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Phylogeny and biogeography of some Cretaceous spatangoid echinoids with special emphasis on taxa from the Western Interior Seaway

Published online by Cambridge University Press:  15 December 2020

Steven Byrum  and
Bruce S. Lieberman Open the ORCID record for Bruce S. Lieberman [Opens in a new window]
Steven Byrum
Affiliation:
Florida Museum of Natural History, University of Florida, Gainesville, Florida32611, USA
Bruce S. Lieberman*
Affiliation:
Department of Ecology and Evolutionary Biology and Biodiversity Institute, University of Kansas, Lawrence, Kansas66045, USA
*
*Corresponding Author
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Abstract

Members of the echinoid order Spatangoida, a highly diverse and abundant marine invertebrate clade, were important denizens of the Cretaceous Western Interior Seaway (WIS), an epicontinental seaway that divided North America in two during an interval of greenhouse conditions between roughly 100 and 65 million years ago. A phylogenetic analysis of spatangoids was conducted using a character matrix of 32 characters from 21 species. Species that occur in the WIS were considered comprehensively, and species from other regions such as South America, Europe, and North Africa were also incorporated into the analysis. Phylogenetic patterns retrieved are largely congruent with preexisting family-level classifications; however, species within several genera, especially Hemiaster and Heteraster, need to be reassigned so that classification better reflects phylogeny. The genera Washitaster and Heteraster are closely related, as are Mecaster, Palhemiaster, and Proraster; Pliotoxaster, Macraster, and Hemiaster; and Micraster and Diplodetus. Biogeographic patterns were also considered using the phylogeny, and several episodes of vicariance and range expansion were identified. These were possibly related to some of the various major episodes of sea-level rise and fall during the Cretaceous. In particular, Valangian–mid-Aptian regressions may have caused vicariance within Heteraster and Washitaster while other early spatangoid vicariance may be related to regressions during the late Aptian–early Cenomanian. Further, vicariance caused by regressions during the mid-Cenomanian–Maastrichtian may have driven diversification within Micraster and Diplodetus. Last, transgressions during the late Aptian–early Cenomanian seem to have spurred prominent range expansions in Mecaster and Hemiaster.

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Journal of Paleontology , Volume 95 , Issue 3 , May 2021 , pp. 613 - 623
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Copyright © The Author(s), 2020. Published by Cambridge University Press

Introduction

Echinoids are a highly diverse group of marine invertebrates known from a plethora of modern and ancient habitats (Smith, Reference Smith1984; Smith and Jeffery, Reference Smith and Jeffery2000; Mooi, Reference Mooi2001; Kroh and Mooi, Reference Kroh and Mooi2018; Arachchige et al., Reference Arachchige, Jayakody, Mooi and Kroh2019). The group experienced a sharp diversity decline at the end of the Paleozoic, when the majority of stem-group echinoids went extinct after the end-Permian mass extinction (Thompson et al., Reference Thompson, Hu, Zhang, Petsios, Cotton, Huang, Zhou, Wen and Bottjer2018). However, their diversity rebounded in the Mesozoic era with the appearance of irregular echinoids (Saucède et al., Reference Saucède, Mooi and David2007). Echinoids were abundant denizens of the shallow benthic communities of the southern parts of the Western Interior Seaway (WIS), in what is today the southwestern United States (e.g., Texas and New Mexico) and northern Mexico (Sonora and Coahuila). This is where the WIS opened into the much larger North Atlantic/West Tethys for much of its duration (Blakey, Reference Blakey2014). Many of the first echinoids found in this region were described in various nineteenth-century studies (e.g., Roemer, Reference Roemer1849, Reference Roemer1852; Clark, Reference Clark1893). Subsequent works include descriptions by Clark (Reference Clark1915) and Cooke (Reference Cooke1946, Reference Cooke1953, Reference Cooke1955) and publications focused on taxa from Texas (Adkins, Reference Adkins1928; Whitney and Kellum, Reference Whitney and Kellum1966; Smith and Rader, Reference Smith and Rader2009; Morgan, Reference Morgan2016; Thompson, Reference Thompson2016). Echinoids have been frequent subjects of phylogenetic analysis (relatively recent publications in this area include Kroh and Smith, Reference Kroh and Smith2010; Mooi et al., Reference Mooi, Kroh and Srivastava2014; Koch et al., Reference Koch, Coppard, Lessios, Briggs, Mooi and Rouse2018; Thompson et al., Reference Thompson, Hu, Zhang, Petsios, Cotton, Huang, Zhou, Wen and Bottjer2018).

Among the irregular echinoids, the most diverse and abundant are the heart urchins of the order Spatangoida, which inhabit all oceans and a wide array of habitats (Stockley et al., Reference Stockley, Smith, Littlewood, Lessios and MacKenzie-Dodds2005). Spatangoids are deposit-feeding burrowers that radiated in the Cretaceous and survive into the present (Mortensen, Reference Mortensen1950; Fischer, Reference Fischer and Moore1966; Smith, Reference Smith1984; Mooi, Reference Mooi2001; Smith and Wright, Reference Smith and Wright2008, Reference Smith and Wright2012; Smith and Kroh, Reference Smith and Kroh2011; Kroh and Mooi, Reference Kroh and Mooi2018). They have also been incorporated into phylogenetic analyses that considered both living and fossil taxa (Villier et al., Reference Villier, Néraudeau, Clavel, Neumann and David2004; Stockley et al., Reference Stockley, Smith, Littlewood, Lessios and MacKenzie-Dodds2005; Saucède et al., Reference Saucède, Mooi and David2007; Kroh and Smith, Reference Kroh and Smith2010; Kroh et al., Reference Kroh, Lukender and Gallemí2014). The goal of the present study is to focus on species-level phylogenetic relationships of Cretaceous spatangoids, with special emphasis on taxa from the WIS, although it was not possible to consider to any great extent the diverse range of taxa present in the Mississippi Embayment, an important and related biogeographic region (see Zachos, Reference Zachos2017), as taxa in this region primarily radiated too late to be relevant to the present analysis. The present study also uses the phylogenetic perspective to consider aspects of the biogeographic relationship between the WIS and other parts of the Cretaceous world.

Materials and methods

Taxonomic sampling

Twenty-one taxa were selected for phylogenetic analysis, comprising representatives of the “Toxasteridae,” Hemiasteridae, and Micrasteridae, the largest and most morphologically diverse clades within the Spatangoida (Smith and Kroh, Reference Smith and Kroh2011). Due to the large number of spatangoid species, not all could be considered in the phylogenetic analysis. Thus, only Cretaceous species, and not Cenozoic and modern forms, were analyzed. All well-preserved Cretaceous WIS species for which at least 75% of the characters used in the phylogenetic analysis could be coded were considered. Additional Cretaceous spatangoid species from outside the WIS were selected using the criteria that: (1) type specimens or quality photographs of these needed to be accessible, (2) species needed to be defined such that at least 75% of characters could be coded, and (3) all major Cretaceous biogeographic regions should be represented. This meant that species that were the subject of more recent systematic treatments were better represented in the analysis. Taxa subjected to phylogenetic analysis and relevant material examined are listed in Table 1. We follow the classification scheme of Smith and Kroh (Reference Smith and Kroh2011) as it has been widely utilized, although an updated classification has recently been presented by Kroh (Reference Kroh and Lawrence2020).

Table 1. Taxa considered in phylogenetic analysis, relevant material examined, and geographic occurrence; KUMIP = Division of Invertebrate Paleontology, Biodiversity Institute, University of Kansas, Lawrence, USA; NMMNH = New Mexico Museum of Natural History, Albuquerque, USA; STIB = Universität Bonn, Germany; USNM = United States National Museum of Natural History, Washington, D.C., USA; UT = Non-Vertebrate Paleontology Laboratory, University of Texas, Austin, USA. For geographic occurrence, 0 = Western Europe; 1 = North Africa; 2 = South America; 3 = the WIS.

Characters

Character and character state descriptions were developed via detailed comparative examination of test features. The preexisting literature was also considered to identify characters that had previously been used to diagnose groups and infer relationships among groups. Villier et al. (Reference Villier, Néraudeau, Clavel, Neumann and David2004), Smith and Stockley (Reference Smith and Stockley2005), and Kroh and Smith (Reference Kroh and Smith2010) were especially helpful in this regard. In total, 32 characters were identified (Appendix), including a combination of qualitative and quantitative characters. Quantitative characters were measured for coding using the program ImageJ; character states are noncontinuous relative to each other, and multistate characters were treated as unordered (Table 2). A variety of means for coding characters exist, and we have followed the precepts and principles presented by Wiley and Lieberman (Reference Wiley and Lieberman2011) for identifying hypotheses of homologies and coding them. No explicit models of character weighting were used, but different morphological systems varied in the number of characters they contained. Further, some characters may display more homoplasy than others; thus, different characters may provide varying degrees of phylogenetic resolution.

Table 2. Character state distributions for taxa used in phylogenetic analysis. ? = missing data.

Phylogenetic methods

A parsimony analysis was implemented using the matrix in Table 2 via PAUP 4.0 (Swofford, Reference Swofford2003), with Toxaster retusus (Lamarck, Reference Lamarck1816) employed as the outgroup; this taxon has been previously treated as an earliest representative of the group by Stockley et al. (Reference Stockley, Smith, Littlewood, Lessios and MacKenzie-Dodds2005). (Nexus files utilized are provided as Supplemental Data.) Data were subjected to a branch-and-bound search (Hendy and Penny, Reference Hendy and Penny1982) to find the most parsimonious tree(s). A bootstrap analysis was conducted using a heuristic search in PAUP 4.0 to determine support for each node appearing in the most parsimonious tree(s) using 100 bootstrap replicates in a stepwise search that employed five random replicated addition sequences per bootstrap replication with one tree held. Jackknife analysis was also used, again using 100 replicates in a stepwise search that employed five random addition sequences per replication, with the percentage of characters removed value chosen at 10%, which equates to three characters, and one tree held. For both support tests, groups were retained that were compatible with the 50% majority rule consensus tree. In addition, a Bremer branch support analysis (Bremer, Reference Bremer1994) was conducted to calculate node support using the difference between the length of the most parsimonious tree and the largest tree in which a node is maintained.

Finally, a Bayesian analysis was conducted using MrBayes 3.2.7, again using Table 2 and Toxaster retusus as an outgroup, to explore how well supported the various nodes in the parsimony tree were. The original file was exported from the simplified nexus file provided as Supplemental Data. No partitioning of the data was implemented as there were no a priori reasons for differentiating among character types or morphological systems. A standard MKV model was employed without a particular starting tree and performed on the CIPRES cluster. The analysis ran for 10 million generations and sampled every 1,000 generations. The posterior probability values for nodes retrieved in the analysis were then compared with those in the parsimony tree (see Cole, Reference Cole2019). Note that there has been extensive discussion about whether a Bayesian or a parsimony approach is better justified for the analysis of morphological character data, and consideration of this topic goes substantially beyond the scope of this contribution. We apply these different methods here only for purposes of considering commonality of results achieved across different methods. The interested reader is referred to discussions by Wiley and Lieberman (Reference Wiley and Lieberman2011) and Wright (Reference Wright2019) for greater amplification of the various issues involved with applying different phylogenetic methods to morphological data.

Biogeographic analysis

Biogeographic patterns were inferred by first substituting the area of occurrence of the species for a given species name. The areas utilized were Western Europe, North Africa, South America, and the WIS (Fig. 1), and these represent major areas of endemism for echinoids during the Late Cretaceous (see e.g., Smith, Reference Smith1984, Reference Smith1992; Rosen and Smith, Reference Rosen and Smith1988; Néraudeau and Mathey, Reference Néraudeau and Mathey2000). Biogeographic states were then mapped to ancestral nodes using the modified version of Fitch parsimony optimization for multistate characters, which treats all transformations between different character states as unordered (described by Lieberman, Reference Lieberman2000). This method minimizes the assumptions about the way biogeographic change happens between different regions. Transitions between ancestral and descendant nodes or ancestral and descendant terminals that involved range contraction were treated as potential instances of vicariance, and equivalent transitions involving range expansion were treated as potential instances of dispersal following Lieberman (Reference Lieberman2000).

Figure 1. Paleogeographic reconstruction for Late Cretaceous ~90 Ma; © 2016 Colorado Plateau Geosystems Inc.

Repositories and institutional abbreviations

Type, figured, and other specimens examined in this study are deposited in the following institutions: Division of Invertebrate Paleontology, Biodiversity Institute, University of Kansas (KUMIP); New Mexico Museum of Natural History (NMMNH); United States National Museum of Natural History (USNM); and Non-Vertebrate Paleontology Laboratory, University of Texas, Austin (UT and UT BEG). High-quality images of type specimens were also provided by the Universität Bonn (STIB).

Systematic paleontology

Order Spatangoida Agassiz, Reference Agassiz1840
Family Hemiasteridae Clark, Reference Clark1917
Genus Hemiaster Desor in Agassiz and Desor, Reference Agassiz and Desor1847

Type species

Spatangus bufo Brongniart, Reference Brongniart1822, Late Cretaceous, Cenomanian, France.

“Hemiaster” texanus (Roemer, Reference Roemer1849)
 Figure 2.1

Reference Roemer1849

Hemiaster texanus Roemer, p. 77, 393.

Reference Cooke1953

Hemiaster texanus; Cooke, p. 33, pl. 13, figs. 1–4 (see for more complete synonymy).

Reference Cooke1955

Hemiaster texanus; Cooke, p. 109, pl. 29, figs. 5–10.

Reference Smith and Bengtson1991

Mecaster texanum; Smith in Smith and Bengtson, p. 13.

Reference Oliveira, Manso, Andrade and Souza-Lima2013

Mecaster texanum; Oliveira et al., p. 11, figs. 7A–C, 9A–I.

Reference Manso and Souza-Lima2017

Mecaster texanus; Manso and Souza-Lima, p. 225, figs. 5, 6.

Holotype

STIB Texas 156, New Braunfels, Texas, Late Cretaceous, Santonian.

Materials

STIB Texas 156; UT BEG 34771, from the Austin Formation, Late Cretaceous, Santonian, Hondo River, 4.0 miles northwest of Hondo, Medina County, Texas, USA.

Figure 2. “Hemiaster” texanus: (1) STIB Texas 156; and “Heteraster” texanus (STIB Texas 157) (2). Scale bars = 1 cm.

Remarks

“Hemiaster” texanus does not share a sister group relationship with Hemiaster bufo (Brongniart, Reference Brongniart1822), but the phylogenetic analysis presented herein suggests that they are relatively closely related. Further, it has been assigned to Hemiaster previously (e.g., Cooke, Reference Cooke1953) and is closely related to other taxa traditionally assigned to Hemiaster (e.g., H. cranium Cooke, Reference Cooke1946; “Hemiaster” batnensis (Coquand, Reference Coquand1862), see Berndt, Reference Berndt2003; and “Hemiaster” estepi Lucas, Reference Lucas2000). Thus, rather than creating a new monotypic genus or lumping the species in with other genera, “Hemiaster” texanus is assigned to a paraphyletic genus following the ‘within quotes’ convention of Wiley (Reference Wiley1979). Photographs of the holotype had not been previously published, and thus one is provided herein.

Figure 3. (1) “Diplodetus” americanus (USNM PAL 76285). (2) Proraster humphreysanus (USNM PAL 331). Scale bars = 1 cm.

Genus Proraster Lambert, Reference Lambert and Grossouvre1895

Type species

Schizaster atavus Arnaud in Cotteau, Reference Cotteau1883; Late Cretaceous, Campanian, near Juillaguet (now Boisné-la-Tude), Charente, and Mensignac, Dordogne, France.

Proraster humphreysanus (Meek and Hayden, Reference Meek and Hayden1857)
 Figure 3.2

Reference Meek and Hayden1857

Hemiaster? humphreysanus Meek and Hayden, p. 147.

Reference Cooke1953

Hemiaster humphreysanus; Cooke, p. 30, pl. 12, figs. 1–4 (see for more complete synonymy).

Reference Hook and Cobban2017

Hemiaster humphreysanus; Hook and Cobban, p. 2.

Holotype

USNM PAL 331, Cedar Creek Anticline, Montana, Pierre Shale, Late Cretaceous, Maastrichtian.

Materials

USNM PAL 331.

Remarks

This species is referred to Proraster on the basis of its close relationship to Proraster dalli (Clark, Reference Clark1891) in the phylogenetic analysis and is a new taxonomic combination. Cooke (Reference Cooke1953) had posited that these species were closely similar. However, since the type species of Proraster could not be included in the phylogenetic analysis, it is possible that the generic assignment of Proraster humphreysanus could change upon subsequent study. Photographs of the holotype had not been previously published, and thus one is provided herein.

Family Micrasteridae Lambert, Reference Lambert1920
Genus Diplodetus Schlüter, Reference Schlüter1900

Type species

Diplodetus brevistella Schlüter, Reference Schlüter1870; Late Cretaceous, Darup, Westphalia, Germany.

“Diplodetus” americanus (Stephenson, Reference Stephenson1941)
Figure 3.1

Reference Stephenson1941

Micraster (Plesiaster) americanus Stephenson, p. 69, pl. 7, figs. 1–4.

Reference Cooke1953

Micraster americanus; Cooke, p. 38, pl. 15, figs. 10–13.

Reference Jagt2000

Diplodetus americanus; Jagt, p. 288.

Holotype

USNM PAL 76285, six miles east of Castroville, Texas, cut in San Antonio road in Bexar County, ‘Navarro group,’ Late Cretaceous, Maastrichtian.

Materials

USNM 76285; UT 83001, cut in San Antonio road in Bexar County, Corsicana Marl, Navarro group, Late Cretaceous, Maastrichtian.

Remarks

“Diplodetus” americanus does not share an exclusive sister group relationship with Diplodetus parvistella (Schluter, Reference Schlüter1900), but the phylogenetic analysis presented herein suggests that they are relatively closely related. Given that Diplodetus parvistella and the type of the genus, Diplodetus brevistella Schlüter, Reference Schlüter1870, are closely related (e.g., Smith and Kroh, Reference Smith and Kroh2011), and to avoid creating a new monotypic genus, “Diplodetus” americanus is referred to a genus that is designated as paraphyletic following the within-quotes convention of Wiley (Reference Wiley1979), though other taxonomic rearrangements are also possible. However, since the type species of Diplodetus could not be included in the phylogenetic analysis, it is possible that the generic assignment of “Diplodetus” americanus could change upon subsequent study. Photographs of the holotype have not been published for several decades, and thus one is provided herein.

Family Toxasteridae Lambert, Reference Lambert1920
Genus Heteraster d'Orbigny, Reference d'Orbigny1855

Type species

Spatangus oblongus Brongniart, Reference Brongniart1821; Late Cretaceous, Valanginian–Hauterivian, France.

“Heteraster” texanus (Roemer, Reference Roemer1849)
Figure 2.2

Reference Roemer1849

Toxaster texanus Roemer, p. 393.

Reference Adkins1928

Heteraster texanus; Adkins, p. 55, pl. 3, fig. 1.

Reference Cooke1946

Enallaster texanus; Cooke, p. 231, pl. 33, figs. 9–12 (see for more complete synonymy).

Reference Cooke1955

Enallaster texanus; Cooke, p. 102.

Reference Villier, Néraudeau, Clavel, Neumann and David2004

Heteraster texanus; Villier et al., p. 267.

Reference Smith and Rader2009

Enallaster texanus; Smith and Rader, p. 9.

Reference Silva-Martínez, Blanco-Piñón, León-González and Rodríguez-Vela2014?

Heteraster texanus; Silva-Martinez et al., p. 388, fig. 6e–g.

Reference Thompson2016

Heteraster texanus; Thompson, p. 100.

Reference Melo, Sánchez, Marín and Figueras2017

Heteraster texanus; Melo et al., p. S120, fig. 7.

Reference Martha, Taylor and Rader2019

Heteraster texanus; Martha et al., p. 246.

Holotype

STIB Texas 157, Fredricksburg, Texas, Late Cretaceous, Albian.

Materials

KUMIP 370536, 370548–370556 (detailed locality information is available for all KUMIP specimens at https://biodiversity.ku.edu/invertebrate-paleontology/collections/collections-search); STIB Texas 157; UT BEG 34165.

Remarks

“Heteraster” texanus does not share an exclusive sister group relationship with Heteraster oblongus (Brongniart, Reference Brongniart1821), but the phylogenetic analysis presented herein suggests that they are relatively closely related. Further, the species has traditionally been assigned to Heteraster (e.g., see Villier et al., Reference Villier, Néraudeau, Clavel, Neumann and David2004, Thompson, Reference Thompson2016; Yavari et al., Reference Yavari, Yadzi, Gahalavand, Kamyabi Shadan, Adabi and Villier2016). Thus, rather than creating a new monotypic genus, or lumping the species within Washitaster Lambert, Reference Lambert1927, “Heteraster” texanus is assigned to a paraphyletic genus following the within-quotes convention of Wiley (Reference Wiley1979). Photographs of the holotype had not been previously published, and thus one is provided herein.

Results

Phylogenetic analysis

Two separate parsimony analyses were run. The first included all taxa except for Linthia variabilis Slocom, Reference Slocom1909, which was coded as missing for 25% of the character states. This analysis resulted in a strict consensus of four most parsimonious trees of length 111, consistency index (CI) of 0.41, and retention index (RI) of 0.60 (Fig. 4), with one character parsimony-uninformative. Then, Linthia variabilis was added and a branch-and-bound analysis performed again. (Both nexus files utilized are provided as Supplemental Data.) This resulted in a strict consensus of 83 trees of length 114, a CI of 0.40, and an RI of 0.60 (Fig. 5). Overall, the topology resembles that of Figure 4, but resolution decreased, possibly due to the high amount of missing data for Linthia variabilis. In terms of tree support statistics, four nodes of the cladogram in Figure 4 were supported by bootstrap values greater than 60, with 15 nodes supported by jackknife values greater than 60 (and seven of these values were above 80). Bremer support was found for four nodes in the tree (Fig. 4). The tree in Figure 4, with the characters used in phylogenetic analysis mapped to the tree, is provided as a figure in Supplemental Data. The Bayesian analysis (not shown) of the matrix without Linthia variabilis contained five nodes with probabilities greater than 60, generally aligning with the results of the parsimony analysis but containing poor resolution of later nodes.

Figure 4. Strict consensus of four most parsimonious trees of length 111 recovered in analysis, excluding relatively incomplete taxon L. variabilis. Numbers above node in bold are bootstrap support values, numbers below node in italics are jackknife support values, and numbers in normal font below node are Bremer support values. *Species occurring outside of the WIS.

Figure 5. Strict consensus of 83 most parsimonious trees of length 114 recovered in analysis, including relatively incomplete L. variabilis. Numbers above node in bold are bootstrap support values, numbers below node in italics are jackknife support values, and numbers in normal font below node are Bremer support values. *Species occurring outside of the WIS.

The analyses suggest several notable relationships relative to traditional groupings and previous analyses. Regarding traditional groupings, the genera Hemiaster, Heteraster, and Proraster are paraphyletic or even polyphyletic (see also Smith and Bengtson, Reference Smith and Bengtson1991; Néraudeau, Reference Néraudeau1994). To make the classification more compatible with the phylogeny, several taxonomic changes were suggested (Fig. 4). For example, Proraster is treated herein as containing Proraster dalli, Proraster humphreysanus, and Proraster bexari (Clark, Reference Clark1915) (Fig. 4). In addition, some taxa were assigned to paraphyletic genera using the within-quotes convention of Wiley (Reference Wiley1979). These are “Heteraster” texanus; “Hemiaster” batnensis; “Hemiaster” texanus; “Hemiaster” estepi; and “Diplodetus” americanus (Fig. 4). At this time, this is preferable to establishing several new monotypic genera as not all relevant species could be considered in the present analysis. The phylogeny indicates that Mecaster fourneli (Aggasiz and Desor, Reference Agassiz and Desor1847) and Palhemiaster Lambert, Reference Lambert1916 are closely related, with Proraster the sister group of these; a paraphyletic set of species referable to Hemiaster and “Hemiaster” represent several successive lineages sister to these. In addition, the following pairs of genera are closely related: Diplodetus plus Micraster Agassiz, Reference Agassiz1836; Washitaster plus Heteraster; and Pliotoxaster Fourteau, Reference Fourteau1907 plus Macraster Roemer, Reference Roemer1888. Further, Washitaster plus Heteraster is the sister group to all spatangoids included here (excluding Toxaster Agassiz, Reference Agassiz1840).

Biogeographic analysis

Biogeographic patterns provide evidence for vicariance within the spatangoids (Fig. 6). For example, following an initial range expansion by the group from being distributed in Western Europe and North Africa to being distributed in these areas and the WIS during the Valanginian, there was subsequent vicariance between northern Africa and the WIS plus Western Europe during the Albian comprising Heteraster and Washitaster. There is further vicariance between Western Europe and the WIS within Heteraster and Washitaster. Another vicariance event occured in the Campanian associated with the divergence of the WIS “Diplodetus” americanus and the Western European Micraster and Diplodetus parvistella. The rest of the diversification within the clade represents speciation occurring within the WIS, although at this time it could not be determined whether this involved smaller scale vicariant events within the WIS or actual sympatric differentiation.

Figure 6. Biogeographic distribution and ages of species mapped to tree shown in Figure 4 along with biogeographic distribution of ancestral nodes, calculated using modified version of Fitch parsimony described by Lieberman (Reference Lieberman2000), and minimum divergence age of nodes, determined using ghost lineage method described by Norell et al. (Reference Norell, Novacek and Wheeler1992) and recently used by Guensburg et al. (Reference Guensburg, Sprinkle, Mooi, Lefebvre, David, Roux and Derstler2019). 0 = Western Europe; 1 = North Africa; 2 = South America; 3 = the WIS.

There also appears to have been range expansion by spatangoids from the WIS into Europe, North Africa, and South America. For example, at some time between the Albian and the Cenomanian, there was a range expansion by the lineage leading to Hemiaster bufo (Fig. 6). There was also a range expansion from the WIS into South America during the Cenomanian associated with the origin of “Hemiaster” batnensis. Finally, sometime between the Albian and the Turonian, there was a range expansion from the WIS into North Africa and South America associated with the origin of Mecaster fourneli.

Discussion

Phylogenetic analysis

In general, the phylogenetic results (Fig. 4) match various aspects of previous interpretations, and genera could be readily assigned to families as per the familial level classification presented by Smith and Kroh (Reference Smith and Kroh2011). For example, Micraster and Diplodetus had been grouped into a monophyletic Micrasteridae due to the shared presence of an ethmophract apical disc and similarly patterned subanal fasciole (Smith and Stockley, Reference Smith and Stockley2005), although Diplodetus was found to be paraphyletic herein. Further, various authors (e.g., Smith and Bengtson, Reference Smith and Bengtson1991; Stockley et al., Reference Stockley, Smith, Littlewood, Lessios and MacKenzie-Dodds2005; Oliveira et al., Reference Oliveira, Manso, Andrade and Souza-Lima2013; Manso and Souza-Lima, Reference Manso and Souza-Lima2017) have, either directly or on the basis of their taxonomic assignments, posited a relatively close relationship between Mecaster and Hemiaster. Some aspects of the phylogeny (Fig. 4) are also in agreement with the phylogeny presented by Stockley et al. (Reference Stockley, Smith, Littlewood, Lessios and MacKenzie-Dodds2005), although they included a much broader range of taxa. They also found Mecaster and Hemiaster to be closely related, although they did not retrieve a monophyletic Hemiasteridae. They also held Toxaster to be the earliest member of the spatangoids.

While the results from the analysis of Villier et al. (Reference Villier, Néraudeau, Clavel, Neumann and David2004) cannot be directly compared with the analysis performed herein, as they focused on Early Cretaceous taxa and had fewer WIS representatives, as well as more representatives from outside of the WIS, some similarities emerge. In particular, Villier et al. (Reference Villier, Néraudeau, Clavel, Neumann and David2004) also retrieved a clade consisting of Heteraster oblongus, “Heteraster” texanus, and Washitaster riovistae (Adkins, Reference Adkins1920) and found that a clade containing Macraster elegans was sister to two clades, one including Micrasteridae and the other including Hemiasteridae. They did not, however, find Macraster to be closely related to Pliotoxaster comanchei (Clark, Reference Clark1915) (treated as Palhemiaster in their study).

Biogeographic patterns

Smith (Reference Smith1984) summarized general patterns of echinoid biogeography and argued that during the Cretaceous, the present-day Mediterranean region (then Tethys) was a biodiversity hotspot for echinoids. The pattern of substantial within-region speciation in the WIS recovered herein suggests that the WIS might also have served as a biodiversity hotspot, at least among spatangoids. The recovery of episodes of range expansion between the WIS and South America matches patterns identified by Néraudeau and Mathey (Reference Néraudeau and Mathey2000) in their study of the biogeography of the Cretaceous of the present-day South Atlantic region. However, it is important to note that not all species of spatangoids could be considered herein. Changing the species sampled, or adding species, could potentially change the biogeographic character optimizations, which might alter the nature or number of episodes of range expansion, or vicariance; moreover, many species that undoubtedly existed are not preserved in the fossil record. Each of these will affect our ability to reconstruct biogeographic patterns (Turner et al., Reference Turner, Smith and Callery2009). Thus, the biogeographic conclusions presented herein focus only on the available data.

Another relevant biogeographic study is that of Rosen and Smith (Reference Rosen and Smith1988), who used parsimony analysis of endemicity (PAE) on Campanian–Maastrichtian echinoids and found the WIS (in particular, a joint region composed of Texas and Mexico) to be the sister region to North Africa plus Europe. This matches the episodes of vicariance and range expansion between the WIS and Western Europe recovered herein. Further, Smith (Reference Smith1992) analyzed biogeographic patterns in Cenomanian echinoids using distance-based methods, cladistic biogeography, and PAE. The PAE indicated Brazil and Angola were closely related biogeographically and sister to the Texas region. Smith's (Reference Smith1992) cladistic biogeographic analysis indicated that Texas could be the sister area of Western Europe and North Africa. Again, this matches some of the episodes posited herein for vicariance among spatangoids.

It is clear that sea-level changes would have had a major impact on the biogeography and evolution of echinoids (Smith and Rader, Reference Smith and Rader2009). Indeed, the WIS evolved significantly during the time interval during which the taxa considered occurred. Multiple studies have been published on Cretaceous sea-level changes (e.g., Hallam, Reference Hallam1992; Haq, Reference Haq2014). Prominent sea-level changes are associated with the following transgressive–regressive cycles seen in the North American Cretaceous carbonate platform series: the Coahuilan (Valanginian–mid-Aptian); the Comanchean (late Aptian–early Cenomanian); and the Gulfian (mid-Cenomanian–Maastrichtian) (Scott, Reference Scott, Simo, Scott and Masse1993; Smith and Rader, Reference Smith and Rader2009). The results from the biogeographic analysis presented herein suggest a possible connection between specific transgressive–regressive cycles and patterns of echinoid vicariance and range expansion. The regressions during the Coahuilan may coincide with the vicariance event that led to the diversification of the WIS “toxasterids,” including Heteraster and Washitaster (Fig. 6); the regression during the Comanchean may have led to the vicariance associated with the divergence at the base of all spatangoids, excluding the “toxasterids” (Fig. 6); another sea-level drop during the Gulfian may coincide with Campanian vicariance in the Micrasteridae (Fig. 6). In addition, sea-level rise associated with the Comanchean could account for the range expansions associated with Mecaster fourneli, Hemiaster bufo, and “Hemiaster” batnensis (Fig. 6). However, a broader range of marine taxa will need to be considered to determine whether these are patterns that consistently emerge in other groups, thereby indicating biogeographic congruence and an overarching earth history control (e.g., Lieberman, Reference Lieberman2000). If, instead, they do not, the patterns may be related to specific aspects of spatangoid ecology. At this time, it is not possible to determine the precise paleogeographic features that served as the specific geographic barriers that caused the episodes of vicariance when sea levels fell.

Acknowledgments

Thanks to L. Appleton and L. Boucher for their help providing access to collections at UT; M. Florence and K. Hollis for their help providing access to specimens and arranging the loan of material at USNM; and N. Ridgewell and S. Lucas for their help providing access to specimens and arranging the loan of material at NMMNH. Thanks to G. Heumann for providing photographs of holotypes at STIB, to T. Ewin for providing photographs of specimens at The Natural History Museum, London, and to J. Kimmig, who assisted with collections-related matters at KUMIP. B. Atkinson provided discussions on Bayesian phylogenetic approaches. C. Beard and L. Smith discussed various aspects of this research and commented on an earlier version of this manuscript. Thanks to A. Kroh, C. Souto, L. Zachos, and R. Mooi for comments on an earlier version of this manuscript. This research was made possible in part by a Panorama grant from the Biodiversity Institute, University of Kansas, and National Science Foundation grant DBI-1602067. In addition, this material is based on work supported while working at the National Science Foundation.

Accessibility of supplemental data

Data available from the Dryad Digital Repository: https://doi.org/10.5061/dryad.6m905qfz2.

Appendix

Apical disc

  1. 1. Plating: genital plate 2 does not extend between the posterior genital plates, referred to as ethmophract (0) / genital plate 2 extends between posterior genital and ocular plates, referred to as ethmolytic (1).

  2. 2. Position on test: posterior of midpoint (0) / at midpoint (1) / anterior of midpoint (2).

Frontal ambulacra (III)

  1. 3. Width is much narrower than (less than two-thirds the size of) central width of paired petals (0) / width is much broader than (more than 1.3 times the size of) central width of paired petals (1) / width is approximately equal to width of central width of paired petals (2).

  2. 4. Frontal ambulacra depth shallow (0) / frontal ambulacra depth substantial (1).

  3. 5. Distal end of frontal ambulacra tapers to form a shallow ‘notch’ present at ambitus (0) / distal end of frontal ambulacra does not form notch at ambitus (1) / very deep notch present at ambitus (2).

  4. 6. Pore pairs of the frontal ambulacra are uniform and uniserial (0) / pore pairs are heterogenous and not uniform (1).

  5. 7. Tubercles located between pore pairs in the poriferous zone: absent (0) / present (1).

Anterior paired ambulacra

  1. 8. Angle axis of petals forms relative to anterior/posterior axis through the apical disc: 6–14° (0) / 18–22° (1).

  2. 9. Relative size of pairs of pores on both axes: equally developed (0) / not equally developed (1).

  3. 10. Termination of paired petals: subpetaloid (0) / divergent (1) / in linear branches (2) / closed (3).

Posterior paired ambulacra

  1. 11. Relative size of pairs of pores on both axes: equally developed (0) / not equally developed (1).

  2. 12. Angle axis of petals forms relative to anterior/posterior axis through the apical disc: 2–7° (0) / 10–11° (1) / more than 14° (2).

  3. 13. Termination of paired petals: subpetaloid (0) / divergent (1) / in linear branches (2) / closed (3).

Fasciole

  1. 14. Subanal fasciole: absent (0) / present (1).

  2. 15. Loosely developed fasciole that does not fully encircle the test (peripetalous parafasciole): absent (0) / present (1).

  3. 16. Fully developed peripetalous fasciole: absent (0) / partial (fascioles are well defined and patterned but do not fully encircle the test) (1) / complete (2).

  4. 17. Multiple fasciole bands on the test: absent (0) / present (1).

  5. 18. Ambulacral plates: decrease in size at peripetalous fasciole (0) / do not decrease in size such that ambulacra are not ‘pinched’ (1).

Interambulacra

  1. 19. Interambulacra on aboral side: flush or level with paired ambulacra (0) / raised above ambulacra (1).

Periproct

  1. 20. In posterior view of the test, the periproct: positioned at the midline (0) / positioned above the midline (1). Note, this character can be influenced by ontogeny since in many spatangoids the periproct will migrate ‘downward’ as plates are added, and this analysis focused on adult specimens.

Plastron

  1. 21. Plates 2a and 2b: asymmetrical (protamphisternous) (0) / symmetrical (mesamphisternous) (1).

Peristome

  1. 22. Labrum: does not project over the peristome (0) / projects over the peristome (1).

  2. 23. Peristome shape: rounded (0) / pentagonal (1).

  3. 24. Peristome: not surrounded by apparent rim (0) / surrounded by apparent rim (1).

  4. 25. Peristome orientation from aboral view: flat (0) / oblique (1) / strongly oblique (2).

Test shape

  1. 26. From an adapical view, posterior end shape: rounded (0) / truncate (1). Note that one needs to pay careful attention with this character and consider a number of specimens as aspects of test shape can vary within species, although no specific polymorphism was found in this instance.

  2. 27. Anterior end in lateral view: has flat angular slope (0) / has inflated curvature (1).

Labrum

  1. 28. Posterior termination of labral plate extends to: second ambulacral plate (0) / third ambulacral plate (1) / fourth ambulacral plate (2).

  2. 29. Sternal suture in aboral view: straight or lightly curved (0) / concave (1).

Measurements

  1. 30. Anterior petal length: length to ambitus ratio: 0.5–0.55 (0) / 0.65–0.68 (1) / 0.72–1.0 (2). Note that for characters 30–32 it was possible to find distinct breaks between the different states such that they are discontinuous.

  2. 31. Posterior petal length: length to ambitus ratio: 0.3–0.47 (0) / 0.51–0.60 (1) / 0.65–1.0 (2).

  3. 32. Anterior ambulacra length/posterior ambulacra length ratio: 0.9–1.3 (0) / 1.54–1.9 (1) / 2.5–2.8 (2).

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Table 1. Taxa considered in phylogenetic analysis, relevant material examined, and geographic occurrence; KUMIP = Division of Invertebrate Paleontology, Biodiversity Institute, University of Kansas, Lawrence, USA; NMMNH = New Mexico Museum of Natural History, Albuquerque, USA; STIB = Universität Bonn, Germany; USNM = United States National Museum of Natural History, Washington, D.C., USA; UT = Non-Vertebrate Paleontology Laboratory, University of Texas, Austin, USA. For geographic occurrence, 0 = Western Europe; 1 = North Africa; 2 = South America; 3 = the WIS.

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Table 2. Character state distributions for taxa used in phylogenetic analysis. ? = missing data.

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Figure 1. Paleogeographic reconstruction for Late Cretaceous ~90 Ma; © 2016 Colorado Plateau Geosystems Inc.

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Figure 2. “Hemiaster” texanus: (1) STIB Texas 156; and “Heteraster” texanus (STIB Texas 157) (2). Scale bars = 1 cm.

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Figure 3. (1) “Diplodetus” americanus (USNM PAL 76285). (2) Proraster humphreysanus (USNM PAL 331). Scale bars = 1 cm.

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Figure 4. Strict consensus of four most parsimonious trees of length 111 recovered in analysis, excluding relatively incomplete taxon L. variabilis. Numbers above node in bold are bootstrap support values, numbers below node in italics are jackknife support values, and numbers in normal font below node are Bremer support values. *Species occurring outside of the WIS.

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Figure 5. Strict consensus of 83 most parsimonious trees of length 114 recovered in analysis, including relatively incomplete L. variabilis. Numbers above node in bold are bootstrap support values, numbers below node in italics are jackknife support values, and numbers in normal font below node are Bremer support values. *Species occurring outside of the WIS.

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Figure 6. Biogeographic distribution and ages of species mapped to tree shown in Figure 4 along with biogeographic distribution of ancestral nodes, calculated using modified version of Fitch parsimony described by Lieberman (2000), and minimum divergence age of nodes, determined using ghost lineage method described by Norell et al. (1992) and recently used by Guensburg et al. (2019). 0 = Western Europe; 1 = North Africa; 2 = South America; 3 = the WIS.