some suggested more recent divergence ages, implying trans-oceanic dispersal events (Mather et al. 2013; Vanschoenwinkel et al. 2012), others suggested divergence times that imply vicariance events ( Korn et al. 2013). Our own divergence time estimates within Notostraca ( Figs. S10, S 11) are similar to the former. One should keep in mind that we predominantly discuss our molecular clock results based on the more conservative calibration point of 255 mya for Limnadiidae + Leptestheriidae + Eocyzicus , the divergence ages inferred using the 380 mya calibration point were even older ( Figs. S13–S 14) and thus even stronger in suggesting vicariance over dispersal.

The divergence between northern and southern hemisphere species of Cyzicidae and Limnadiidae (with the exception of Eulimnadia ) probably predates the break-up of Pangea in the early to mid-Jurassic, suggesting that geographic distances might have separated these clades already on Pangea. If Limnadia and Imnadia are indeed nested among the two southern continental clades, a southern hemisphere origin can be hypothesized for Limnadiidae . From there the northern hemisphere could have been colonized when Gondwana and Laurasia were still joined in Pangea. Also for Leptestheriidae , a southern hemisphere origin can be assumed, based on the phylogenetic relationships among its taxa. But here multiple independent colonization events of northern hemisphere continents have to be assumed. The strongly diverging relationships within Leptestheriidae among most analyses do not allow establishing detailed biogeographic hypotheses for this taxon. Most inferred clades within Leptestheriidae do not correspond to geographic regions (e.g., African or Indian species do not form monophyletic groups each) and many deep splits within Leptestheriidae were dated to be 90 mya or older, which roughly coincides with the separation between Africa and South America (~100 mya; McLoughlin 2001) and predates the final break-off of Australia (~50 mya; Beaulieu et al. 2013). Ancestral leptestheriid lineages were probably once widely distributed across Gondwana and when Gondwana broke apart, several of these lineages survived on more than one continent. For example, one leptestheriid clade that was consistently recovered comprised Leptestheria nobilis from India, Leptestheria sp. (specimen M 124) from Madagascar and L. brevirostris from Botswana with an estimated clade age of 95 mya. A Madagascar-India-Seychelles block was the first landmass that broke-off from Gondwana around 120 mya (Ali and Aitchinson 2008, McLoughlin 2001) and this leptestheriid clade probably evolved on this landmass and dispersed from Madagascar to continental Africa subsequently. Clades with similar African and Indian distributions were also recovered for the notostracan Triops (e.g., clades 18 and 26 in Korn et al. 2013; see also Modak et al. 2018). While some of these distributions might be due to the geological processes (e.g., clade 18 in Korn et al. 2013), others appear to be younger and thus possibly due to more recent dispersal and colonization events (e.g., clade 26 in Korn et al. 2013).

The only spinicaudatan example of repeated transoceanic dispersal is Eulimnadia . This genus probably evolved in South America and successfully dispersed to virtually all other continents (there are no extant records from Antarctica, whether Antarctica was historically inhabited is unknown), as well as many oceanic islands ( Bellec and Rabet 2016), probably during the last 50 mya. Bellec and Rabet (2016) dated the onset of this global distribution to only 30 mya. Eulimnadia is special among Spinicaudata due to its very fast development and short life-cycle, which enables them to survive in short-lived habitats and by which they might escape competition and predation from slower developing taxa, as well as the presence of hermaphrodites instead of females ( Bellec and Rabet 2016). The latter enables the colonization of new habitats from single resting eggs, which greatly improves dispersal effectiveness.

The Australian fauna is noteworthy not only because of its exceptional diversity ( Schwentner et al. 2015b), but it apparently evolved independently from other regions even when Australia was still connected to other Gondwanan landmasses. This seems to have been the case for the Limnadopsis - Paralimnadia - Australimnadia clade within Limnadiidae and the Australian Ozestheria clade. In both cases, age estimates suggest that the Australian clades (134 and 94 mya, respectively) evolved long before the final separation of Australia from the rest of Gondwana ( Beaulieu et al. 2013), which occurred around 50 mya. In this relatively long timespan, apparently no exchange between Australia and other continental masses occurred. Such patterns of ancestral regionalization were also found in many other old Gondwanan lineages (e.g., Murienne et al. 2014). Australia was linked to the rest of Gondwana primarily via Antarctica, the only continent without extant Spinicaudata but with a rich fossil record of these animals ( Shen 1994). Antarctica may have restricted exchange between Australia and other regions of Gondwana simply by geographic distance (isolation by distance). The age of the Australian Eocyzicus clade roughly coincides with the final break-off of Australia, implying that Australia was colonized by Eocyzicus around that time. Again, Eulimnadia may be the only taxon that colonized Australia after it separated from Gondwana, potentially as Australia drew closer to Asia. It has been suggested that Eoleptestheria cf. ticinensis invaded Australia recently from China ( Timms 2009a); however, our phylogenetic analyses, as well as previous analyses of the genetic diversity within the Australian populations ( Schwentner et al. 2015b), suggest a longer presence of this taxon in Australia. Notably, for the large branchiopod taxa Cyclestherida and Notostraca , younger ages have been assumed for the divergence of Australian and non-Australian species ( Schwentner et al. 2013; Mathers et al. 2013). In these taxa, dispersal to or from Australia apparently occurred more recently, probably after Australia and Asia moved closer. The case of the notostracan Triops is particularly interesting as the putatively closest relatives to the Australian fauna occur in North America ( Mathers et al. 2013).

The age estimates presented herein may also help to improve our understanding of the relationships between fossil and extant taxa ( Astrop and Hegna 2015). We provide the first molecular clock based age estimated for all extant families and many genera. Our results suggest that within each extant family only one to three extant lineages date back more than 150 million years and the main divergence took place within the last 100 million years. Thus the majority of fossil families probably went extinct without any extant representatives; this may be particularly true for the rich Permian and Carboniferous fauna as crown-group Spinicaudata may have only originated around that time. The similar carapace shapes of Cyzicidae a n d E o c y z i c i d a e m i g h t g o b a c k t o c o m p a r a b l e c a r a p a c e s h a p e s a l r e a d y k n o w n f r o m d i ff e r e n t Euestheriidae fossils since the Permian ( Astrop and Hegna 2015). Nevertheless, peculiar similarities in carapace shape between fossil and extant taxa may be generally due to convergence rather than evolutionary stasis. For example, the carapace of the fossil Palaeolimnadiopseidae Defretin-Le Franc, 1965 has large similarity to extant species of Limnadopsis and Australimnadia (for example, compare Gallego and Breitkreuz 1994 and Gallego 2005 with Timms 2009b or Timms and Schwentner 2012) and it has been suggested that Palaeolimnadiopseidae are the ancestors of Limnadopsis ( Zhang et al. 1976, but questioned by Astrop and Hegna 2015). Palaeolimnadiopseidae date back as far as the Upper Permian (summarized in Gallego 2005) long before the inferred evolution of Limnadiidae , potentially even before the evolution of crown-group Spinicaudata . Of course, it is possible that some younger species that have been assigned to Palaeolimnadiopseidae belong to the stem lineage of extant Limnadiidae and are not related to the older taxa ( Astrop and Hegna 2015). Our divergence time estimates may help to improve the current hypotheses of how such fossil and extant taxa may have been related.

Species Diversity of Spinicaudata

Detailed population-based molecular genetic and integrative taxonomic approaches have revealed much higher species diversities for Spinicaudata (e.g., Schwentner et al. 2011 2014 2015a b; Weeks et al. 2009) and other ‘large Branchiopoda’ like Triops (e.g., Korn et al. 2010; Mathers et al. 2013; Meusel and Schwentner 2017; Vanschoenwinkel et al. 2012). Several species that were assumed to be morphologically variable could be shown to represent an amalgam of multiple, morphologically differentiated, species (e.g., Korn et al. 2010; Schwentner et al. 2012b; Meusel and Schwentner 2017; Tippelt and Schwentner 2018). However, extensive overlaps of intraspecific variability and interspecific variation are prevalent also in these species and their initial delimitation would have been difficult based solely on morphological characters. The majority of these studies have been conducted on the Australian fauna, which now appears to be the continent with the highest extant clam shrimp diversity, harbouring roughly one third of all spinicaudatan species ( Schwentner et al. 2015b). However, the spinicaudatan fauna of other continents have not been studied as extensively. In the analyses presented herein, many species were represented by single individuals or were studied from a few populations only. Despite this relatively sparse intraspecific sampling, instances of putatively cryptic species were revealed in Africa, North America and Europe; in the case of Leptestheria rubidgei even within a single population. On the one hand, this suggests that the species diversity of Spinicaudata may be underestimated on local and global scales, on the other hand it shows that the species level taxonomy of many spinicaudata taxa requires revision. Taxonomic revisions that combine detailed morphological and molecular genetic data will become indispensable to assess the true diversity of Spinicaudata and will probably reveal many more currently cryptic species.

Acknowledgments: This work ( urn:lsid:zoobank.org:pub:665CB83B-7A2A-445F-9154-B18BDCCA573C ) and Eocyzidae fam. nov. ( urn:lsid:zoobank.org:act:CF3B4318-483E-47B8-A391-DA6A9E77400F ) have been registered with ZooBank. We are greatly indebted to Brian V. Timms, who has made this research possible through his relentless efforts in studying Australian Branchiopoda and has taken MS, NR, SR, and DCR on numerous collecting expeditions across Australia. We thank Mikhail Son (Institute of Marine Biology, Odessa, Ukraine) and Federico Marrone (Università degli Studi di Palermo, Palermo, Italy) for providing specimens as well as two anonymous reviewers for their helpful comments and Thomas Hegna for his patience and support. We thank also Daniela Lupi, Eric Gallerne, Eric Quéinnec, Marco Pagni, Michaël Manuel, Pascal Lluch, Sébastien Lacau for the help in the field or soil sampling. Sequencing costs were financed via internal funds from the Museum of Comparative Zoology and the Faculty of Arts and Sciences to G.G. and from the Center of Natural History (Universität Hamburg) to M.S. and via Emergence project of UPMC and ATM gave to N.R. This work benefited from equipment and services from the iGenSeq core facility, at ICM, Sorbonne-Unversité and SSM in MNHN (Paris).

Authors’ contributions: M.S. and D.C.R. conceived and planned the study. M.S. performed all Sanger sequencing and phylogenetic analyses. D.C.R. updated the diagnoses of the taxa. M.S., D.C.R. and N.R. drafted the manuscript. All authors contributed to the final version of the manuscript.