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Crustacean Paleobiology, I

Room: Grand Ballroom South

10:30 - 12:00

Chair(s): Rod Feldmann, Carrie Schweitzer

THAM-1.1  10:30  UNKNOWN UNKNOWNS: WHAT WOULD WE NOT KNOW ABOUT PANCRUSTACEA WITHOUT FOSSILS. SCHRAM F.R.*, Deptment of Invertebrate Paleontology, Burke Museum of Natural History & Cultures, University of Washington, Seattle, WA

Everyone recognizes that fossils do tell us a lot about evolution within Arthropoda. But if we had no fossil record, what would we have missed amongst the Pancrustacea? We would never know – so they could not be missed. Nevertheless, it is interesting to play the game of “unknown unknowns.” First of all, our understanding of biodiversity would be depleted. Exact numbers remain vague, but as an example, in the Class Oligostraca the ostracods have around 14,500 living species and subspecies, and estimates indicate that another 56,000 ostracods are extinct species residing only in the fossil record. That is a great deal of missing diversity. Second, our understanding of body plan disparity would suffer. For instance, we would not know about several members of Class Multicrustacea such as: Cyclida (an infraclass of the Subclass Hexanauplia), Pygocephalomorpha (an extinct peracaridan order of Infraclass Eumalacostraca), and Thylacocephala (an enigmatic infraclass, probably of Multicrustacea). Gaps in understanding the evolution of living crustaceomorphs have also fallen prey to the effects of unknowns. Before World War II, branchiopods served as a kind of ancestral type. That is until 1943 when Mystacocarida were discovered. Before 1955, Cephalocarida were unknown. Not until 1981, were Remipedia unveiled. Each of these groups in turn served as a succession of avatars for ancestral types of crustaceans. Then between 2010-2013, DNA sequence data overturned all our old ideas: Hexapoda united with the paraphyletic crustaceomorphs to present us with a new reality, class Pancrustacea, which included the merger of three of the “old ancestors” of the crustaceans—branchiopods, cephalocarids, and remipedes—into a new subclass Xenocarida. How are we now to make sense of the evolution of hexapods from crustaceans? Can new “unknowns” provide insight? Such is indeed, and continues to be, the case. We not only have new insights about the radiations of crustaceans based on real fossils (Briggs et al., this meeting), but also from newly recognized crustaceomorphs we can now perceive what crustaceans evolved into (insects) and how they might have done it (via xenocaridans).


The inclusion of insects within a newly defined Pancrustacea has challenged traditional morphological synapomorphies for crustaceans. Exceptionally preserved fossils from the Cambrian (542-485 million years ago), the time when major pancrustacean lineages are inferred to have diverged, complicate the situation. Previously proposed ladder-like patterns of character evolution leading up the putative stem-lineage make inadequate assumptions about the developmental expression of morphology in fossils. Trunk tagmosis, among other characters, may be quite evolutionarily labile. Micro-CT study of new bivalved arthropod material from the Chengjiang biota reveals the putative pancrustacean stem-group bears previously believed crown-group synapomorphies, such as tritocerebral (second) antennae and epipodites on the basipodal or coxal trunk limb segment. A complex evolutionary history, with substantial character loss in the crown-group of early diverging lineages (e.g. the less-studied Oligostraca), is proposed in the context of recent phylogenomic hypotheses.

THAM-1.3  11:15  CRUSTACEANS FROM FOSSIL COLD SEEP ENVIRONMENTS. KLOMPMAKER A.A.*, Department of Integrative Biology & Museum of Paleontology, University of California, Berkeley, 1005 Valley Life Sciences Building #3140, Berkeley, CA 94720, USA; NYBORG T., Department of Earth and Biological Sciences, Loma Linda University, Loma Linda, CA 92350, USA; BREZINA J., South Dakota School of Mines and Technology, Rapid City, SD 57701, USA; ANDO Y., Mizunami Fossil Museum, 1-47, Yamanouchi, Akeyo-cho, Mizunami, Gifu, 509-6132, JAPAN

Crustaceans including decapods, copepods, amphipods, cumaceans, tanaidaceans, ostracods, and isopods are major components of modern marine methane seeps, where they play a key role in structuring these hotspots of diversity in relatively deep waters. There is every reason to suspect they were common too in ancient seeps, but relatively few studies have focused on crustaceans from fossil seep deposits thus far. We hypothesize that crustaceans can be commonly found in Meso-Cenozoic seeps when many of the aforementioned groups were present and/or radiated. To this end, we review the global fossil record of crustaceans in seeps for the first time using the primary literature and newly collected specimens from the Late Cretaceous of South Dakota, USA. We find that seep crustaceans are much more common than previously known, are found on each continent, and occur more frequently starting in the Jurassic. Decapod crustaceans are represented by body fossils and traces (coprolites, repair scars in mollusks, and burrows), whereas only body fossils of ostracods and barnacles are known. Other groups are lacking. While modern seep decapods are dominated by galatheoid squat lobsters, alvinocaridid shrimps, king crabs, and true crabs, the fossil record is consisting primarily of callianassid ghost shrimps and true crabs thus far. Preservation and recognition are likely to have influenced this discrepancy. Finally, the relatively unexplored fossil record of seep crustaceans provides many opportunities for systematic and paleoecological research.

THAM-1.4  11:30  THE FOSSIL RECORD OF BRANCHIOPOD CRUSTACEANS. Hegna T.A.*, Department of Biology, Western Illinois University, Macomb, IL 61455 USA

The higher-level phylogeny of modern branchiopod crustaceans is relatively well understood. However, we do not have a firm grasp on divergence dates or character state transitions between the major clades. Rehbachiella from the upper Cambrian of Sweden is pretty firmly established as a stem group branchiopod. The anostracans and notostracans are two of the earliest branching lineage of branchiopods; they have stem group representatives in the Devonian known from the Rhynie Chert and the Strud Lagerstätte. Possible crown group representatives for both are known from the Mesozoic. The understudied Triassic Grès à Voltzia Lagerstätte contains possible stem group phyllopods and diplostracans. Laevicaudatans have a published record in the Permian and possible soft-part preservation known from the Jurassic. However, owing the character-poor nature of these fossils, it is impossible to tell if they represent crown-group laevicaudatans. Spinicaudatans have a rich record of carapaces. The earliest known total group spinicaudatans come from the Devonian (though there is reference to some possible late Silurian fossils). The leaiids are an enigmatic extinct diplostracan lineage thought to be closely related to the spinicaudatans. They have a record that extends from the Devonian to the Triassic. Cyclestherids have an enigmatic fossil record. There are no examples of cyclestherids preserved with soft-parts, so the only character used to assign fossils to this lineage is the shape of the carapace. According to that metric, cyclestherids have a record that begins in the Devonian. Cladocerans are late-comers, with a record that begins in the late Jurassic.

THAM-1.5  11:45  BRING OUT YOUR DEAD: RECONSTRUCTING A DECAPOD BIOCOENOSIS THROUGH QUANTITATIVE TAPHONOMY AND FUNCTIONAL MORPHOLOGY. Kornecki K.M.*, Department of Earth and Environmental Science, Rensselaer Polytechnic Institute, Troy, NY 12180 USA

The Blue Springs locality (BSL) of the Maastrichtian Coon Creek Formation (CCF) crops out in Mississippi in what was a large bay of the Western Interior Seaway. The BSL is fossiliferous, with excellent preservation of a diverse assemblage of decapod taxa (Kornecki et al., 2017). A recent paleo reconstruction of CCF “Dakoticancer australis Assemblage” community (Bishop, 2016), proposed trophic relationships of major fossil groups and included some discussion of individual species’ predator-prey relationships. Whereas Bishop (2016) cited modern intertidal zones and “warm sea bottoms” as modern analogs, isotopic and biomarker analyses (Vrazo et al., 2018).suggested that the depositional environment was a shallow, nearshore marine environment further north in Tennessee and postulated that environments within the bay to the south would be offshore. These hypotheses were tested using quantitative taphonomy (corpse vs. molt preservation, articulated vs. disarticulated) and functional morphology of associated decapods to provide further insight on the CCF paleo community, particularly regarding depositional environment vs. the “living” assemblage. The ratio of molts to corpses of Dakoticancer australis Rathbun, 1935 and Tetracarcinus subquadratus Weller, 1905 suggests that molts may have been deposited in what was an intertidal zone and do not represent the dominant living taxon, though they may migrate up-slope for mating or molting events. The presence and accumulation of flat, phosphatic concretions (proposed to be large “rip-up clasts”), glauconite, and other factors support an interpretation for a reducing environment and potential for anoxic episodes or pockets, increasing preservation potential of molts and less calcified decapods, such as mud shrimps.

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