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Journal of Paleontology 92(2):170–182 Fossil record of Homarus and Hoploparia
The fossil record of Homarus is equivocal, depending on which species are referred to the genus. Six fossil species recognized herein, all European, give the genus a fossil record extending back to the Early Cretaceous (ca. 100 Ma). These are: Homarus benedeni Pelseneer, 1886 (Albian of France; Fig. 1), H. morrisi (Eocene of southern England), H. klebsi (late Eocene–late Oligocene of western Europe), H. percyi Van Beneden, 1872 (Oligocene of Europe), H. lehmanni Haas, 1889 (early Oligocene of Germany), and H. hungaricus n. sp. (late Oligo- cene of Hungary). Schweitzer et al. (2010) listed eight species of Homarus, but their list contains several differences from ours (Appendix 1). Its oldest, and sole Cretaceous, occurrence (H. benedeni,of
Albian age) is based on material that we have not been able to examine first hand. Line drawings in Pelseneer (1886, p. 49–50) show that H. benedeni is very Homarus-like and not at all suggestive of Hoploparia. Based on these line drawings, we are confident that Homarus is known from the Albian (ca. 100 Ma). Some of the Paleogene occurrences exhibit a combination
of Homarus- and Hoploparia-like morphological features (Appendix 1) (e.g., a ventral extension of the branchiocardiac groove [H. klebsi, H. morrisi] or a long subdorsal carina [H. klebsi]). Table 1 presents the stratigraphic distribution of species of
Homarus. Homarus first appears in the fossil record in the Early Cretaceous (Albian) and then is not known from the Late Cretaceous, despite the fact that nephropid lobsters in general are well known from that time interval. This stratigraphic distribution makes one wonder if we are looking at one lineage, or if the Paleogene species are convergent with the Albian form. Hoploparia is the best known fossil genus of clawed lob-
ster, with a record extending from the Lower Cretaceous (Valanginian) to the Neogene (Miocene). Fifty-eight species are known: 18 from the Lower Cretaceous, 33 from the Upper Cretaceous (31, plus one carryover from the Lower Cretaceous and one extending into the Paleogene), and nine Paleogene and Neogene (eight, plus one carryover from the Upper Cretaceous) (Table 2). Hoploparia was cosmopolitan in geographic range, extending from Canada (i.e., the Late Cretaceous H. bennetti Woodward, 1900 from Vancouver, and H. albertanensis Tshudy et al., 2005 from Alberta) and Greenland (i.e., the early Eocene H. groenlandica Ravn, 1903) to the Antarctic Peninsula (i.e., the Campanian–Maastrichtian H. antarctica, the
Table 1. Stratigraphic distribution of species of Homarus.
Recent H. americanus Milne Edwards, 1837 H. gammarus (Linnaeus, 1758)
Paleogene: Oligocene (33.9–23.03 Myr) H. hungaricus n. sp. H. klebsi (Noetling, 1885) (Eocene–Oligocene) H. lehmanni Haas, 1889 H. percyi Van Beneden, 1872
Paleogene: Eocene (56.0–33.9 Myr) H. klebsi (Noetling, 1885) (Eocene–Oligocene) H. morrisi Quayle, 1987
Cretaceous: Albian (113–100.5 Myr) H. benedeni Pelseneer, 1886
Campanian–Paleocene H. stokesi (Weller, 1903), and the early Miocene H. gazdzicki Feldmann and Crame, 1998).
Nephropid lobster diversity through time
Moving beyond alpha taxonomy and phylogenetic studies, some recent papers have examined lobster diversity through time (Tshudy, 2003 for the family Nephropidae; Schweitzer and Feldmann, 2014, 2015 for all lobsters). We revisit that herein for the family Nephropidae. A tally of fossil nephropid species per geological age is presented in Appendix 2, and diversity through time is graphed in Figure 6 (data and calculations in Appendix 3). The known diversity of fossil nephropids really only
equates to known shelf-depth diversity (i.e., marginal shelf and epicontinental sea diversity) (Tshudy, 2003). Lobsters that lived on the continental slope and at greater depths are never collected as fossils. High sea levels, and therefore epicontinental seas, increase
known fossil lobster diversity in two ways: (1) they increase lobster habitat, and (2) they fossilize lobsters where they can be collected today (those fossilized below modern sea level are not collected). Effect #1 produces a real increase in diversity (i.e., a real signal) and so, of course, should not be corrected for. Effect #2 biases the record in favor of time intervals (geological ages here) of high sea levels and, thus, we should at least attempt to correct for it. In the absence of published information on rock exposure
area per geological age, Tshudy (2003) normalized known species diversity for area of epicontinental sea coverage by using the sea level curve by Vail et al. (1978). Figure 6 herein updates the results of Tshudy (2003) in the light of species-level taxonomic additions and changes (since 2003)—the Oligocene species count has increased from two to four, and the Turonian– Coniacian–Santonian count from 15 to 22. Figure 6 shows known species diversity (white bars) for clawed lobsters of the family Nephropidae and normalized diversity for epicontinental sea coverage (black bars). As in the 2003 study, both known species diversity
(i.e., raw numbers) and normalized numbers indicate that the diversity of shelf-dwelling nephropids was highest during the Late Cretaceous; higher than in the Early Cretaceous and the post-Cretaceous. Tshudy (2003) interpreted the reduction in the Paleogene and Neogene as resulting not from the end- Cretaceous extinction, but largely from the nephropid general abandonment of shelf depths in the Paleogene. Schweitzer and Feldmann (2014) compiled and interpreted
(all) lobster diversity through time at the levels of infraorder, superfamily, family, and genus. Among the potential biases that they recognized was rock volume (first discussed by Raup, 1976a, b; later by Signor, 1985; and discussed in the context of nephropid lobsters by Tshudy, 2003, p. 179). Tshudy (2003) considered rock volume as less important than rock exposure area in biasing the lobster record through time, in that only exposed rocks yield lobster fossils (with very few exceptions, e.g. the new species described here, lobsters are unrecognizable in drill cuttings). Schweitzer and Feldmann (2015, p. 635) again noted the importance of the “uneven rock record” in interpreting diversity through time, this time acknowledging
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