Poposauroids (Pseudosuchia; Paracrocodylomorpha) are one of several lineages of ecologically unique archosauriform reptiles that diversified throughout the Triassic Period. Fossils of the poposauroid lineages Shuvosauridae and Poposaurus are well known from Late Triassic deposits across western Pangaea, represented by abundant materials in the southwestern United States in the Chinle Formation and the Dockum Group and rarer occurrences in the Ischigualasto Formation of Argentina. However, Late Triassic records of these lineages are currently unknown in other parts of Pangaea. Here, we extend the geographic range of Late Triassic poposauroids by reporting two incomplete femora from the Newark Supergroup of the eastern United States. Both specimens are from the Chatham Group of North Carolina, the first (NCSM 27054) from a quarry near Wadesboro in Anson County (Cumnock Formation), and the second (AMNH FR 24431) from a quarry near Gulf in Chatham County (Pekin Formation). Both possess diagnostic character states of Poposauroidea and the more complete specimen, NCSM 27054, lacks character states of shuvosaurids. We conclude that these femora represent poposauroids similar (if not identical) to Poposaurus gracilis among known taxa in the clade. Our findings expand the Late Triassic range of Poposauroidea into the central/eastern Pangaean Atlantic rift zone. NCSM 27054 and AMNH FR 24431 may represent some of the oldest Late Triassic poposauroid fossils and help fill a gap in the stratigraphic range of Poposauroidea.
1. Introduction
The Triassic Period represents a time of great diversification for archosauriform reptiles (birds, crocodiles, and their closest relatives; Nesbitt 2011; Cabreira et al. 2016; Ezcurra et al. 2017, 2020). The Poposauroidea, a lineage of paracrocodylomorph stem-crocodylians, exemplify this diversity, exhibiting broad ecological and morphological variation (including hypertrophy of the neural spines, loss of teeth, and bipedalism in various taxa) throughout the Triassic (Nesbitt 2007, 2011; Butler et al. 2011; Wang et al. 2009). Two lineages of large-bodied bipedal poposauroids occur in the Late Triassic: the beaked, edentulous shuvosaurids (e.g., Effigia, Shuvosaurus) and the toothed Poposaurus (Long & Murray 1995; Nesbitt & Norell 2006; Parker & Nesbitt 2013; Schachner et al. 2020). Nearly all Late Triassic poposauroids are from the northwestern fringes of Pangaea (the current-day western USA; Nesbitt & Norell 2006; Parker & Nesbitt 2013) with the exception of the southwestern Pangaean (Argentinian) shuvosaurid Sillosuchus longicervix Alcober & Parrish 1997. This latter occurrence highlights the potential for a much more widespread presence of poposauroids in the Late Triassic. One of the primary barriers to the dispersal of these poposauroids (and all terrestrial life) was enormous arid belts across the mid-latitude of Pangaea (Whiteside et al. 2011; 2015; Griffin et al. 2022). In spite of this, several aspects of northern and southern Pangaean faunas suggest that various tetrapod clades crossed these arid belts, as indicated by the presence of closely-related taxa on both sides (e.g., aetosaurs, Desojo et al. 2013; parasuchid phytosaurs, Kammerer et al. 2016; Stocker et al. 2017; lagerpetid ornithodirans, Nesbitt et al. 2009; metoposaurid temnospodyls, Chakravorti & Sengupta 2018). Sillosuchus was from south of these belts, with the remainder of known Late Triassic Poposauroidea residing north of them.
There is an exception to the otherwise latitudinallycontinuous barrier formed by the arid belts: the Atlantic rift zone, which formed a terrestrial northeast-southwest-trending, higher humidity zone during the Late Triassic Period that connected what is now eastern North America and northwestern Afro-Eurasia to present-day South America and western Africa prior to the origin of the central part of the Atlantic Ocean (Sellwood & Valdes 2006; Khalifa 2007; Dunne et al. 2021). Fossils from this rift zone are thus critical in filling biogeographic gaps in Late Triassic ecosystems. Archosauriform trackways are abundant throughout this region (e.g., Olsen 1980; Olsen & Baird 1986), but these are difficult to place to more specific groups due to shared plesiomorphies across the pes of archosaurs and their closest relatives (=Eucrocopoda; Demathieu & Demathieu 2004; Padian et al. 2010; Ezcurra 2016). The body fossil record of the Newark Supergroup, particularly from the Deep River Basin, has greatly expanded in recent years (Heckert et al. 2015; Sues et al. 2003; Zanno et al. 2015), filling out and clarifying the diversity of these low-latitude faunas.
Fig. 1.
Map of eastern North America, with highlighted box (B) showing North Carolina and the places of discovery of the poposauroid specimens NCSM 27054 and AMNH FR 24431. Orange represents the Newark Supergroup depositional basins; AMNH FR 24431 was found within the Durham Sub-basin while NCSM 27054 was found within the Wadesboro Sub-basin, together making up the larger Deep River Basin. Map constituent references: Newark depositional basins (marcostrat.org), US map (‘USA_States_ Generalized’: https://www.arcgis.com/home/item.html?id=8c2d6d7df8fa4142b0a1211c8dd66903&sublayer=0), Canada map (‘Territorial_Evolution_1867_-2003_TE_’: https://www-tlstest.arcgis.com/home/item.html?id=c705639d23e34f8691d97edc653a29a7)
Here we add to this diversity and document the first occurrences of Poposauroidea from the Late Triassic Chatham Group of the Deep River Basin, an Atlantic rift basin located in current-day North Carolina. We further explore its biogeographic implications for Late Triassic corridors prior to the split of Pangaea.
Abbreviations: AMNH FR, Fossil Reptile Collections at the American Museum of Natural History, New York City, USA; NHMUK (formerly BMNH), Natural History Museum, London, U.K.; GR, Ghost Ranch Ruth Hall Museum of Paleontology, Abiquiu, USA; MCP, Museu de Ciências e Tecnologia da Pontifícia Universidade Católica do Rio Grande do Sul, Porto Alegre, Brazil; NCSM, North Carolina Museum of Natural Sciences, Raleigh, USA; NMT, National Museums of Tanzania, Dar es Salaam, Tanzania; PVL, Paleontología de Vertebrados, Instituto “Miguel Lillo”, San Miguel de Tucumán, Argentina; SMNS, Staatliches Museum für Naturkunde, Stuttgart, Germany; TTU-P, Paleontological Collections at Texas Tech University Museum, Lubbock, USA; UCMP, University of California Museum of Paleontology, Berkeley, USA; YPM, Yale Peabody Museum of Natural History, New Haven, USA; ZPAL, Institute of Paleobiology of the Polish Academy of Sciences, Warsaw, Poland.
2. Geologic setting and age
Both femora were collected in exposures attributed to the Upper Triassic Chatham Group. More specifically, AMNH FR 24431 was collected from the Pekin Formation and NCSM 27054 was collected from the overlying Cumnock Formation. The sediments in these formations represent terrestrial deposits within the Deep River Basin (NCSM 27054 from the Wadesboro sub-basin, AMNH FR 24431 from the Sanford sub-basin), a rift basin that formed along with several others across eastern North America and Afro-Eurasia with the initiation of spreading between the North American and African tectonic plates in the late Permian Period (Campbell & Kimball 1923; Olsen et al. 1991; Clark et al. 2001). Radioisotopic ages are absent for this basin and nearly all circum-Atlantic rift basins, but correlations with the absolutely-dated deposits of the Newark Basin were employed (Olsen & Whiteside 2008; OLSEN et al. 2011). Correlation of the Newark Basin was accomplished when Olsen (1997) demonstrated a series of four tectonostratigraphic sequences (TS) that can be found throughout circum-Atlantic rift basin deposits of eastern North America and Morocco referred to as, from lowest to highest, TS I, TS II, TS III, and TS IV. The depositional initiation of each TS is thought to have occurred as a result of an extensional event associated with the aforementioned break-up of Pangaea and the opening of the Atlantic Ocean. Olsen et al. (2000) demonstrated extraordinarily similar sequence stratigraphy between the only two basins known to preserve all four TS, the western-Atlantic Fundy Basin (Nova Scotia) and the eastern-Atlantic Argana Basin (Morocco), further supporting the existence of the four TS proposed by Olsen (1997). The units of the Fundy Basin had previously been correlated with those of the absolutely-dated Newark Basin, and so the newly-identified boundary between TS II and TS III was subsequently dated to 228.5 Ma (Olsen & Whiteside 2008; Olsen et al. 2010), making this an intercontinentally-locatable age constraint on early Late Triassic strata of the Atlantic rift basins.
With this established, the upper part of the Pekin Formation is the lowest unit in TS III of the Deep River Basin, having been correlated with the Stockton Formation of the well-studied and magnetostratigraphically-dated Newark Basin. This provides us with an estimated age restriction between 222.5–228.5 Ma for the material described here.
3. Systematic paleontology
Archosauria Cope, 1869 sensu Gauthier & Padian, 1985
Pseudosuchia ZITTEL, 1890 sensu Gauthier & Padian, 1985
Poposauroidea Nopcsa, 1923 sensu Nesbitt, 2011
Fig. 2A–E
Specimen: NCSM 27054, left femur with complete proximal and distal ends missing a ∼3 cm section.
Locality and horizon: NCPALEO 0401 (precise location on file at NCMNS), near Wadesboro, Anson County, North Carolina. Cumnock Formation, Chatham Group, Newark Supergroup. SJN visited the quarry with Vince Schneider (who discovered the specimen) and determined it was from the southwestern side of the deepest side of the pit, stratigraphically high on the quarry wall. Although the exact position of the femur was not recorded during collection, these strata consist entirely of Cumnock exposures, per Schneider.
Age: Late Triassic, Carnian or early Norian, between 222.5–228.5 Ma (see Geological Setting and Age above).
Description: NCSM 27054 is comprised of two pieces representing much of the left femur. The proximal piece is represented by the proximal portion (=head) and the proximalmost section of the shaft, just distal to the transition between the head and the shaft (Fig 2A, upper images of 2B–E), whereas the distal portion is represented by ∼2/3 of the length of the element (lower images of 2B–E). Measurements include a maximum proximal width (trending anteromedially) = 37.7 mm, a maximum distal width (trending mediolaterally) = 32.8 mm and an estimated length of ∼190 cm. The preservation of both pieces is excellent and clear muscle scars are present.
The proximal surface possesses a transverse groove that trends anteromedially-posterolaterally parallel to the long axis of the surface (see Nesbitt 2011 [character 31], to understand the complex distribution of this feature), a feature common in poposauroids (Nesbitt 2011). As in other archosaurs, the proximal end of the femur features three prominent tubera easily viewed proximally: the anteromedial tuber, the posteromedial tuber, and the anterolateral tuber. The anteromedial tuber is about the same size as the posteromedial tuber, a character state shared among suchians (Nesbitt 2011). The anteromedial tuber possesses a distinct posteromedial expansion to form a head, like that in other poposauroids as well as saurischian dinosaurs (Nesbitt 2011). The medial surface of the anteromedial tuber is rounded, unlike the straight surface in silesaurids (e.g., Asilisaurus kongwe, NMT RB159; Silesaurus opolensis, ZPAL AbIII/457). The ventral margin of the tuber is well defined and a ridge at the distal margin can be traced posterolaterally to just distal of the posteromedial tuber. The bone surface distal to this ridge is a porous surface consisting of pits and small ridges (in contrast to smooth compacta), similar to Poposaurus gracilis (YPM 57100), Shuvosaurus inexpectatus (TTU-P9001), and saurischians (e.g., Tawa hallae, GR 244). In posteromedial view, the anteromedial tuber of NCSM 27054 is rounded, rather than hooked as in early pterosauromorphs (e.g., Dromomeron romeri, GR 218; Arcticodactylus cromptonellus, Jenkins et al. 2001).
The posteromedial tuber is separated from the anteromedial tuber by a concave depression visible in proximal view (Fig. 2A). The distal margin of the posteromedial tuber is defined by a change in bone surface texture from proximally compact to distally more porous bone, like that distal to the anteromedial tuber. An anterolateral tuber is present on the proximal end of the femur (Fig. 2A) as a slightly rounded ridge extending proximodistally along the anterolateral surface, as is typical of many archosaurs (e.g., Tawa hallae, GR 244; Poposaurus gracilis, YPM 57100). However, a homologous ridge is absent in Shuvosaurus inexpectatus (TTU-P9001; Nesbitt 2011) and Effigia okeeffeae (AMNH FR 30588; Nesbitt 2007).
The posterolateral portion of the proximal articular surface descends distally from the rest of this surface (Fig. 2D), resulting in a gradually depressed area often referred to as the ‘fossa trochanterica’ in ornithodirans (e.g., Dromomeron romeri, GR 218; Tawa hallae, Nesbitt 2011), but which is also present in poposauroids (e.g., Poposaurus gracilis, Schachner et al. 2020; Shuvosaurus inexpectatus, TTU-P9001). This sloped articular surface originates at the posteromedial tuber and extends posterolaterally to the end of the proximal surface. In anterolateral view, the profile of the proximal end is arched like that of other pseudosuchians (e.g., Poposaurus gracilis, YPM 57100) and this is in contrast to the more flattened morphology of dinosauriforms (e.g., Silesaurus opolensis, DZIK 2003; Saturnalia tupiniquim, Langer 2003).
The medial expansion of the head of the femur of NCSM 27054 is not as medially-expanded as that of Shuvosaurus inexpectatus (TTU-P9001; Nesbitt 2011) and Effigia okeeffeae (AMNH FR 30588), but it is nearly identical to that of Poposaurus gracilis (YPM 57100). The femur possesses a smooth transition between the head and the shaft on the medial portion of the bone, as is typical of early archosaurs and differs from the distinct notch or concave emargination found in ornithodirans (e.g., Dromomeron romeri, GR 218; Asilisaurus kongwe, NMT RB159).
The anterolateral surface of the proximal portion of the femur is largely smooth and featureless, lacking the ridge often referred to as the dorsolateral trochanter found in this location in some crocodylomorphs (Terrestrisuchus gracilis, NHMUK PV R 7562; Nesbitt 2011; Kayentasuchus walkeri, UCMP 131830; Clark & Sues 2002) and most dinosauriforms (e.g., Silesaurus opolensis, ZPAL Ab III/460/1; Saturnalia tupiniquim, Langer 2003). At the distal part of the preserved portion of the proximal end, slight rugosities are present (Fig. 2C). Slight rugosities are present in the same place in shuvosaurids (e.g., Effigia okeeffeae, AMNH FR 30588; Shuvosaurus inexpectatus, TTU-P18309) and in other pseudosuchians such as ornithosuchids (Riojasuchus tenuisceps, PVL 3827) and some early diverging crocodylomorphs (Kayentasuchus walkeri, UCMP 131830). This area bears the anterior trochanter in avemetatarsalians (e.g., Teleocrater rhadinus, NHMUK PV R6795; Saturnalia tupiniquim, MCP 3846-PV). A proximal condylar fold, as is found in some loricatans (Fasolasuchus tenax, PVL 3851; Nesbitt 2011), including some crocodylomorphs (e.g., Hesperosuchus agilis, AMNH FR 6758), is absent in NCSM 27054.
Based on comparisons with other poposauroids (e.g., Poposaurus gracilis, YPM 57100; Effigia okeeffeae, AMNH FR 30588; Shuvosaurus inexpectatus, TTUP9001), the gap between the preserved proximal and distal portions of NCSM 27054 should represent approximately 3 cm of missing bone. Breaks in both the proximal and distal pieces show that the bone was largely hollow, with a large medullary cavity that has a cortex thickness varying from 2.14 mm to 4.24 mm across a diameter of 14.9 mm at its shortest, to 22.14 mm at its longest. This cortical thickness is only found in the crocodylomorph Terrestrisuchus gracilis, Ornithodira, and Poposauroidea among early members of Archosauria (see character 323 of Nesbitt 2011). It is not clear if NCSM 27054 lacked a ridge for the attachment of the M. caudifemoralis as in Shuvosaurus inexpectatus (TTU-P9001; Nesbitt 2011) and Effigia okeeffeae (AMNH FR 30588). A deep pit lies on the medial portion of the distal portion of the femur and is likely homologous with the deep pit associated with the attachment of the M. caudifemoralis in most archosaurs (Fig. 2E). If homologous, this pit is deeper in NCSM 27054 and shuvosaurids (Shuvosaurus inexpectatus, TTU-P18309) and the medial border of the shaft is slightly medially bowed at the proximodistal level of (and thus medial to) this pit, a character state exclusive to poposauroids (Nesbitt 2005; 2007; 2011). Just lateral to the pit, the surface of NCSM 27054 is slightly rugose, a feature that is absent in a well-preserved femur of Shuvosaurus inexpectatus (TTU-P18309). The slightly rugose surface of NCSM 27054 swells and forms the posterolateral edge of the femur. This rugosity is also present in Poposaurus gracilis (YPM 57100) and is likely the attachment for the M. iliofemoralis externus (Nesbittet al. 2017; 2018). From this swollen area, a ridge that defines the posterolateral border emerges and extends to the crista tibiofibularis (Fig. 2C–E).
Distally, the posterior portions of a crista tibiofibularis and the fibular condyle are broken off. However, enough of the bases of the lateral condyle and crista tibiofibularis are preserved to demonstrate that these structures form roughly a right angle to each other in distal view, a feature differing from the obtuse angle in Dinosauriformes (Nesbitt 2011) but shared with Effigia okeeffeae (AMNH FR 30587; Nesbitt 2007), Poposaurus gracilis (YPM 57100), and some loricatans (e.g., Batrachotomus kupferzellensis, Gower & Schoch 2009; Postosuchus kirkpatricki, TTU-P9000; Dromicosuchus grallator, NCSM 13733). In distal view, the lateral condyle tapers posterolaterally and the distal surface of this condyle extends more distally than any of the other condyles. A small groove is present between the lateral condyle and the crista tibiofibularis in distal view. Anteriorly, there is a proximodistally oriented fossa that separates the medial and lateral sides of the distal end.
Fig. 2.
NCSM 27054, a left poposauroid femur in (A) proximal, (B, upper two) anterolateral, (B, lower) distal, (C) lateral, (D) posteromedial, and (E) medial views. Scale bar is 5 cm. alt = anterolateral tuber, amt = anteromedial tuber, ctf = crista tibiofibularis, ‘ft’ = ‘fossa trochanterica’, gr = groove, mc = medial condyle, lc = lateral condyle, pit = pit, pmt = posteromedial tuber, r = ridge, rs = rugose surface.
Fig. 3.
AMNH FR 24431, a distal left poposauroid femur in (A) anterolateral, (B) lateral, (C) posteromedial, (D) medial, and (E) distal views. Scale bar is 5 cm. ctf = crista tibiofibularis, gr = groove, mc = medial condyle, lc = lateral condyle.
Poposauroidea indet.
Fig. 3
Specimen: AMNH FR 24431, distal ∼25% of the left femur (Fig. 3).
Locality and horizon: Pekin Formation, Pomona Pipe Products Pit, near Gulf, Chatham Co., North Carolina. Collected April 17th, 1965.
Age: Late Triassic, Carnian or early Norian, between 222.5–228.5 Ma (see Geological Setting and Age above).
Description: The lateral condyle and crista tibiofibularis are preserved, whereas the medial condyle is missing. The shaft has thin walls (∼5 mm or less) and is caved in (Fig. 3). We estimate the length of the femur to be ∼24 cm based on the width (estimated at 55 mm) of AMNH FR 24431 and comparisons to Poposaurus gracilis (YPM 57100). The lateral condyle is posterolaterally directed, rounded and separated from the crista tibiofibularis by a groove on both its medial and distal surfaces (Fig. 3E). The crista tibiofibularis tapers posterolaterally to a blunt point and the distal surface is slightly more proximal than the rest of the distal end.
The shape of the distal end is consistent with that of Poposaurus gracilis (YPM 57100) and Effigia okeeffeae (AMNH FR 30587) in that the crista tibiofibularis is pointed and separated from the lateral condyle by a groove, the lateral condyle is tapered posterolaterally and rounded, and that there is a 90o–100o angle between the posterior portion of the lateral condyle and the lateral side of the crista tibiofibularis (Fig. 3B, E). However, some of these character states are also present in other pseudosuchians. For example, the 90o–100o angle between the posterior portion of the lateral condyle and the lateral side of the crista tibiofibularis is also present in some loricatans (e.g., Batrachotomus kupferzellensis, Gower & Schoch 2009; Postosuchus kirkpatricki, TTU-P9000; Dromicosuchus grallator, NCSM 13733).
4. Discussion
4.1. Identification
The nearly complete femur NCSM 27054 preserves a combination of poposauroid character states previously tested in phylogenetic analyses (Nesbitt 2011, and iterations of the same dataset, e.g., Butler et al. 2011; NESBITT et al. 2020; Smith et al. 2024). This combination includes the following character states: a proximal surface with a transverse groove that trends anteromedially-posterolaterally parallel to the long axis of the surface (broadly present in Paracrocodylomorpha); three prominent tubera clearly visible proximally (the anteromedial tuber, the posteromedial tuber, and the anterolateral tuber); the posterolateral portion of the proximal articular surface is depressed (=‘fossa trochanterica’); a low or absent ridge for the attachment of the M. caudifemoralis; absence of a distinct ridge in the anterolateral surface of the proximal end (= anterior trochanter area); thin cortex with an open medullary cavity; a medially-bowed border of the femur adjacent to a pit for the attachment of the M. caudifemoralis medially; approximately a 90° angle in distal view between the lateral condyle and crista tibiofibularis; and a small groove present between the lateral condyle and the crista tibiofibularis in distal view. Similarly, AMNH FR 24431 preserves character states consistent with those of the distal end of NCSM 27054. The combination of character states listed above supports a placement within Poposauroidea for both specimens. The lack of clear shuvosaurid character states (e.g., an enlarged and hooked anteromedial tuber of the proximal end, a less arched proximal profile in posterolateral view; Nesbitt 2011; Nesbitt & Chatterjee 2023) indicates that NCSM 27054 does not belong to that clade, but a more precise and well-supported position within Poposauroidea is not inferable based on this material alone. However, we do note that the character state combination in NCSM 27054 is consistent with that of Poposaurus gracilis (e.g., YPM 57100), although NCSM 27054 and Poposaurus gracilis do not appear to possess any clear synapomorphies exclusively. Therefore, we identify the Chatham Group poposauroid only as a non-shuvosaurid poposauroid, albeit noting the possibility that it could be a close relative of or even referable to Poposaurus.
4.2. Biogeography
NCSM 27054 and AMNH FR 24431 narrow a significant biogeographic gap between the western North American and southern South American fossil records of poposauroids, demonstrating that this group was present in the Late Triassic of the Atlantic rift basins of eastern/central Pangaea. Their Carnian to early Norian age makes these among the oldest poposauroid fossils from the Late Triassic, coeval or near coeval with the Carnian Sillosuchus longicervix (Alcober & Parrish 1997; Desojo et al. 2020) and the possibly Carnian-aged holotype specimen of Poposaurus gracilis (Lovelace et al. 2024). Because of the fragmentary remains of these specimens, their indeterminate phylogenetic position within Poposauroidea, and the paucity of Carnian vertebrate-bearing units globally (Lovelace et al. 2024), it is difficult to say whether these specimens might represent earlier-diverging, biostratigraphically-informative forms of the Poposaurus lineage.
The presence of poposauroids adds to a growing record of tetrapod fossils whose geographic ranges are connected by Atlantic rift basin occurrences, including Pekin Formation aetosaurs (e.g., Gorgetosuchus pekinensis; Heckert et al. 2015), crocodylomorphs (Dromicosuchus grallator, Carnufex carolinensis; Sues et al. 2003; Zanno et al. 2015), dicynodonts (Placerias hesternus; Baird & Patterson 1968), and traversodontid cynodonts (Boreogomphodon jeffersoni; Sues & Olsen 1990; LIU et al. 2017). The presence of these poposauroid femora and the discovery of several well-preserved tetrapods from the Pekin Formation (e.g., Boregogomphodon jeffersoni; LIU et al. 2017) suggest better-represented and thus more informative amniote material likely awaits discovery within the Deep River Basin.
5. Conclusion
NCSM 27054 and AMNH FR 24431 represent the first confirmed poposauroid materials from the Chatham Group and the Atlantic rift region of Late Triassic Pangaea. Their occurrence in an Atlantic rift zone basin fills biogeographic gaps in the Late Triassic poposauroid fossil record, previously restricted to northwestern Pangaea (western North America) and southwestern Pangaea (Argentina). Their occurrence supports the continuity of faunal representation between North and South Pangaean vertebrate assemblages.
Acknowledgements
We thank Vince Schneider (NCMNS, retired) for the collection of NCSM 27054 and Sean Moran for arranging the loan of NCSM 27054. We thank Marilyn Fox and Vannessa Rue (YPM) and Patricia Holroyd at UCMP for facilitating comparisons with other Poposaurus material. We thank William Parker and Nate Smith for their considerate and helpful reviews of this publication. Funding for SJN was provided by NSF EAR 1943286.
