A New Genus of Therian Mammal from the Late Cretaceous "El Gallo Formation," Baja California, Mexico

A New Genus of Therian Mammal from the Late Cretaceous "El Gallo Formation," BajaCalifornia, MexicoAuthor(s): Jason A. LillegravenSource: Journal of Paleontology, Vol. 50, No. 3 (May, 1976), pp. 437-443Published by: SEPM Society for Sedimentary GeologyStable URL: http://www.jstor.org/stable/1303524 .

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JOURNAL OF PALEONTOLOGY, V. 50, NO. 3, P. 437-443, 1 PL., 1 TEXT-FIG., MAY 1976

A NEW GENUS OF THERIAN MAMMAL FROM THE LATE CRETACEOUS "EL GALLO FORMATION,"

BAJA CALIFORNIA, MEXICO

JASON A. LILLEGRAVEN* Department of Zoology, San Diego State University,

San Diego, California 92182

ABSTRACT-Detailed descriptions are presented for the lower cheek teeth and mandibles of Gallolestes pachymandibularis, a new genus and species of therian mammal from the principally nonmarine Late Cretaceous (Campanian) "El Gallo Formation" of Baja California del Norte, Mexico. Phylogenetic relationships of the species are unclear, but it seems most likely to represent a new family (not named) of placental (Infraclass Eutheria) as based upon molar morphology. The species is placed within the Order Insectivora for reasons of conservatism. G. pachymandibularis has four molariform lower teeth, the most anterior of which is more heavily worn than the next posterior. Possible morphological and phylogenetic interpretations are that the species is: (1) a eutherian and the molariform teeth represent homologues of P4M1-3 of typical Cenozoic kinds; (2) a marsupial with M1-4 represented; and (3) a eutherian, but with a cheek tooth formula or eruption sequence differing from that typical of Cenozoic kinds. Only the second possibility is considered unlikely.

INTRODUCTION

HE Late Cretaceous (late Campanian) "El Gallo Formation" of Baja California,

Mexico, has been explored for land verte- brates for a number of years. The "El Gallo Formation" was proposed by Kilmer (1963, unpubl. Ph.D. dissertation, Univ. California, Berkeley). No description of the type section has ever been published, nor is the dissertation available through the microfilm services. Thus, until described in publication form, the unit must be considered informally. The rocks are extensively exposed in badlands of gently- dipping strata northwest of the village of El Rosario near the western shoreline of the peninsula (see Lillegraven, 1972, p. 2 for map). Although producing localities are far from rich when compared with many known from other parts of the world, efforts have not gone unrewarded. Morris (1973), for example, has described some of the dinosaur fauna. I published a preliminary study of the few mammalian specimens then known (1972). Addi- tionally, good frog, turtle, alligator, lizard, and bird remains have been recovered and are now under study by a variety of individuals.

During the summer of 1973 Messers. Harley Garbani and Michael Greenwald discovered a small mammalian mandible (LACM 42633 from

LACM loc. 3302) of exceptional interest described in the present paper. The specimen was not found intact, but rather was disrupted along the surface as a scattering of tooth bits plus the dentary itself. I consider it a tribute to the eyes of the collectors that when pre- pared, the specimen included at least parts of five teeth (P1. 1). Possible homologies of the included teeth are discussed in the "Affinities" section at the end of the paper. For purposes of description, I have chosen a conservative approach and considered the teeth in LACM 42633 to represent P3-4M1-3 and have consis- tently referred to them as such within quotation marks below. Good fits between the roots preserved in the mandible and two teeth ("P," and "M2") were discovered during preparation. Other roots were battered by weathering before collection; thus the association between

"P4, 1"M," and "M " and jaw cannot be proven. However, all teeth are from the same side of the animal, are of the same color of preservation, are of the proper size relation, show appropriate stages of wear, and were found in a small area on an otherwise nearly barren rock exposure; I have little doubt that all the parts are from the same individual. Preparation of the specimen and its measurements (Table 1) were done by me. Measure- ments were made with an EPOI Shopscope using the orientations specified by Lillegraven

* Present address: Department of Geology, The University of Wyoming, Laramie, Wyoming 82071.

Copyright @ 1976, The Society of Economic Paleontologists and Mineralogists

437



438 JASON A. LILLEGRAVEN

TABLE 1-Measurements (mm) of holotype (LACM 42633) of Gallolestes pachymandibularis gen. et sp.

nov.

w "P3" 1.022 (est.)

W-TRI LTH-TRI "P4" 1.068 1.130

A-P W-TRI LTH-TRI W-TAL "M1" 2.288 1.607 1.051 1.670 "M2" 2.520 1.681 1.227 1.634 "M3" - 1.512 1.112

Depth of mandible below "M2" 3.729. Transverse width of mandible at center of "M2"

2.337.

(1969, p. 16). Abbreviations of measurements are as follow:

A-P Anteroposterior length

W Greatest width

W-TRI Width of trigonid

W-TAL Width of talonid

The abbreviation "LACM" is used through- out to indicate specimens catalogued in the Natural History Museum of Los Angeles County (Section of Vertebrate Paleontology). "LACM loc." refers to vertebrate fossil locality numbers, detailed descriptions of which are on file at that museum.

The two specimens previously described as "Family indefinite, new genus" (Lillegraven, 1972, p. 7) seem clearly referable to the same species as LACM 42633. The newly discovered

jaw also shows that I erred in 1972 in calling LACM 27599 an "M,"; it is almost certainly homologous to the tooth on LACM 42633 here described as "M,." Unfortunately, no ele- ments of the upper dentition of this taxon have yet been recovered.

SYSTEMATIC PALEONTOLOGY

Infraclass EUTHERIA Gill, 1872 Order INSECTIVORA Illiger, 1811

Family indefinite, probably new Genus GALLOLESTES new genus

Diagnosis of genus.-Small therian with four robustly-constructed, strongly-rooted molariform lower teeth that show progressively less wear from first to last; first molariform tooth markedly smaller than second; sharp break in morphology present between simply-constructed last premolariform and first molariform teeth;

paraconids much smaller than proto- and meta- conids and labially placed on last three molariform teeth; hypoconulid set markedly closer to entoconid than to hypoconid; hypoconulid developed as salient posterior projection on last molariform tooth; wear on molariform teeth is directly on apices of cusps and their crests rather than along more vertical shear surface; mandibular construction heavy and deep relative to tooth sizes; mental foramen directly below anterior root of first molariform tooth.

Distribution of genus.-Known only from "El Gallo Formation," Baja California del Norte, Mexico.

Etymology of generic name.-Latin gallus, a cock; Greek lestes, a robber.

Type locality.-LACM loc. 3302, middle one- third of "El Gallo Formation," Baja California del Norte, Mexico.

GALLOLESTES PACHYMANDIBULARIS new species

(P1. 1, Text-fig. 1)

Holotype.-LACM 42633, right mandible with posterior accessory cusp of "P,," trigonid of "P4," complete "M,-2," and trigonid of "M,."

Referred specimens.-"M,," LACM 27599 (LACM loc. 7172); mandible with worn talonid of "M1" plus complete but worn

"M2-3," LACM

27600 (LACM loc. 7172); talonid of molar, LACM 42635 (LACM loc. 3302).

Etymology of, trivial name.-Greek pachys, thick; latin mandibula, a mandible.

Diagnosis of species.-As for genus. Description.-Mandible. The mandibles both

on the holotype and LACM 27600 are heavily constructed, being remarkably deep and wide (Table 1) with respect to the size of the teeth. A single mental foramen is placed midway down the side of the mandible directly below the posterior edge of the anterior root of "P4." The jaw is widest posteriorly, tapering anteriorly up to the "P3" where the jaw width again markedly increases anteriorly. The anterior expanded area shows no indication of fine cracks in the bone and seems not to be a taphonomic artifact. All preserved roots are surprisingly robust considering the size of the teeth.

"P3." Only the posterior root and posterior accessory cusp of the doubly-rooted "P3" are

preserved. The posterior accessory cusp is a

simple blade with an anteroposterior crest placed about two-thirds the distance across the width of the tooth from the labial surface. A broad somewhat concave wear surface extends labioventrad from the crest. A lingual cingulum



CRETACEOUS MAMMAL FROM BAJA CALIFORNIA 439

mm

0

12

2

TEXT-FIG. 1-Scanning electron stereogram in occlusal view of talonid of "P3" and trigonid of "P4" on LACM 42633, holotype of Gallolestes pachymandibularis gen. et sp. nov. Extensive wear on protoconid and metaconid of "P4" is readily visible. x47.

parallels the main crest at lower elevation to near the posterior extreme of the cusp. Wear is prominent along the main crest of the posterior accessory cusp and along its labial shearing surface.

"P4." Unfortunately, only the trigonid of the at least partly molariform "P4" is preserved. Although the apices of the trigonid cusps are worn (Text-fig. 1), they seem to have been arranged to form a nearly equilateral triangle. A paraconid is distinctly developed but is much smaller and lower than both the proto- and metaconid. The protoconid is a broadly rounded cone. The metaconid is distinctly smaller than the protoconid and has a flattened lingual surface. The apices of the proto- and metaconid are obliterated by wear. Wear on these two cusps reduced their apices to flat surfaces and a broad confluence of exposed dentine exists. Wear on the anterior surface of the protoconid is restricted to a narrow vertical band which, before wear, must have been a weak ridge (paracristid). The protoconid was undoubtedly taller than the metaconid before wear. The small paraconid is less worn, projects anteriorly to a greater degree than on any of the more posterior molariform teeth, and is set almost

to the extreme lingual surface of the tooth. It shows some anteroposterior compression, though not to the degree seen on the "molar" paraconids. The wear surface on the paraconid is transversely elongated, slopes anteroventrad, and is weakly connected with the narrow wear band running down the anterior face of the protoconid. An anterior cingulum is represented by only the slightest bump on the anterior surface of the protoconid. Although most of the talonid has been lost from the specimen, the tiny remnant of its anterolingual corner suggests it was a fairly wide structure with a flattened basin. The tooth's two roots are strongly developed.

"Molars." The lower "molars" are robust with rather bulbously-constructed cusps. The "molars" of the holotype follow in most re- spects with the published description for LACM 27599 and 27600 (Lillegraven, 1972, p. 7-9) thus will only be amplified here. The talonid of "M3" is missing from the holotype but fortunately is present on LACM 27600.

As is the usual therian condition, the paraconid is most labially set in "Mi" and becomes progressively more lingual in "M2-3." The anterior cingulum is weak in all three "molars"




but is significantly better developed in "M2,_,3" than in "M,." As is also the typical therian condition, the relative width of the trigonid to the talonid progressively increases from "M," through "M3." The hypoconulid is close to the entoconid but cannot be said to be "twinned" with it as in the sense of primitive marsupials. The hypoconulid is the lowest of the talonid cusps in "M,," equals or slightly exceeds the height of the entoconid in "M2," and is the tallest of the talonid cusps in "M3." The hypoconulid protrudes posteriorly somewhat more in "M2" than in "M," and dra- matically more so in "M3" than in "M2." The apex of the hypoconid is set slightly anterior to that of the entoconid in "M1-2" and markedly anterior to it in "M3." The cristid ob- liqua strikes the base of the protoconid progressively more lingually from "M," through "M3." A posterior cingular shelf' is developed on "M1-2" that descends steeply labioventrad from the apex of the hypoconulid to the pos- terolabial base of the hypoconid. It is significantly stronger on "M2" than on "Ml," but its presence cannot be determined on "M3" because of excessive wear to the only available specimen.

A complete series of wear stages is now available for the lower "molars" with LACM 27599 (unworn), LACM 42633 (moderately worn), and LACM 27600 (extremely worn). Wear is dominantly on the apices of the cusps and crests rather than along vertical shear surfaces. Wear patterns quickly become con- fluent along the protocristid (sharp ridges connecting apices of proto- and metaconid). Wear is much less pronounced on the paracristid of the protoconid. A transversely elongated teardrop-shaped wear facet is formed on the very crest of the paraconid but the extent of wear is far less than that for the metaconid and protoconid. Talonid wear is by far the heaviest initially on the apex and projecting crests of the hypoconid. The apices of the entoconid and hypoconulid remain distinct until rather advanced wear stages for the

trigonid. Affinities.--The true dental formula and

phylogenetic affinities of Gallolestes pachymandibularis are as yet impossible to assess and a number of alternatives exist. Three of the more reasonable interpretations are discussed here and only the second do I consider unlikely. The genus is retained within the Order Insectivora purely for reasons of conservatism.

1. The species is a "typical" Cenozoic-style eutherian and the teeth in LACM 42633 are

P3-4M1-3 as used in the description above. Two difficulties exist with such an interpretation. First, the molariform "P4" is more heavily worn than "M,," a highly unusual situation in that the permanent M, usually erupts before the permanent P4 and is thus more heavily worn. Nevertheless, eruption of P4 before M1 does occur in rare cases among Cenozoic eutherians such as Mephitis mephitis and Conepatus sp. (see Slaughter, Pine, and Pine, 1974, Table 1, p. 116). The second difficulty is that if the tooth here designated as "P4" is indeed a P,, it would be far-and-away the most molariform lower premolar yet described from any Cre- taceous therian. Many have intuitively sus- pected that the evolution of Late Cretaceous eutherians involves tendencies toward molari- zation of posterior premolars through time from a more primitive simpler premolar design; to find such an "advanced" molariformity in "P4" in Gallolestes in sediments as old as the late Campanian would thus be rather startling. Certainly our ideas of directions of evolution in Late Cretaceous eutherian premolars are in need of reevaluation.

2. The species is a marsupial and the teeth in LACM 42633 should be interpreted as Pl3M-4. The increasing degree of wear from the first through the last molariform teeth certainly fits this interpretation as does the marked break in morphology between the rather simply constructed last premolariform tooth and the fully molariform "M,." However, the construction of the individual molars themselves are most unlike any Cretaceous marsupial known to me with their reduced, labially-set paraconids, the rather distantly set (for a marsupial) entoconid and hypoconulid, and the saliently-projecting hypoconulid on the terminal molar; the molars are typically eutherian in basic design.

3. The species is a eutherian, but with a dental formula that differs from that typical of Cenozoic kinds. Two possibilities come to mind and there are undoubtedly others equally prob- able. First, the species simply has four molars as were present in Deltatheridium (see Butler and Kielan-Jaworowska, 1973) and Deltather- oides (Kielan-Jaworowska, 1975b), both of which are known from older Late Cretaceous beds of Mongolia. As I discussed in another paper (1974), it seems quite unlikely that these two genera are true marsupials. The second possibility is that the tooth in LACM




42633 described above as "P4" may be in ac- tuality a paedomorphously retained molariform deciduous premolar, most likely dP,. The tooth is proportionately smaller relative to the defini- tive molars than are most molariform permanent P4's known from younger beds, thus suggesting the possibility of a deciduous nature; it is, however, strongly rooted be-lund the condition seen in most deciduous premolars.

The ideas discussed in "3" above border upon heresy from the point of view of our existing concepts of Late Cretaceous eutherian evolution. Nevertheless, when one considers the variability in dental formulae in Asiatic Late Cretaceous eutherians now being docu- mented by Kielan-Jaworowska (1975a; see also 1975b) and that seen even in North Ameri- can kinds (e.g. see Clemens, 1973, p. 25) they rather suddenly fall within the realm of possibility. The Late Cretaceous was, indeed, a time of major experimentation among eutherians in the evolution of dental formulae.

Gallolestes pachymandibularis shows no spe- cific relationships with any other known Late Cretaceous or early Tertiary therian. I be- lieve the species to be the only known repre- sentative of a hitherto undescribed family, although I refrain here from naming such because of the extreme question of affinities and because so few unequivocal features for family-level diagnosis are as yet available.

If LACM 42633 is to be interpreted as pos- sessing P3-4M1,3, the specimen does have some features that would be expected in the transi- tion between a typical Cretaceous "insectivore" and an early condylarth. Among these are: (1) robustly-developed (though small) molars with reduced paraconids; (2) relatively massive mandibular construction; (3) markedly projecting hypoconulid on M3; (4) approximation of hypoconulid toward the entoconid; and (5) wear function dominantly on the crests of the molars rather than shearing along the sides of the teeth. Similarities seem strongest with early Paleocene hyopsodontids such as Choero- claenus or Oxyacodon and much less with Protungulatum (the oldest known arctocyonid) or early periptychids such as Conacodon or Hemithlaeus. Even the similarities with the hyopsodontid condylarths, however, are not great; compelling arguments for the ancestry of the group through Gallolestes cannot be mustered. The major necessary modifications needed to derive any early condylarth from known parts of Gallolestes would be to at least partly demolarize the P4, markedly shorten the talonid length relative to the trigonid, lower

the relative molar cusp height, further reduce the paraconid development, and increase the overall size. Fossils showing such intermediate adaptations have not yet been recovered from any continent.

ACKNOWLEDGMENTS

Sincere thanks are specially offered to th( following individuals and institutions: Dr. Wil- liam J. Morris (Department of Geology, Oc- cidental College), for inviting me to participate in the El Gallo project; National Geographic Society, for its financial aid in the collecting phase of the project; Ing. Diego A. Cordoba M. (Instituto de Geologia, Universidad Nacional Autonoma de Mexico), for his generosity of interest and cooperation; Dr. David P. Whistler (Natural History Museum of Los Angeles County), for providing logistical support and permission to study the specimens; Dr. Zofia Kielan-Jaworowska (Polish Academy of Sci- ences), for allowing my access to manuscripts in press; Mr. Michael J. Novacek (Depart- ment of Paleontology, University of California, Berkeley), for aid in electron microscopy; Dr. Richard C. Fox (Departments of Geology and

Zoology, University of Alberta), for sugges- tions on interpretation of specimens; Dr. Wil- liam A. Clemens (Department of Paleontology, University of California, Berkeley), for allowing access to pertinent specimens for purposes of comparison; and my wife, Mrs. Bernice A. Lillegraven, for her many assis- tances in manuscript preparation.

REFERENCES

Butler, P. M., and Z. Kielan-Jaworowska. 1973. Is Deltatheridium a marsupial? Nature 245:105- 106.

Clemens, W. A., Jr. 1973. Fossil mammals of the type Lance Formation Wyoming. Part III. Eu- theria and summary. Univ. California Publ. Geol. Sci. 94:vi + 102 p.

Kielan-Jaworowska, Z. 1975a. Preliminary description of two new eutherian genera from the Late Cretaceous of Mongolia. Results Polish- Mongol. Palaeont. Exped.-Part VI, Palaeont. Polonica, No. 33:5-16.

1975b. Evolution of the therian mammals in the Late Cretaceous of Asia. Part I. Deltatheridiidae. Results Polish-Mongol. Palaeont. Exped.-Part VI, Palaeont. Polonica, No. 33:103- 132.

Lillegraven, J. A. 1969. Latest Cretaceous mammals of upper part of Edmonton Formation of Alberta, Canada, and review of marsupial-Placen- tal dichotomy in mammalian evolution. Univ. Kansas Paleont. Contribs., Art. 50 (Vertebrata 12), 122 p.

-. 1972. Preliminary report on Late Cretaceous mammals from the El Gallo Formation, Baja




California del Norte, Mexico. Nat. Hist. Mus. Los Angeles Co., Contribs. Sci., No. 232, 11 p.

1974. Biogeographical considerations of the marsupial-placental dichotomy. Annual Rev. Ecol. Syst. 5:74-94.

Morris, W. J. 1973. A review of Pacific Coast hadrosaurs. Jour. Paleontology 47:551-561.

Slaughter, B. H., R. H. Pine, and N. E. Pine.

1974. Eruption of cheek teeth in Insectivora and Carnivora. Jour. Mammalogy 55:115-125.

MANUSCRIPT RECEIVED MAY 28, 1974 REVISED MANUSCRIPT RECEIVED OCTOBER 23, 1974

San Diego State University Foundation contributed $450 toward publication of this paper.

EXPLANATION OF PLATE 1

FIGS. 1-3--Stereograms of Gallolestes pachymandibularis gen. et sp. nov. LACM 42633. Holotype. Right mandibular fragment with "P3-MI-3." 1, Labial view; 2, Occlusal view; 3, Lingual view. All x6.5.




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A New Genus of Therian Mammal from the Late Cretaceous "El Gallo Formation," Baja California, Mexico