This is Mazon Monday post #251. What's your favorite Mazon Creek fossil? Tell us at email:[email protected].
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The Mazon Creek fossil fauna has a new member!. Hot off the internet presses, we present to you... Tardisia broedeae gen. et sp. nov (new species and new genus). The animal was described in the paper "A possible vicissicaudatan arthropod from the Late Carboniferous Mazon Creek Lagerstätte" authored by Victoria E. McCoy, Fabiany Herrera, Jack Wittry, Paul Mayer, and James C. Lamsdell. All of those authors will be familiar to ESCONI members with interest in fossils.
Abstract
The Vicissicaudata, a group of artiopods, originated and reached their highest diversity during the Cambrian period. However, relatively few vicissicaudatan species are known from the Paleozoic. Here we report a new species of vicissicaudatan arthropod, sister to the cheloniellids, from the Late Carboniferous Mazon Creek Lagerstätte. The two specimens preserve a small eyeless head, a trunk comprising seven tergites with wide pleural lobes, a narrower postabdomen bearing two long, posteriorly directed caudal appendages, and a short, shield-shaped telson. This new species not only extends the stratigraphic range of the Vicissicaudata into the Late Paleozoic but also represents an intermediate morphology between the cheloniellids and other vicissicaudatans.
Tardisia broedeae was named for ESCONI's own Irene Broede, long-time member (since January 1956!) and a long-time contributor on the ESCONI Board. Irene is standing in the middle of Karen Nordquist and Eileen Mizerk in the photo above from the 2018 ESCONI show. Irene's contributions to ESCONI are many. Most recently, she has served as the 2nd Vice President. Here's an example from the past in a Throwback Thursday post about Steven J. Gould's visit to the Field Museum in 2000 (see Throwback Thursday #224).
Congratulations Irene!!!
Tardisia broedeae is a vicissicaudatan arthropod. It belongs to artiopoda an extinct group of arthropods from the Paleozoic. They reached their greatest diversity during the Cambrian. The group includes trilobites, cheloniellids, and other odd arthropods. From Wikipedia.
The Artiopoda is a grouping of extinct arthropods that includes trilobites and their close relatives. It was erected by Hou and Bergström in 1997[5] to encompass a wide diversity of arthropods that would traditionally have been assigned to the Trilobitomorpha. Trilobites, in part due to abundance of findings owing to their mineralized exoskeletons, are by far the best recorded, diverse, and long lived members of the clade. Other members, which lack mineralised exoskeletons, are known mostly from Cambrian deposits.[6]
Tardisia broedeae is known from just two specimens. Both were collected by giants in Mazon Creek history. FMNH PE928, the holotype, was collected by George Langford. He mentions it in his third book "Additions to the Mazon Creek Flora and Fauna: The Wilmington Coal Flora and Fauna from a Pennsylvanian Deposit in Will County, Illinois, ed Wittry" on page 19. There's a photo of both part and counterpart. The specimen is listed as undescribed due to too few examples. Langford donated many of his specimens to the Field Museum and the Illinois State Museum, unfortunately, only one half of the concretion could be located. The second specimen, FMNH PE88856, was collected by Thomas V. Testa and subsequently donated to the Field Museum by Jack Wittry.
Drawing by George Langford from "Additions to the Mazon Creek Flora and Fauna: The Wilmington Coal Flora and Fauna from a Pennsylvanian Deposit in Will County, Illinois, ed Wittry"
Description from the paper.
Etymology. The genus name Tardisia (feminine) is inspired by the TARDIS time machine in the TV show Dr. Who, and refers to the large stratigraphic gap between this species and the next youngest members of the Vicissicaudata.
Diagnosis. Head shield smaller than the first thoracic tergite, no eyes. Seven thoracic tergites with wide pleural lobes ending in posteriorly directed points. Eighth trunk segment differentiated into a postabdomen without wide pleural lobes, bearing a pair of long, posteriorly directed, caudal appendages. Short, wide, shield-shaped telson overlaps the bulbous base of the caudal appendages where they insert into the postabdomen.
Description. The holotype, specimen FMNH PE928 (Fig. 1a–c) comprises one-half of a siderite concretion with the fossil preserved in negative relief (Fig. 1a). The overall shape of the body approximates an elongated oval, with its widest point at the third thoracic tergite (Fig. 1a–c). This specimen is missing the anterior-most portion of the head shield, but the preserved portion of the body is 10.6 mm long from the anterior-most preserved part of the head to the posterior-most tip of the telson, and 6.2 mm wide at its widest point in the third trunk tergite. According to Langford’s photograph and line drawing (Langford, Reference Langford and Wittry2018), the head shield was fully preserved in the half of the concretion that is now lost and was rounded and sub-semicircular (∼106° arc), smaller than the first thoracic tergite and lacking eyes (Fig. 1b).
The trunk comprises seven tergites, a postabdomen and a telson (Fig. 1a–c). Seven tergites comprise the thorax and terminate in posteriorly directed points. In four of these, the posterior margin of the tergite overlaps the anterior margin of the next tergite, suggesting that each tergite overlaps the posterior one (Fig. 1a, b). The eighth trunk segment is differentiated into a narrower postabdomen, lacking wide pleura, which bears a pair of caudal appendages. The seven trunk tergites and the postabdomen are all approximately 1 mm long. From the first to the seventh trunk tergite, their respective widths are 5.7 mm, 5.9 mm, 6.2 mm, 5.7 mm, 4.9 mm, 3.9 mm and 3.0 mm; the postabdomen is 2.5 mm wide. The telson is shield-shaped (Fig. 1a–c), 1.6 mm long and 2.1 mm wide at its widest point (its anterior end). The axis is not clearly differentiated in this specimen.
The caudal appendages on the postabdomen are the only appendages preserved in this specimen, either on the concretion half we have, or on the half that is now lost (Langford, Reference Langford and Wittry2018); no appendages preserved inside the concretion were observed in the CT scan. The caudal appendages on the postabdomen are not fully preserved, both lack their posterior-most end, and the region where the left appendage intersects the telson is obscured by some concretionary material (Fig. 1a–c). The right appendage is 0.5 mm wide where it intersects the telson, and the left appendage (which has a longer preserved length) extends 4.5 mm beyond the telson. In the fossil (Fig. 1a, b), both appendages narrow continuously from anterior to posterior. In the CT scan (Fig. 1c), it is apparent that these appendages have bulbous anterior ends where they meet the postabdomen, which is covered by the telson. Based on the CT scans (Fig. 1c), the appendages are about 1 mm wide where they meet the postabdomen, and the maximum preserved appendage length is 6.1 mm. These appendages are straight, and there is no evidence as to whether they were flexible or movable. Their preservation in these specimens when the trunk biramous appendages were not preserved suggests the caudal appendages were heavily sclerotized, with similarly robust cuticles as the dorsal exoskeleton.
The paratype specimen FMNH PE88856 (Fig. 1d–f) also comprises one-half of a siderite concretion with the fossil preserved in negative relief (Fig. 1d). The overall shape of the body is oval, with its widest point at the third trunk tergite (Fig. 1d–f). This specimen is missing the telson, and the postabdomen is poorly preserved (Fig. 1d–f), but the preserved portion of the body measures 8.6 mm long from the head to the posterior-most preserved part of the postabdomen, and 5.6 mm wide at its widest point in the third thoracic tergite. The head shield of this specimen is completely preserved. It is smaller than the first trunk tergite and bears no eyes. In the fossil, the head shield appears to have a straight anterior margin (Fig. 1d). However, in the CT scan, it is clear that the head is rounded and sub-semicircular (∼94° arc) (Fig. 1f).
The trunk comprises seven tergites, a poorly preserved postabdomen, and the telson is not preserved (Fig. 1d–f). The first seven tergites, which form the thorax, terminate in posteriorly directed points, although these are less pronounced than in FMNH PE928 (Fig. 1). The curvature of some of the trunk tergites changes abruptly, which likely delineates the axis of the body (Fig. 1d–f). The axis is ∼2 mm wide in all the segments where it can be measured. The first three tergites have tergopleurae that curve slightly towards the anterior of the body (Fig. 1d–f); this might just be due to flattening the three-dimensional body at an angle (Van Roy et al. Reference Van Roy, Rak, Budil and Fatka2022). The eighth trunk segment is too poorly preserved to confirm its differentiation into the postabdomen (Fig. 1d–f); however, it does bear a pair of caudal appendages. The first four trunk tergites are ∼1 mm long, the fifth through seventh trunk tergites are each ∼0.6 mm long and the postabdomen is too poorly preserved to measure. From the first to the seventh trunk tergites, their respective widths are 5.0 mm, 5.4 mm, 5.6 mm, 5.4 mm, 4.6 mm, 3.8 mm and 2.9 mm. The telson is not preserved.
As with FMNH PE928, the caudal appendages on the postabdomen are the only appendages preserved in this specimen and no appendages preserved inside the concretion were observed in the CT scan. In this specimen as well as in FMNH PE928, there is no evidence as to whether the caudal appendages were flexible or movable. The appendages are not fully preserved, and both lack their posterior-most end (Fig. 1d–f). The bulbous base of the appendages, where they meet the postabdomen, is clearly visible for both appendages; this was obscured by the telson in FMNH PE928 (Fig. 1a–c) and indicates that the telson is not preserved in FMNH PE88856. The appendages are ∼0.6 mm wide where they meet the postabdomen and extend 4.7 mm posterior to the postabdomen. These appendages have a striated appearance (Fig. 1d). These appendages are preserved with high relief and are semicircular in cross section (Fig. 1f inset); however, the appendages may have been compressed during fossilization, in which case they could have originally had more relief, potentially to the point of being circular in cross section. The cross section (Fig. 1f inset) also shows that the two appendages are separate. Based on the features revealed in the CT scan – two separate structures that are semicircular to circular in cross section – we can discard a tailspine interpretation for these appendages.
Specimens
From the paper
Figure 1. Tardisia broedeae gen. et sp. nov. specimens, line drawings, and CT scans. (a–c) Holotype, FMNH PE928. (a) Fossil specimen, photographed under cross-polarized light. (b) Line drawing of FMNH PE928, black dotted lines indicate overlap between the tergites, brown dotted lines show where the specimen is overlain by concretionary material, and the red dotted line at the anterior of the head indicates the anterior margin of the head as reconstructed by Langford. HS is for head shield, TT is for thoracic tergite, PA is for postabdomen, t is for telson, and CA is for the caudal appendages. (c) CT scan of FMNH PE928, reconstructed as if looking through the concretion, and showing the surface of the fossil currently obscured by the concretion. The arrows indicate the bulbous bases of the appendages. (d–f) Paratype FMNH PE 88856. (d) Fossil specimen, photographed under cross-polarized light. (e) Line drawing of FMNH PE 88856, grey dotted lines indicate where the tergite curvature changes, which likely delineates the axis. Brown dotted lines indicate the preserved edges of the postabdomen, which likely do not reflect the complete original shape. HS is for head shield, TT is for thoracic tergite, PA is for postabdomen, t is for telson, and CA is for the caudal appendages. (f) CT scan of FMNH PE88856, reconstructed as if looking through the concretion, and showing the surface of the fossil currently obscured by the concretion. Note the rounded anterior margin of the head. Inset is a digital slice of the CT scan, at the point in the fossil indicated by the black line, showing the appendages in cross section. All scale bars = 1 mm.
From "Additions to the Mazon Creek Flora and Fauna: The Wilmington Coal Flora and Fauna from a Pennsylvanian Deposit in Will County, Illinois, ed Wittry"
