Basal flange

A costa-like structure presented as a prominent shelf-like projection, which extends around the element parallel to, and slightly above, the basal margin to define the upper limits of the basal surface. This character is well developed in species of Cooperignathus, Jumudontus, and Histiodella, as well as in some species of Protoprioniodus, Oelandodus (likely a junior synonym of Protoprioniodus), and Fahraeusodus.

Basal surface

The surface adjacent to the basal margin of the elements located between the basal margin and the basal flange.

Makellate element

A term referring to most types of the elements taking the M position in the apparatus. The most distinctive feature of a makellate element is its orientation as an antero-posteriorly compressed ramiform, protoramiform, or coniform element (Sweet, Reference Sweet1988; Nicoll, Reference Nicoll1990, Reference Nicoll1992).

Protoprocess

Adentate anterior, posterior, or lateral projections of protoramiform, ramiform, protopectiniform, or pectiniform elements.

Protocarminate element

A blade-like protopectiniform element with a prominent cusp, posterior and anterior protoprocesses, and a straight or nearly straight basal margin.

Protoanguliplanate element

An arched protopectiniform element with the upper surface developed as a thin, antero-posteriorly extended, blade-like crest, which lacks a recognizable cusp and denticles; typically represented by the P element of Cooperignathus nyinti (Cooper, Reference Cooper1981) and C. aranda (Cooper, Reference Cooper1981).

Type species

Histiodella altifrons Harris, Reference Harris1962.

Diagnosis

Seximembrate (or septimembrate) apparatus of adentate (in early species) to denticulate (in later species) elements, including geniculate makellate M element, protoramiform or ramiform S elements, which form a symmetry transition series from alate prototriform or triform Sa, asymmetrical prototertiopedate (or modified tertiopedate) Sb, to asymmetrical and laterally compressed protobipennate or modified bipennate Sc and possibly protoquadriramate or quadriramate Sd, and protocarminate or carminate P elements typically with well-defined basal surface between the basal flange and basal margin (modified after McHargue, Reference McHargue1982, p. 1430–1431, and Stouge, Reference Stouge2012, p. 82).

Remarks

Two species of Histiodella, including the type species of the genus (H. altifrons) and H. serrata, were erected by Harris (Reference Harris1962) as form species based on the Pa elements (protocarminate for H. altifrons and carminate for H. serrata) from the Joins Formation exposed in the Arbuckle Mountains of southern Oklahoma. Harris (Reference Harris1962, p. 209) distinguished the latter by its larger size with denticulate upper margin of the anterior and posterior processes. Mound (Reference Mound1965) subsequently erected H. minutiserrata and H. triquetra as two additional form species from the Joins Formation of the Arbuckle Mountains, Oklahoma. The former was based on the protocarminate Pa element differing from H. altifrons in having a minutely serrated upper margin, and H. triquetra was based on the prototriform alate element. Histiodella sinuosa (Graves and Ellison, Reference Graves and Ellison1941) was established as a form species based on the carminate Pa element characterized by having a denticulate anterior process and adentate posterior process. The type material of H. sinuosa, which is from the Fort Peña Formation of West Texas (Graves and Ellison, Reference Graves and Ellison1941; Bradshaw, Reference Bradshaw1969), was also reported from upper part of the Joins Formation and lower and middle part of the Oil Creek Formation in Arbuckle Mountains of southern Oklahoma (McHargue,1982; Bauer, 2010) and correlative units in Utah (Ethington and Clark, Reference Ethington and Clark1982) and Nevada (Harris et al., Reference Harris, Bergström, Ethington and Ross1979; Sweet et al., Reference Sweet, Ethington and Harris2005) of North America. Bradshaw (Reference Bradshaw1969) adopted a broader species concept of H. sinuosa and synonymized all the known species of Histiodella at the time, including H. altifrons Harris, Reference Harris1962, H. serrata Harris, Reference Harris1962, H. minutiserrata Mound, Reference Mound1965, and H. triquetra Mound, Reference Mound1965.

McHargue (Reference McHargue1975, Reference McHargue1982) was the first to apply a multielement species concept to the species of Histiodella based on the study of a large collection from the Joins Formation in the Arbuckle Mountains, southern Oklahoma. Although Pb (= short bryantodontiform of McHargue, Reference McHargue1982), ramiform S (including Sa, Sb, and Sc = trichonodelliform, zygognathiform, and twisted bryantodontiform, respectively, of McHargue, Reference McHargue1982) and geniculate makellate M (= oistodontiform of McHargue, Reference McHargue1982) elements were recognized to form the species apparatuses (McHargue, Reference McHargue1982, text-fig. 2), the seximembrate species concept and distinction among the four species (H. altifrons, H. minutiserrata, H. serrata, and H. sinuosa) recovered from the Joins Formation were mainly based on the protocarminate or carminate Pa elements (= bryantodontiform of McHargue, Reference McHargue1982) of these species, because the S elements were much less frequently represented in the Joins Formation samples and were difficult to assign confidently to species based on their morphology and known stratigraphic range (McHargue, Reference McHargue1982, table 1).

Table 1.

Data matrix for the 10 species of Histiodella and outgroup, Cooperignathus nyinti (Cooper, Reference Cooper1981) (composite coding, with 12 binary characters, N = inapplicable). See text for descriptions of characters 1–12.

Ethington and Clark (Reference Ethington and Clark1982) accepted the species concept proposed by McHargue (Reference McHargue1982) and recovered H. altifrons, H. minutiserrata, and H. sinuosa from the Kanosh Shale, and a new species, H. holodentata Ethington and Clark, Reference Ethington and Clark1982, from the overlying Lehman Formation of Utah. Similarly, the 516 original type specimens defining H. holodentata from the Ibex area belong to the carminate Pa element, except for a single ramiform element assignable to the triform Sa element. Subsequently, three species of Histiodella, including H. bellburnensis Stouge, Reference Stouge1984, H. kristinae Stouge, Reference Stouge1984, and H. tableheadensis Stouge, Reference Stouge1984 (a junior synonym of H. holodentata) were erected based on the material from the Table Head Group of Newfoundland. Stouge (Reference Stouge1984, p. 87) suggested that the apparatuses of the Histiodella species from the Table Head Group consisted of six morphologically distinctive elements, but the Pb (= ozarkodiniform element of Stouge, Reference Stouge1984), geniculate M, and ramiform S elements were relatively rare, smaller in size, and were neither adequately described nor illustrated. Among these three species recognized in the Table Head Group, H. bellburnensis is represented by only the carminate Pa element (Stouge, Reference Stouge1984, p. 87, pl. 17, figs. 20, 21), H. tableheadensis by the carminate Pa (Stouge, Reference Stouge1984, p. 87, 88, pl. 18, figs. 8, 12–14) and unillustrated geniculate M and ramiform S elements, and H. kristinae by the carminate Pa (Stouge, Reference Stouge1984, p. 87, pl. 18, figs. 2, 3, 6, 7), geniculate M (Stouge, Reference Stouge1984, p. 87, pl. 18, figs. 1, 4), and ramiform S (Stouge, Reference Stouge1984, p. 87, pl. 18, figs. 5, 9–11) elements.

Several species of Histiodella were also erected from the middle Darriwilian of North China, but they were based solely on the carminate Pa elements. Histiodella triangularis (Wang and Lou, Reference Wang and Lou1984) and H. wuhaiensis Wang, Bergström, Zhen, Zhang, Wu, and Chen, Reference Wang, Bergström, Zhen, Zhang, Wu and Chen2013, were characterized by having an inconspicuous cusp and are considered as valid, showing closer phylogenetic ties with H. bellburnensis. Histiodella infrequensa An in An et al., Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983, from the Beianzhuang Formation of North China is considered as a junior synonym of H. holodentata, and H. intertexa An in An et al., Reference An, Du, Gao, Chen and Li1981 (illustrated only as a nomen nudum in An et al., Reference An, Du, Gao, Chen and Li1981, Reference An, Du and Gao1985, and formally defined in An, Reference An1987) from the Kuniutian Formation in South China as a junior synonym of H. kristinae. A likely new species recently reported from middle Darriwilian cherts in New South Wales and referred to as Histiodella sp. cf. H. kristinae is also solely based on the carminate Pa element, which is like that of H. kristinae, but with a more extended anterior process (Zhen and Rutledge, Reference Zhen and Rutledge2020, fig. 10g–i; Zhen et al., Reference Zhen, Percival, Gilmore, Rutledge and Deyssing2021b, fig. 5s).

It is now generally agreed that Histiodella species have a seximembrate or septimembrate apparatus, a view supported by recovery of Pb, M, and S elements in association with various species of Histiodella, as discussed above (e.g., Ethington and Clark, Reference Ethington and Clark1982; McHargue, Reference McHargue1982; Stouge, Reference Stouge1984, Reference Stouge2012; Sweet, Reference Sweet1988; Zhen and Percival, Reference Zhen and Percival2004; Du et al., Reference Du, Zhao, Huang, Tan, Wang, Yang, Zhang and Xiao2005; Bauer, Reference Bauer2010; Feltes et al., Reference Feltes, Albanesi and Bergström2016). However, their morphologic relationships within an apparatus, particularly between the rare and delicate ramiform S elements and the constituent Pa element, are still poorly understood and often variably interpreted. Therefore, differentiation of a Histiodella species is practically still based mainly, if not solely, on the Pa element.

Histiodella donnae Repetski, Reference Repetski1982, from the lower part (upper Tremadocian) of the El Paso Group of westernmost Texas, is represented by a symmetrical element (figured paratype, Repetski, Reference Repetski1982, pl. 8, fig. 7; interpreted herein as representing the Sa element) and an asymmetrical element (holotype, Repetski, Reference Repetski1982, pl. 8, fig. 6; interpreted herein as representing the Sb element). Two additional elements belonging to this species subsequently were illustrated by Repetski et al. (1998, fig. 2I–L), including a geniculate M (Repetski et al., Reference Repetski, Loch, Ethington, Santucci and McClelland1998, fig. 2J) and a laterally compressed coniform Sc (Repetski et al., Reference Repetski, Loch, Ethington, Santucci and McClelland1998, fig. 2L). A posteriorly curved cusp and a mid-carina on the broadly concave posterior face of the Sa and Sb elements of H. donnae suggest a closer phylogenetic relationship with Rossodus Repetski and Ethington, Reference Repetski and Ethington1983. However, because the P elements have not been recovered for H. donnae, the morphological and phylogenetic relationship between these leaf-shaped blades of H. donnae and Histiodella sensu stricto from the Middle Ordovician remains unclear. Therefore, H. donnae is excluded herein from Histiodella sensu stricto. Histiodella angulata Moskalenko, Reference Moskalenko and Sokolov1982 (see Moskalenko, Reference Moskalenko1983; Tolmacheva et al., Reference Tolmacheva, Dronov, Alekseev, Danukalova, Larionov, Obut, Sennikov and Kipriyanova2019), reported from lower Darriwilian of the Siberian Platform, is considered herein as a possible species of Jumudontus Cooper, Reference Cooper1981. Histiodella levis Tolmacheva, Reference Tolmacheva2014, from the lower Darriwilian of central Asia is an adentate species consisting of a seximembrate apparatus that is comparable with that of H. altifrons, but the Pa element of H. levis (see Tolmacheva, Reference Tolmacheva2014, pl. 22, figs. 1–9, 18) has a much shorter anterior protoprocess, a more open basal cavity, a prominent platform-like projection on the outer-lateral side, and surface granular ornamentation on the basal flange and basal surface, which resemble typical Jumudontus species. More specifically, the strong resemblance between the granular ornamentation (Tolmacheva, Reference Tolmacheva2014, pl. 22, fig. 18) of H. levis and the small pustules developed on the P elements of Jumudontus gananda, as documented by Nicoll (Reference Nicoll1992), suggests a closer relationship of these species.

Species assigned to Histiodella

Given the remarks above, only the 10 named species discussed below are confirmatively included in Histiodella.

Histiodella altifrons Harris, Reference Harris1962.—Middle Ordovician, holotype (OMC 115, figured by Harris, Reference Harris1962, pl. 1, fig. 4) from the horizon 11.89 m (39 feet) above the base of the Joins Formation at the Simpson section located along the west side of U.S. Highway 77, on the south side of Arbuckle Mountains, Oklahoma (= Histiodella triquetra Mound, Reference Mound1965).

Histiodella bellburnensis Stouge, Reference Stouge1984

Middle Darriwilian, holotype (ROM 39716, figured by Stouge, pl. 17, fig. 20) from the Middle Member of the Table Head Group exposed at Table Point of the Great Northern Peninsula, Newfoundland.

Histiodella holodentata Ethington and Clark, Reference Ethington and Clark1982.—Middle Darriwilian, holotype (UMC 1094-2, figured by Ethington and Clark, Reference Ethington and Clark1982, pl. 4, fig. 3) from the Lehman Formation in the Ibex area of western Utah (= Histiodella infrequensa An in An et al., Reference An, Zhang, Xiang, Zhang, Xu, Zhang, Jiang, Yang, Lin, Cui and Yang1983; = Histiodella tableheadensis Stouge, Reference Stouge1984). A morphologically transitional form between H. holodentata and H. kristinae is characterized by having the tips of the cusp and the tallest denticle on the anterior process at more or less the same height. This form was treated by Stouge (Reference Stouge2012) as a separate species, referred to as Histiodella cf. holodentata, while other authors (e.g., Zhen, Reference Zhen2020; Zhen et al., Reference Zhen, Normore, Dent and Percival2020a) considered it as representing an advanced variant of H. holodentata.

Histiodella kristinae Stouge, Reference Stouge1984

Middle Darriwilian, holotype (ROM 39718, figured by Stouge, pl. 18, fig. 3) from the Middle Member of the Table Head Group exposed at Table Point of the Great Northern Peninsula, Newfoundland (= H. intertexa An in An et al., Reference An, Du, Gao, Chen and Li1981, which was introduced as a nomen nudum).

Histiodella labiosa Bauer, Reference Bauer2010

Early Darriwilian, lectotype (hereby specimen OSU 52034, originally figured by Bauer, Reference Bauer2010, pl. 2, fig. 13, is designated as lectotype) from sample 72SC-500 at the horizon 152.4 m above the base of the Oil Creek Formation in section 72SC exposed in southern Oklahoma.

Histiodella minutiserrata Mound, Reference Mound1965

Middle Ordovician, holotype (USNM 146971, figured by Mound, Reference Mound1965, pl. 3, fig. 2) from the Joins Formation at the Simpson section (same locality and section of Harris, Reference Harris1962, as indicated by Mound, Reference Mound1965, p. 2) located along the west side of U.S. Highway 77, on the south side of the Arbuckle Mountains, Oklahoma.

Histiodella serrata Harris, Reference Harris1962

Middle Ordovician, holotype (OMC 118, figured by Harris, Reference Harris1962, pl. 1, fig. 3) from the horizon 57 m (187 feet) above the base of the Joins Formation at the Simpson section, located along the west side of U.S. Highway 77, on the south side of the Arbuckle Mountains, Oklahoma (= ?Histiodella triquetra Mound, Reference Mound1965, which might represent the Sa element of H. serrata).

Histiodella sinuosa (Graves and Ellison, Reference Graves and Ellison1941)

Middle Ordovician, holotype (MSM 7125, G414, figured by Graves and Ellison, Reference Graves and Ellison1941, pl. 2, fig. 13) from the Fort Peña Formation exposed on east side of road in bed of Alsate Creek, six miles southwest of Marathon, Texas.

Histiodella triangularis (Wang and Lou, Reference Wang and Lou1984)

Middle Darriwilian, holotype (NIGP 52199, figured by Wang and Lou, Reference Wang and Lou1984, pl. 12, fig. 17) from the Zotzeshan Formation of Inner Mongolia, North China.

Histiodella wuhaiensis Wang, Bergström, Zhen, Zhang, Wu, and Chen, Reference Wang, Bergström, Zhen, Zhang, Wu and Chen2013

Middle Darriwilian, holotype (NIGP 156446, figured by Wang et al., Reference Wang, Bergström, Zhen, Zhang, Wu and Chen2013, pl. 2, fig. 8) from the lower Klimoli Formation of Inner Mongolia, North China.

Type series

Lectotype (designated herein OSU 52034, originally figured by Bauer, Reference Bauer2010, pl. 2, fig. 13) and paralectotypes (OSU 52031 = M from sample 72SC-500; OSU 52032 = Sa from sample 72SC-470; OSU 52035 = Pb from sample 72SC-620, originally figured by Bauer, Reference Bauer2010, pl. 2, figs. 10, 11, 14, respectively).

Diagnosis

Species of Histiodella consisting of carminate Pa and Pb elements, geniculate makellate M element, and ramiform S elements; Pa and Pb elements triangular in outline with a prominent cusp, a longer anterior process and with well-defined basal flange; Pa element with a narrow platform-like projection developed near mid-length on the outer-lateral side, and confluent denticles on anterior and posterior processes; axes of the denticles on both processes parallel or sub-parallel to the axis of cusp; Pb element having a denticulate anterior process and a shorter posterior protoprocess with minutely serrated upper margin and a weakly developed platform-like projection on the outer-lateral side; M element typically with short denticles along inner-lateral margin (modified after Bauer, Reference Bauer2010, p. 13).

Description

Species consisting of likely seximembrate apparatus of denticulate elements, including geniculate and typically denticulate makellate M, ramiform S (alate triform Sa, modified bipennate Sc, and possibly also including a modified tertiopedate Sb) and carminate Pa and Pb (both with a well-developed basal flange) elements. Carminate Pa element triangular in outline (H/L ratio varying from 0.52–0.86) with a prominent cusp and the denticulate anterior and posterior processes triangular in outline (Fig. 4); cusp suberect to weakly reclined, forming the apex of the element, laterally compressed and biconvex with sharp anterior and posterior edges, and width of three to four times as wide as the adjacent denticles in lateral view; anterior process longer, bearing 7–10 long, confluent denticles in full-growth specimens; posterior process slightly shorter, bearing 5–9 long, confluent denticles; cusp and denticles ornamented with fine striae; denticles on the two processes highest next to the cusp and progressively shorter distally; axes of denticles extending parallel or sub-parallel to the axis of cusp (more or less perpendicular to basal margin); platform-like projection prominent on the outer-lateral side, triangular in outline in apical view (Fig. 4.2, 4.8, 4.12); basal cavity shallow, slightly expanded underneath cusp and extending as a narrow groove to the distal end of the processes (Fig. 4.7).

Carminate Pb element shorter triangular in outline and shorter in length compared to the Pa (H/L ratio varying from 0.74–0.96) with a prominent cusp, a denticulate anterior process, and a shorter posterior protoprocess (Figs. 3.2, 5); cusp suberect to weakly reclined to form apex of the element, laterally compressed and biconvex with sharp anterior and posterior edges, and with a width of three to four times that of the adjacent denticles on the anterior process in lateral view; anterior process longer with a slightly convexly curved upper margin, bearing 4–9 long, confluent denticles that are highest next to the cusp and progressively shorter towards distal end; axes of denticles extending parallel or sub-parallel to axis of cusp; posterior margin of the cusp and upper margin of the posterior protoprocess extending smoothly into each other, straight (Fig. 5.5, 5.7) to slightly concavely curved (Fig. 5.6, 5.10) and minutely serrated in lateral view (Fig. 5.4); surface of cusp and denticles ornamented with fine striae; platform-like projection weakly developed on the outer-lateral side (Fig. 5.8).

Geniculate M element antero-posteriorly compressed with a robust and outer-laterally recurved cusp and a low, short base (Figs. 3.1, 6.2–6.9); cusp short and slightly bowed posteriorly, biconvex with sharp outer-lateral and inner-lateral margins, a more broadly convex anterior face and a broad carina on the posterior face (Fig. 6.2, 6.3, 6.5); inner-lateral margin gently curved and typically ornamented with 3–8 short denticles, less commonly denticles may be rudimentary (Bauer, Reference Bauer2010, pl. 2, fig. 10) or even absent; base expanding posteriorly to form a triangular outline in apical or basal view (Fig. 6.4) with a basal cavity of moderate depth (Fig. 6.2).

Ramiform S elements represented by alate triform Sa and modified bipennate Sc elements; Sa element bearing a denticulate blade-like lateral process on each side and sharp blade-like costa on the posterior face (Bauer, Reference Bauer2010, pl. 2, fig. 11); Sc element strongly compressed laterally and asymmetrical with a prominent cusp and an anterior process bearing five denticles (Fig. 6.1); cusp biconvex with sharp anterior and posterior margins.

Material illustrated

Thirty topotypes (OSU 54951–54980), including Pa (11 specimens, Fig. 4) and Pb (10 specimens, Fig. 5), M (eight specimens, Fig. 6.2–6.9), and Sc (one specimen, Fig. 6.1) elements from the Oil Creek Formation, section 72SC, southern Oklahoma.

Remarks

Bauer (Reference Bauer2010) recognized carminate Pa and Pb elements, geniculate M, and ramiform Sa and Sb elements, but S elements are extremely rare with only eight specimens represented among a total of 997 specimens in his original study of H. labiosa. Reexamination of the current topotype material from sample 89JA-178 produced a single specimen representing the modified bipennate Sc (Fig. 6.1) of this species, supporting a possible seximembrate species apparatus for H. labiosa. However, Bauer (Reference Bauer2010) included and described the Sb element as like alate Sa but with asymmetrically disposed lateral processes, but it was neither illustrated by him from the original type material nor recovered from the current samples of this study.

The Pa element of H. labiosa morphologically resembles that of H. serrata and H. holodentata, but differs from the latter two species by having a longer posterior process and by having denticles on the posterior process extending with their axes parallel or nearly parallel to the axis of the cusp. Bauer (Reference Bauer2010) also noted the development of a narrow platform on the outer-lateral side of the Pa (Fig. 4.2, 4.8, 4.12) and Pb (Fig. 5.8) elements of H. labiosa—a distinctive character that has not been reported occurring in any other species of Histiodella.

Cladistic analysis

To test the validity of these inferred phylogenetic relationships, the 10 species of Histiodella, plus Cooperignathus nyinti acting as the outgroup, were scored using the following 12 binary characters of the Pa elements and were then cladistically analyzed by applying reductive coding strategy (Table 1).

Character 1.—Basal flange and basal surface: absent (0), present (1). All 10 species of Histiodella and the out-group are coded as (1).

Character 2.—Protopectiniform or pectiniform: absent (0), present (1). All 10 species of Histiodella and the out-group are coded as (1).

Character 3.—Presence of cusp: absent (0), present (1). This is a character shared by all 10 species of Histiodella, with C. nyinti coded as (0).

Character 4.—Denticulated or serrated: absent (0), present (1). This is a character shared by nine species of Histiodella that are coded as (1), with H. altifrons and C. nyinti coded as (0).

Character 5.—Denticles on anterior process: absent (0), present (1). This is a character shared by eight species of Histiodella that are coded as (1), with H. minutiserrata, H. altifrons, and C. nyinti coded as (0).

Character 6.—Denticle on posterior process: absent (0), present (1). This character is shared by seven species of Histiodella that are coded as (1), with H. sinuosa, H. minutiserrata, H. altifrons, and C. nyinti coded as (0).

Character 7.—Axes of denticles on posterior process parallel to axis of the cusp: absent (0), present (1). This character features H. labiosa coded as (1). It is inapplicable for H. minutiserrata, H. altifrons, and C. nyinti, which are coded as (N). Others are coded as (0).

Character 8.—Reclined denticles on posterior process: absent (0), present (1). This character is shared by six species of Histiodella that are coded as (1), with H. labiosa coded as (0). It is inapplicable for H. sinuosa, H. minutiserrata, H. altifrons, and C. nyinti, which are coded as (N).

Character 9.—Rectangular outline in lateral view: absent (0), present (1). This character is shared by five species of Histiodella that are coded as (1), with the other five species of Histiodella coded as (0). It is inapplicable for C. nyinti, which is coded as (N).

Character 10.—Apex of the unit is located toward anterior end: absent (0), present (1). This character is shared by four species of Histiodella that are coded as (1), with the other six species of Histiodella coded as (0). It is inapplicable for C. nyinti, which is coded as (N).

Character 11.—Small or inconspicuous cusp: absent (0), present (1). This character is shared by three species of Histiodella that are coded as (1), with the other seven species of Histiodella coded as (0). It is inapplicable for C. nyinti, which is coded as (N).

Character 12.—Strongly reduced anterior process: absent (0), present (1). This character features in H. triangularis, which is coded as (1), with other species of Histiodella coded as (0). It is inapplicable for C. nyinti, which is coded as (N).

Histiodella cladogram

The cladogram produced is highly resolved (Fig. 8). It depicts the origin of this biostratigraphically valuable genus in the Middle Ordovician and its close phylogenetic affinity with late Floian (Early Ordovician) Cooperignathus, suggesting that both genera might share a common adentate ancestor (within Oistodontidae). It also supports the monophyletic grouping of the 10 Histiodella species and the evolutionary trends that have been revealed from the analyses of morphological changes and morphometrics among these species (e.g., McHargue, Reference McHargue1982; Feltes and Albanesi, Reference Feltes, Albanesi, Liao and Valenzuela-Rios2017). Our study is consistent with the results derived from the geometric morphometric analysis of five Histiodella species conducted by Feltes and Albanesi (Reference Feltes, Albanesi, Liao and Valenzuela-Rios2017), demonstrating that the Histiodella species evolved through continuous changes in shape of the element outlines and size and height of denticles of the Pa elements.