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Lungfishes from the trinity group cretaceous of north texas.

Abstract.--Most Ceratadus specimens from the Aptian/Albian of north Texas belong to a small species that is characterized by distinctive sharp dental crests, denticulate in unworn tooth plates. These blade-like crests, indicative of a cutting and chopping dentition, distinguish this lineage from the crushing dentitions of most other North American Cretaceous dipnoans. This species is described and named herein as Ceratodus texanus. The functional adaptation of the dentition of this species (cutting/chopping) is indicative of predation on small invertebrates. The species is recorded from the Antlers Formation at Butler Farm (Wise County) and Greenwood Canyon (Montague County), and from the Twin Mountains Formation at the Jones Ranch Locality (Hood County). Although the natural history of dipnoans in the eastern North American subcontinent remains largely unknown, it is now certain that eastern lungfish populations, after formation of the central seaway, included lineages with crushing dentitions and lineages with sectorial dentitions.

Lungfish in the Geological Record.--The long and extensive paleontological record of lungfishes presents familiar challenges and limitations. Preservation is strongly facies-controlled and linked to their adaptations for survival, primarily in freshwater environments of periodic stress. Lungfish fossils are often discovered incidentally in deposits that yield large terrestrial vertebrates. The distinctive tooth plates are by far the most common lungfish fossils, and anatomical evidence from other parts of the body is limited. However, the gradual accumulation of records has revealed surprising complexity. Such is the case in the Cretaceous record of North America, summarized in the benchmark study of Kirkland (1987), who not only summarized the known record at the time of that publication, but also anticipated much of the information in the present work. The additional specimens now available and the perspective of other research during the past two decades are adequate reasons for a review of lungfish from the Aptian/Albian of Texas.

Fossil dipnoans are most often known from pterygopalatine (upper) and prearticular (lower) tooth plates. Mesozoic specimens are generally referred to the form genus Ceratodus in view of the inability to distinguish genera from tooth plates alone. Standard descriptions emphasize the crests of the tooth plates, typically five above and four below. The configuration of ridges, ridge angles, and the presence of either sharp or blunt crests can be used to divide lungfish taxa into functional modalities. Although there is a spectrum of adaptations, it is generally possible to state that a specimen has either a cutting or a crushing adaptation of the plates.

Lungfish specimens from north-central Texas have been known for many years, and frequently were recovered from excavations in which large terrestrial vertebrates were being sought. This association is known elsewhere worldwide (Churcher 1995; Churcher & Dc Iuliis 2001; Grandstaff 2006). Prior to the discovery or a specimen from the Campanian of New Jersey (Parris et al. 2001; 2004), these Texas specimens were among the latest in age from North America. The present work is an expansion of earlier research (Parris et al. 2007) to include all known specimens of Trinity Group lungfishes from north-central Texas.

Cretaceous Paleoenvironnient of North-Central Texas.--The lithostratigraphic units from which these specimens were collected are all part of the Trinity Group of Texas (Winkler et al. 1989). The units described herein are all correlated approximately to the Aptian/Albian Stages, chronostratigraphic units of the Lower Cretaceous (Comanchean) of Texas.The lithostratigraphy has been described extensively by other authors (Thurmond 1974; Winkler et al. 1989; Winkler et al. 1990; Winkler & Rose 2006) and the specimens come from localities cited in their work (Fig. 1). These specimens come from units that were deposited as the epicontinental seaway was becoming established (Young 1986; Smith et al. 1994), and are thus associated with what would become the eastern subcontinent that existed during much of the Cretaceous Period (Fig. 2). All are interpreted as freshwater deposits, although it has long been recognized that the Trinity Group consists of units ranging from terrestrial to marine marginal to marine, a paleogeographical situation in which the sequences contain marine interbeds indicative of intermittent transgressions by epicontinental marine waters.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

The established interpretation of the paleoenvironment of the Texas lungfish localities is terrestrial in all cases, although admittedly the presence of lungfish specimens contributes to such an interpretation. This interpretation was succinctly stated by Winkler & Rose (2006) in their description of the Jones Farm, the lowest site stratigraphically and presumed to be the earliest (Aptian-Albian Boundary). Its fauna is dominated by sauropod remains, a primary indicator of a terrestrial paleoenvironment. Supporting fossil evidence includes substantial plant and charcoal remains, in strata that appear to represent river channel deposits. The two other localities for lungfish specimens, Butler Farm and Greenwood Canyon, also lack marine fossils.

MATERIALS AND METHODS

The specimens described in this study were examined under a binocular microscope for principal features and preliminarily attributed to the same species on that basis. Biometrics have been discussed previously by Vorob'yeva & Minikh (1968), Martin (1980; 1981; 1982; 1984), Kirkland (1987), and Kemp, (1997). The methods of Kirkland (1987), employed herein (Figs. 3A, B), are based on those of Vorob'yeva & Minikh (1968), and enable recognition of species-level distinctions between populations.

[FIGURE 3 OMITTED]

The specimens were digitally photographed with a Panasonic Lumix DCM-FZ2, downloaded, and viewed by computer projection. Lines tangent to plate edges and parallel to ridge crests were superimposed onto the projected images using Adobe Illustrator, and the resulting images printed. Biometric angles were then measured on the printed images with a protractor. Dimensions were measured directly on the specimens with a hand micrometer. Morphologic and biometric data are summarized in Tables 1 and 2. Although this study lacks the large samples available to Vorob'yeva & Minikh (1968) the number of specimens is sufficient to confirm that the assemblage is a distinct species. The interpretation is conservative in that none of the observations are considered to be indicative of generic or greater distinction.
Table 1. Standard angular measurements of Texas Trinity lungfish
tooth plates.

                                Biometric angles (degrees)

Tooth plates               ABC     C1Cp    C2Cp   C3Cp   C4Cp

Pterygopalatines
  SMU 70434                 141     111     76      37     15
  YPM PU 18861              149     108     73      45     13
  FMNH PF629                133      90     61      32     15
  SMU 76360                 134      97     58      28     10
                    ave.  139.3   101.5     67    35.5   13.3
Prearticulars
  SMU 70531                 129      92     50      21
  SMU 72464                 131      96     45      22
  FMNH PF628                127      95     52      19
  FMNH PF627                131      90     60      29
                    ave.  129.5    93.3   51.8    22.8

Table 2. Linear measurements of Texas Trinity lungfish tooth plates.

                     Ridge lengths (mm)*   Lengths on embayments (mm)

                    BC1   BC2   BC3   BC4   BCp   Bfl   Bf2   Bf3

Pterygopalatines

  FMNH PF629        12.7  19.3  20.7  21.7  20.9  16.9  15.2  18.4

  FMNH PF627         9.4  12.5  10.2  10.7  14.2    10   9.8  10.9

  SMU 76360         12.7  17.3  18.6  21.2  19.7    12    15  18.7

  SMU 70434          3.6   4.3   5.3   4.0   3.6   3.1   3.1   3.3

  YPM PU 18861       7.7    10   9.3   9.8   9.0   7.2   6.4   6.9

Prearticulars

  SMU 72464          5.1   3.9   4.8         7.6   3.1   3.4   4.3

  SMU 70531          7.9   6.8   8.4        10.3   4.6   5.3   8.3

  FMNH PF628         3.3   3.1   6.0         8.3   2.6   3.8   5.8

                    Ridge length ratios

                      BC1/BCp  BC2/BCp

Pterygopalatines

  FMNH PF629           061      0.92

  FMNH PF627           0.66     0.88

  SMU 76360            0.64     0.88

  SMU 70434            1.00     1.19

  YPM PU 18861         0.86     1.11

Prearticulars

  SMU 72464            0.67     0.51

  SMU 70531            0.77     0.66

F  MNH PF628           0.40     0.37

* Size rank is based on BCp lengths; smaller plates have lower rank
numbers.


The following institutional abbreviations are used herein: FMNH, Field Museum of Natural History, Chicago, Illinois; SMU, Shuler Museum of Paleontology, Southern Methodist University, Dallas, Texas; YPM PU, Yale Peabody Museum (Princeton University Collections), Yale University, New Haven, Connecticut.

SYSTEMATIC PALEONTOLOGY

Subclass DIPNOI Mailer 1845 Order CERATODONTIFORMES Berg 1940 Family CERATODONTIDAE Gill 1872 Genus CERATODUS Agassiz 1838 CERATODUS TEXANUS sp. nov.

Etymology: named for the state of Texas, formerly the Texas Republic.

Holotype: SMU 70434 (Figs. 3B, C; 4A), a small left pterygopalatine tooth plate with crest CI broken, from the Antlers Formation at Butler Farm, near Decatur, Wise County, Texas.

[FIGURE 4 OMITTED]

Paratypes: SMU 72464 (Fig. 4F), a small left prearticular tooth plate; SMU 70531 (Fig. 4G), a right prearticular tooth plate, both from the same locality as the holotype.

Referred specimens: FMNH PF627 (Fig. 4C), a left pterygopalatine tooth plate with broken crests, FMNH PF628 (Fig. 4E), a right prearticular tooth plate, FMNH PF629 (Fig. 4D), a left pterygopalatine tooth plate, all from the Antlers Formation at Greenwood Canyon, near Forestburg, Montague County, Texas (SMU Locality 21); and SMU 76360, a left pterygopalatine tooth plate from the Twin Mountains Formation at Jones Ranch, Hood County, Texas (SMU Locality 282). Also referable is YPM PU 18861 (Fig. 4B), a left pterygopalatine plate with crest Ci broken, attributed to Forestburg, Montague County, Texas, which is probably from the Greenwood Canyon Locality or one of several others not far away (Thurmond 1974).

Other specimens, SMU 70530 and FMNH PF649, are minor fragments of little diagnostic value, but were collected at Butler Farm and Greenwood Canyon respectively, and may pertain to the new species. SMU 76525 (right prearticular) and SMU 76526 (right pterygopalatine) from the Twin Mountains Formation at Jones Ranch, Hood County, are also tentatively referred.

Type Locality: The holotype and paratype specimens were collected from SMU Locality 20 (Butler Farm, near Decatur, Wise County, Texas).

Type Horizon: Antlers Formation.

Diagnosis: Small species of Ceratodus with five sharp crests on pterygopalatine dental plate; occlusal view of proportions in pterygopalatine dentition generally ovate, with inner angle (ABC of standard biometrics) very obtuse; crests C, and [C.sub.3] notably elongate and subequal in length; angles between C, and [C.sub.3] and between [C.sub.3] and [C.sub.4] relatively wide. Referred lower tooth plates are consistent in size, proportions, and occlusal morphology.

Distinguishable from Ceratodus guentheri Marsh 1878 [non Kirkland 1987] (=Potamoceratodus guentheri Pardo et al. 2010) and Ceratodus fossanovum Kirkland 1998 [= C. guentheri Kirkland 1987] by the greater inner angle, more derived cutting edges of crests, and by generally smaller size. Distinguishable from Cenomanian lungfish tooth plates from the Woodbine Formation of Texas (Main pers. comm.) by larger inner angles (ABC) in the Trinity plates (in both pterygopalatine and prearticular plates), larger intercrest angles in the Trinity pterygopalatine plates, and by larger C [C.sub.1]p and smaller [C.sub.3]Cp angles in the Trinity prearticular plates. Distinguishable from all other taxa of the Cretaceous of North America by the presence of sharp crests which have an acute angle between the mesial and distal faces of the crest, interpreted as adaptations to cutting/slicing habits. By contrast, other Cretaceous taxa in North America have more obtuse angles between the mesial and distal faces of the crests.

Description: As noted, the pterygopalatine tooth plates have a generally ovate or elliptical aspect in occlusal view, but this is partly attributable to the breakage of the [C.sub.1] crest in all of the specimens thus far found. However, it seems apparent from a close examination of the holotype that this crest was not large; only a small portion seems to be missing from the holotype. Its proportions were different from those of other species that have cutting/slicing adaptations (Kirkland 1987); pterygopalatine tooth plates referred to C. guentheri and C. .fossanovum have relatively prominent [C.sub.1] crests. Cutting edges of crests are enhanced by denticles in early growth (and slight wear) stages; minor denticle remnants are present on crest [C.sub.3] of the holotype SMU 70434, and on crest [C.sub.3] of FMNH PF627.

Prearticular tooth plates consistently have a more elongate aspect in occlusal view, with [C.sub.1], the first of the four crests, being especially prominent. Denticles were not observed on any of the prearticular plates.

DISCUSSION

Although considerable variation in size and proportions may be found in tooth plates from a single locality, the basic anatomy of tooth plates from a site and horizon (representing dietary adaptations) is generally consistent. As recognized by Peyer (1968), it is probable that specimens from any given locality of a given age are the same species. This view is consistent with observations of lungfish dental anatomy, including Tomes (1882), Peyer (1925), Bemis (1984), and Smith & Krupina (2001), all of whom noted the sustained growth and redeposition of dental plates in Dipnoi.

The term "ptychoceratodontiC has been applied to specimens with cutting/chopping adaptations (Martin 1982), but Kirkland (1987) cautioned that a familial epithet for taxa with such adaptations cannot be applied formally to specimens known only from dentitions. Nonetheless, Ceratodus texanus has a dentition that is functionally "ptychoceratodontid", and could ultimately be referred to a formal taxon based on this dental feature and supporting evidence in skull anatomy. Adaptations of this form have been judged to be of generic value in some studies (Cione et al. 2007). Parris et al. (2007) speculated that these dental adaptations suggest predation primarily upon small invertebrates (as opposed to the more opportunistic faunal exploitation enabled by large size and crushing dentitions).

An approximation of occlusal position based on actual specimens is shown in Fig. 5. Despite minor irregularities, it appears to represent a functionally cutting/chopping system with interlocking of the ridges on upper and lower tooth plates when the plates are in occlusion.

[FIGURE 5 OMITTED]

The welcome discovery and publication of cranial material of Ceratodus guentheri has justified referral of that species to a new genus, Potamoceratodus (Pardo et al. 2010). As we have noted herein (Fig. 6), this species was potentially ancestral to the Texas lineage that includes C. texanus, a clade with cutting/chopping dental adaptations. However, since knowledge of C. texanus is limited to the dental series, referral of any Texas specimens to the new genus is as yet unjustified. Therefore, we maintain the traditional practice of assigning isolated lungfish tooth plates from the Cretaceous of North America to Ceratodus.

[FIGURE 6 OMITTED]

Biogeographic Implications.--Small lungfishes with narrow, sectorial crests suggestive of a distinctive ecological niche have now been found in sediments from the Aptian to Cenomanian of eastern Texas (Kirkland 1987; Parris et al. 2007; Main pers. comm.). Cenomanian specimens representing yet another new species of Texas lungfish with sectorial crests are under study elsewhere. These lungfishes lived on the eastern shores of the newly-forming epicontinental seaway, and by Cenomanian time would become isolated from western populations. They differ markedly in morphology from contemporaneous lungfishes on the western side of the seaway (C. frazieri and C. gustasoni, see Fig. 6), and more closely resemble lungfish taxa (C. guentheri, C. fossanovum) from the Morrison Formation (Kirkland 1987; 1998). Interestingly, this ecomorph is not known from later Cretaceous deposits in the eastern subcontinent; the latest known lungfish plate from eastern North America is of the more robust "ceratodontid' morphology (Parris et al. 2001; 2004). It appears that these small Texan lungfishes either died out as a lineage, or that their descendants later adopted a more durophagous habit.

ACKNOWLEDGEMENTS

We thank D. Winkler and L. Jacobs (SMU), who generously supported this work from the beginning, lending many of the specimens. Other loans were facilitated by D. Brinkman (YPM PU) and W. Simpson (FMNH). We benefited from discussions with colleagues R. Cifelli, P. Murry, R. Purdy, J. Kirkland, C. McNulty, D. Main, K. Shimada, T. Lipka, and L. Banks. We also thank P. Gaspari-Bridges and P. Entin of the Princeton University Library, who were of great assistance in obtaining reference materials.

LITERATURE CITED

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Churcher, C. S. 1995. Giant Cretaceous lungfish Neoceratodus tuberculutus from a deltaic environment in the Quseir (= Bans) Formation of Kharga Oasis, Western Desert of Egypt. J. Vertebr. Paleontol., 15 (4):845-849.

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DCP at: david.parris@sos.ni.gov

David C. Parris(1), Barbara Smith Grandstaff(2), and Nathan T. Banks(3)

(1) New Jersey State Museum, 205 West State Street, Trenton, New Jersey. 08625-0530

(2) Department of Animal Biology, School of Veterinary Medicine, University of Pennsylvania, 3800 Spruce Street, Philadelphia, Pennsylvania 19104-6046

(3) Shuler Museum of Paleontology, Southern Methodist University, Dallas, Texas 75275
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Author:Parris, David C.; Crandstaff, Barbara Smith; Banks, Nathan T.
Publication:The Texas Journal of Science
Date:Aug 1, 2011
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