Lotagnostus

Taylor, John F., Loch, James D. & Repetski, John E., 2024, Taxonomy and stratigraphic distribution of Lotagnostus (Agnostida: Agnostidae) and associated trilobites and conodonts in the Upper Cambrian (Furongian) of Laurentia, Zootaxa 5422 (1), pp. 1-66 : 21-23

Deficiencies of Lotagnostus for defining the base of Cambrian Stage 10

Contrary to the claim of Peng et al. (2015, p. 302) that “… Lotagnostus americanus provides the same favorable characteristics as other agnostoids such as Ptychagnostus atavus , Lejopyge laevigata , Glyptagnostus reticulatus , and Agnostotes orientalis … ”, multiple studies on Laurentian occurrences of Lotagnostus , including our own, have established that it lacks several of the indispensable attributes that made those four species suitable for defining the bases of global Cambrian stages. First of all, the taxonomy of Lotagnostus is far more controversial. Secondly, in addition to the strongly divergent opinions among specialists regarding the acceptable range of morphologic variation within L. americanus , which in our view is best restricted to the holotype for the reasons already noted and discussed in greater detail in Systematic Paleontology, Lotagnostus was not as broadly distributed environmentally. Unlike the four agnostoids already in use as indices for global stages, whose environmental tolerances allowed them to occupy shallower waters where they were preserved in association with the endemic taxa of many paleocontinents, Lotagnostus in Laurentia was largely restricted to very deep/cold waters of lower slope and basinal settings ( Figure 6 ). We know not only within which zone the First Appearance Datum (FAD) of those other agnostoid species lie in Laurentia, but where within those zones, often to subzone level. In contrast, neither the fauna from the Hales, nor our rich faunas from the Windfall Formation, contain any taxa that constrain the position of the equivalent horizon in shallow Laurentian successions to less than approximately half a stage Figure 8 ). This is an unacceptable level of imprecision on one of the major Cambrian paleocontinents for a global stage boundary. Selection of a Lotagnostus - based GSSP for the base of Cambrian Stage 10 despite these shortcomings would be particularly unfortunate given the availability of an alternative horizon that does not suffer from such deficiencies; the FAD of the conodont Eoconodontus notchpeakensis ( Landing et al., 2011; Miller et al., 2011, 2015).

Systematics

Repositories. Types and illustrated specimens collected by J.F. Taylor and Loch are housed in the Carnegie Museum of Natural History ( CM), Pittsburgh , Pennsylvania or National Collection of Invertebrate and Plant Fossil Types ( GSC) in Ottawa, Canada. Re-illustrated specimens from collections of The Natural History Museum, London bear the prefix NHM. Trilobites collected by M.E. Taylor and conodonts illustrated herein are reposited at the Smithsonian National Museum of Natural History ( USNM): conodont specimens from collections 5/22/08C and 5/22/08D are assigned USNM locality numbers USGS CO-12121 and USGS CO-12122 , respectively .

Methods and Terminology. Trilobite specimens were inked and whitened with magnesium oxide in preparation for photography. Abundance data are provided in a cranidia-pygidia-librigenae (C-P-L) format. Measurements used in morphologic comparisons were made digitally on enlarged images, utilizing dimensions acquired with a calibrated microscope ocular. Ratio values are calculated means; minimum and maximum values are presented parenthetically. Morphologic terms used are those recommended by Whittington (1997) with additional reference to Robison (1982) and Shergold et al. (1990) for aspects of agnostoid morphology. Usage of open nomenclature follows that recommended by Bengtson (1988) wherein “cf.” denotes possible but uncertain assignment to the species designated, and “aff.” is used to identify a similar but definitely separate species.

In describing the partitioning of the pygidial axis in Lotagnostus , we interpret the furrows that bound the central lobe of the tripartite M1 as forward extensions of the longitudinal furrows that trisect M2, rather than following Westrop & Landing (2016) and Tortello (2018), who suggested that they represent the anterior deflection of F1 to intersect the articulating furrow. We note that Innitagnostus Opik (1967 , text-fig. 12, pl. 58, figs 3–4), another member of the Subfamily Agnostinae , exhibits a complete, transaxial F1 furrow while the anterior lobe still appears trisected. Moreover, F1 shallows over the midline in larger pygidia of L. rushtoni (e.g. Plate 12.11 ).

Length ratios for basal glabellar lobes: The utility of the length of basal glabellar lobes for discrimination of species within Lotagnostus has been debated at length. Some ( Ludvigsen et al., 1989; Rushton (2009); Westrop et al., 2011) consider it a useful character while others ( Peng et al., 2015) dismiss the variation in that feature as primarily taphonomic. In their critique, Peng et al. (2015, fig. 10) utilized a ratio designated the BLL:PGL, calculated by dividing the total length of the basal lobe by the length of the posteroglabella. In the present study, we employed a superior metric referred to herein as the EBL/APL ratio, which compares the lengths of the basal lobe and posteroglabella measured along different lines ( Figure 10.1 ). The Exsagittal Basal lobe Length (EBL) is the distance from the anterior-most point (the tip) of the basal lobe to the posterior margin (back of the basal lobe) measured parallel to the midline. It differs from the Basal Lobe Length (BLL) of Peng et al. (2015, fig. 10) in being measured along an exsagittal line, rather than spanning the total length from the anteriormost to posteriormost points regardless of whether they lie along the same line parallel to the midline. The Axial Posteroglabella Length (APL) is the distance along the midline from the center of F3 to the glabellar culmination. For specimens with a node at the glabellar termination, the measurement was to the base of the node. It differs from the Posteroglabellar Length (PGL) of Peng et al. (2015) in being measured along the midline, rather than exsagittally from the back of the basal lobe to the intersection of F3 with the axial furrow.

The EBL/APL ratio ( Figure 10.2 ) is typically higher than the BLL/PGL ratio for the same cephalon because the denominator is greater, due to 1) the anterior convexity of F3, which places its junction with the axial furrow junction farther forward than where it crosses the midline and 2) the most posterior point at the back of the basal lobe commonly lying some distance behind the glabellar culmination. Conversely, the length of the basal lobe measured exsagittally (EBL) is commonly somewhat less than the total length, not constrained to a line parallel to the axis (PGL), but that difference is small compared to the difference between the APL and PGL.

The EBL/APL ratio has several advantages over the BLL/PGL ratio. First, it is based on two totally independent lengths of basal lobe and posteroglabella, whereas the length of the posteroglabella in the PGL is commonly in part controlled by the length of the basal lobes. The PGL also can be affected by the position of the basal lobes if they are situated far enough back that their posterior margins lie behind the glabellar culmination. This relationship becomes more problematic when, as is commonly the case, the basal lobes are displaced by compaction and/or tectonic deformation. Such displacement does not distort the EBL/APL ratio. An advantage of the EBL over the BLL is that measurement of the latter is problematic in some agnostoids whose basal lobes merge adaxially with the occipital band behind the axis with no clearly defined boundary between those two elements. Consequently, a specific, posteriormost point on the back of the basal lobe is not determinable.

Phylum Arthropoda Siebold & Stannius, 1845

Figures

FIGURE 6. Reconstruction of northern Laurentian platform margin showing environmental settings interpreted for formations that have yielded Lotagnostus, and pie charts showing relative abundances of Lotagnostus, other agnostoids, olenids, and other trilobites. Position of Windfall Formation on the profile is based on the lithofacies documented in the Antelope Range. Rimmed platform profile and lithofacies patterned after reconstruction of Appalachian margin by James et al. (1989, fig. 12).

FIGURE 8. Estimated distribution of Lotagnostus species within the trilobite and conodont zonations. Probable range (black bar) delineates interval within which occurrences of a species probably lie. Maximum range (dashed lines) denotes full possible range. Abbreviations: C., Corbinia; Elk., Elkanaspis; Ellips., Ellipsocephaloides; I., Irvingella; N., Noelaspis; P., Phylacterus; S., Stigmacephalus; Skull., Skullrockian; Step., Steptoean; Trem., Trempealeauan. Zonal successions from: 1) Westrop, 1995; 2) Miller et al., 2003, 2015; 3) Westrop, 1986; 4) Ludvigsen et al., 1989.

FIGURE 10. Morphometric comparisons of some Lotagnostus species. (1) Dimensions measured on cephala. Abbreviations: AAL, Axial Anteroglabella Length; APL, Axial Posteroglabella Length; EBL, Exsaggital, Basal lobe Length; PFL, Preglabellar Field Length. (2) Scatter plot of EBL/APL ratio versus length of the cephalon for 3 species of Lotagnostus. L. “trisectus” refers to species identified by that name from Sweden in previous studies. (3) Scatter plot of PFL/AAL ratio versus length of the cephalon for 4 species of Lotagnostus. (4) Graphic comparison of posteroaxis length/length of axis as a function of pygidial length (excluding articulating half ring) for 3 species of Lotagnostus.

PLATE 12. Lotagnostus rushtoni n. sp. Each white scale bar represents 2mm. 1-3. Cephalon, CM 41383, from 5/22/08D, dorsal, anterior, and left lateral views. 4-6. Cephalon, USMN 775739, from D3383-CO, dorsal, anterior, and right lateral views. 7. Cephalon, USMN 775740, from D3383-CO, dorsal view. 8. Cephalon, USMN 775741, from D3383-CO, dorsal view. 9. Cephalon, USMN 775742, from D3383-CO, dorsal view. 10. Cephalon, CM 41384, from 5/22/08D, dorsal view. 11- 14. Pygidium, holotype, CM 41385, from 5/22/08D, dorsal, posterior, right lateral, and posterior oblique views. 15. Pygidium, USMN 775743, from D3383-CO, dorsal view. 16. Pygidium, USMN 775744, from D3383-CO, dorsal view. 17-19. Pygidium, USMN 775745, from D3383-CO, dorsal, left lateral, and posterior views. 20. Pygidium, CM 41386, from 5/22/08D, dorsal view. 21. Pygidium, USMN 775746, from 3D3381-CO, dorsal view.