Prolapsus, A Large Sciuravid Rodent: And New Eomyids From the Late Eocene of Trans-Pecos Texas

36 Pages·1991·10.1 MB·

by  Wilson, John Andrew

Pearce-Sellards Series
48
PROLAPSUS
,
A LARGE SCIURAVID RODENT
AND NEW EOMYIDSFROM
THE LATE EOCENE OFTRANS-PECOS TEXAS
JohnAndrew Wilson
and
Anthony C.Runkel
September 5,1991
TEXASMEMORIALMUSEUM,THE UNIVERSITYOFTEXASATAUSTIN
CONTENTS
Text 1
Illustrations 15
Tables 19
References 28
JohnAndrewWilson
VertebratePaleontology Laboratory
Texas MemorialMuseum
The University ofTexas atAustin
10100BurnetRoad
Austin, Texas78758
AnthonyC. Runkel
2300HowardStreet,NE
Minneapolis, Minnesota55418
ThePearce-SellardsSeriesisanoccasional, miscellaneousseriesofbriefreportsofMuseumandMuseumassociated
fieldinvestigationsandotherresearch. Allmanuscriptsaresubjected toextramuralpeerreviewbeforebeingaccepted.
Theseries titlecommemoratesthefirsttwodirectorsoftheTexasMemorialMuseum:Dr.J.E.Pearce,Professorof
Anthropology,TheUniversityofTexasatAustin,andDr.E.H.Sellards,ProfesssorofGeology,TheUniversityof
TexasatAustin. Bothprofessorsarenowdeceased.
©1991 by Texas Memorial Museum
The University ofTexas atAustin
All Rights Reserved
Printed in the United States ofAmerica
This ispublication No.-N.S. 35 of theTexas MemorialMuseum Notprinted with state funds
1991 Prolapsus: Sciuravidae
1
ABSTRACT
The questionablyhystricognathous and hystricomorphousrodent Prolapsus is assigned,
on thebasisofits dentalcharacters, to theFamily Sciuravidae. Thecomplete dentitionof
Prolapsus sibilatorisis now known butP. junction!sis still represented by isolated teeth
only. Prolapsus sp.ofWood(1973) is now identifiedasa speciesofPauromys. Prolapsus
isfound in the earlyUintan (Uinta B) WhistlerSquat local faunaand lateUintan(Uinta
C)Serendipitylocalfaunaatseveral localities inTrans-Pecos Texasand is closely related
to a new eomyid genus, Aguafriamys, here described, that occurs in sediments of
Duchesneanage inthesamearea. IfProlapsusis interpretedasbeing trulyhystricognathous
itfollows thatthischaracterhas evolved more than once. A new species of Yoderimysis
described from theCoffeeCup local faunaofChadronian age.
INTRODUCTION
Wood(1972, 1973)reported thediscovery of,and describedin detail, the Eocenerodent
Prolapsus from sedimentsthen assigned to the PruettFormationbutnow included in the
,
Devil’s GraveyardFormation (Stevens et al., 1984)of Brewster County, Texas. Wood
(1973) recognized thatthe teethofProlapsus stronglyresembled thoseofmembersof the
Family Sciuravidae but did not assign Prolapsus to that family because of the fully
hystricognathous character of the mandible. Later, without including Prolapsus in a
family. Wood (1975, p. 78-79) included the genus in the new infraorderFranimorpha
along withotherNorth Americanhystricognathsand later (Wood 1977, 1981)statedthat
a newly discovered skull (TMM41672-11)was hystricomorphous.
Since 1973,the Prolapsus sample has more than doubledand now includes two partial
skulls, several more lower jaws (figs. 1-3)and many isolated teeth. One ofthe partial
skulls, TMM 41672-11 isbeing studied byDr. A. E. Woodand is theonereferred toby
Korth (1984:7). Thispaper will onlyrefer toTMM41672-11 forpurposes ofdiscussing
thedentition so as not to interferewithWood’s studyofthemorphologyofthe skull. We
will mention, however, that the infraorbital foramen on the right side of the skull
fragment ofTMM41672-11 (fig. 2A)is incomplete on thedorsalmargin,but asprepared,
and judgingfrom the ventraland lateral edgesof theforamen, its diameterwas aboutthe
size ofMl. Following the suggestion of Ferrusquia-Villafranca (1989) this would be
considered a small infraorbital foramen. However, the process of the maxillary that
formstheexternal margin ofthe infraorbitalforamenis very thin and similarto thatfound
in the chinchilla. It is much larger than the infraorbital foramen in the cylindrodonts
Pseudocylindrodon texanus and Ardynomys occidentalis from the early Chadronian of
theVieja area.
The Prolapsus material that wasavailable toWood (1973) was collected from the basal
Tertiary conglomerate markerbed (localities TMM 41444 and 41443) and theWhistler
Squat quarry(41372). Theirstratigraphic positions in the Devil’s GraveyardFormation
are shownin Stevens etal.(1984:figs.5, 6). Thefaunasfrom thesethree localitieswere
combined by Wilson (1984) to form the Whistler Squat local fauna and are associated
withatuffthathasyielded adateof46.9± 1.0Ma(Henry etal., 1986:23).TwoProlapsus
2 Pearce-Sellards Series No.48
teeth (42953-6, P. sihilatorisand 42953-7, P. junctionis)collected from the basal Big
Yellow Sandstone Member, Canoe Formation, in the Tornillo Flat area of Big Bend
National Park are associated with afauna thatcorrelatesmost closely with theWhistler
Squat local fauna (Runkel, 1988). In addition, Runkel (1988) reported on a locality on
thenortheastflankofDogie Mountainfrom whicha numberofProlapsus teethwerere-
covered. This locality is directlybeneathanash datedat46.29 ±0.04 Maby theA 40/39
method at theUniversity ofCalifornia GeochronologyCenter.
Isolated Prolapsus teeth from the Serendipity locality (41745) and isolated teeth and
skull and jaw fragments from the Purple Bench locality (41672) are additions made
subsequenttoWood’s 1973paper. These localities arecombinedto form theSerendipity
local faunaand lie between rocks dated at43.9± 0.7 and 42.7 ± 1.6 Ma(Wilson, J.A.,
1986:360). TheSerendipitylocal faunawas assigned anage oflateUintan (Uinta C)by
Wilson (1986).
The new materialallows ustoreidentify two upperP4s, 41372-278and -297,assignedby
Wood to Prolapsus sihilatoris (1973:figs. 7A, B) as belonging to a new undescribed
genusand species and not toProlapsus. Also newly known sincethat paperis theM 3of
Prolapsus junctionis. Furthermore, a large number of isolated teethofWood’s (1973)
Prolapsus sp., the very small form, have been restudiedby Walton (1986) and are now
identified as Pauromys. Two new lower jawfragments are of eomyids, one of which
seemsto berelated toProlapsus.
Abbreviations All specimens ofProlapsus are currentlyfound in thecollectionsof the
-
Texas Memorial Museum(TMM). Specimennumbers withoutprefixesbelong toTMM;
such numbers preceded by a hyphen are abbreviated, and include the five digit locality
numberpreceding, e.g., 41372-1,-2. Detailed descriptionsoflocalitiesare onfileat the
VertebratePaleontology Laboratory,Balcones ResearchCenter, 10100BurnetRd., Aus-
tin, Texas78758-4497.
CM Carnegie Museum,Pittsburgh
FMNH Field Museum ofNatural History, Chicago
SDSM South Dakota School ofMines and Technology
1991 Prolapsus: Sciuravidae 3
Family Sciuravidae Miller and Gidley, 1918
Prolapsus Wood, 1973
Genotype Prolapsus sibilatorisWood, 1973
-
Referredspecies P. junctionsWood, 1973
-
Emended Diagnosis [modified fromWood(1973)] - DentalformulaI */,, C°/0, P2/j,M
V 3. Questionablyhystricognathous with angle arising entirely laterad of incisive alveo-
lus; lower premolar smaller than molars; lower cheek teeth with transversely elongate
mesoconid that may be single, double, or lophate; molar metalophid extending from
protoconid to rear surfaceofmetaconid,closing rear oftrigonidbasin; anteriorcingulum
with very small anteroconid or without anteroconid, usually a forward opening of
trigonid basin at one or both ends of anterior cingulum; lower teeth with partial or
complete crests from entoconidtoward hypoconid, with a cusp-like enlargement in the
center of the crest on some teeth; posterolophid reaching to or nearly to entoconid,
hypoconulids present orabsent; P3is a simplepeg,P4 withorwithout ahypocone;M 1and
M 2 quadrate, M 3 subtriangular; M 1 and M 2 with large and distinct hypocone, M 3 with
hypocone close to protocone; protoloph usually complete aftermoderate wear; anterior
cingulum large with or without protostyle; anterior arm of hypocone on M 1and M 2
widely separate fromprotocone and protoloph and continuous intocentral basin,almost
to paracone,forming apseudomesoloph; single ormultiple metaconulesconnecting with
hypocone in varying manners; strong posterior cingulum; prominent mesostyle or
mesostyles;lowercheekteethwithtworoots,oneanteriorand oneposterior;uppercheek
teeth with one lingual and two buccal roots; upper and lower incisors with flat anterior
face, enamel extending well ontolateralside; incisorenamel uncertain*.
Distribution. UintaBandC, middleEocene, Devil’s GraveyardandCanoeFormations,
-
Big Bendregion ofTexas.
Prolapsus sibilatois Wood, 1973
Figures 1-5,Tables 1-4.
Type. - TMM41372-179,right lowerjaw withM,.3, the alveolus ofP4, a broken incisor,
and a broken angularprocess.
Material. 41372-778(Wood’s -241C), I,; -299,LP -284,LM,; -291,RM,; -266,RM
- 4; 2;
-143,LM,; -265.RM,;-269,RM,;-381,RM,;41672-8,LP -M,;-9,RM,-M -14,RM,;-15,
4 2;
LM,; -85RM,, M,;-86. RM,; -106,leftlowerjawfragment with P-M,; -198,leftlowerjaw
4
fragment with P -M,; 41745-84,LP -58, RM, or M,; -89,LM, orM,; -142,LM, orM,;
4 4;
-272,RM,;-278,LM,orM,;-307,LM, orM 2;-308,LM, orM,;-309,LM, orM,;-310,LM,
orM,;-311.RM, orM,; -314,RM, orM,; -315,LM, orM 2; -317,LM, orM 2; -318,LM, or
M,;-319,RM, orM 2;-316,RM,;-371,RM,; -377,RM,; -378,LM,; -379,LM,;-380,RM,;
*Wood(1973.p. 22, 27)identifiedtheincisorenamelas pauciserial. In 1985(p. 481,482) hedescribeditas
“conditionintermediatebetweenpauciserialand multiserial.”ThomasMartin, UniversitatBonn, Germany
(personalcommunication),October 11, 1990 statedthatProlapsushaspauciserial enamel.
4 Pearce-Sellards Series No.48
41372-782(Wood’s -241G),RI1; -256,LM 1; -262,RM'; -285, RM 1; -300,LM 1; 252, RM2;
-263,LM2; -295,RM2; -304,LM2; 41672-3,skullfragmentwithRP4-M',LP3,alveolusofP4-
Ml lingual halfofM 2 -11,skullfragment withRLI, RP3-M2;-93,RM'-M2; 41443-70,LM 1
orM, 2;41745-40,RP4; -31,RM1orM 2;-60,RM 1orM 2;-134,RM 1orM 2; -147,LM' orM 2;
-282,RM 1orM 2;-283,RM 1 orM2;-284,RM 1 orM2;-285,RM1 orM2;-286,RM 1or M 2;
-289,LM 1or M 2;-290,LM1 orM 2; -292,LM 1orM 2; -294,LM' orM 2; -295,RM 1orM 2;
-297,RM1 orM 2; -298,RM 1orM 2; -299,LM1 orM 2;-300,LM' orM 2; -301,LM1 orM 2;
-302LM1 orM 2; -303,LM1 orM 2; -305,LM3;41549-3,RM3; 42953-6,LM1 orM 2.
Stratigraphic position. Lower member of the Devil’s Graveyard Formation, basal
-
Tertiary conglomerate(41443), WhistlerSquat quarry(41372), AguaFria area; Hannold
Drawarea. BigYellowSandstone Member, CanoeFormation, CrusherSection, BigBend
NationalPark(42953). Middlemember Devil’s GraveyardFormation, Purple Bench lo-
cality (41672), Serendipity locality (41745), Margaret’s Bonebed (41549), Agua Fria
area.
Description. The mesoconidson P. sihilatorisfrom theSerendipity andPurple Bench
-
localities show more variability than those from the Whistler Squat quarry. From the
former localities it is usually a single but lophate crest with only three out of 24
specimens showing double cusps. The lophatemesoconid may show a short and weak
connection with the protoconid; it may also have a short, weak connection with the
hypoconid or thehypolophid, or itmay not connect with any othercusp. On theM 3the
mesoconidconnects withthe entoconidon threespecimensanddoesnot connectwith the
entoconid on three others. Of seventeen isolated lower first or second molars from
Serendipity locality, 12 have a complete hypolophid with no hypoconulid, two have a
very small hypoconulidoncomplete hypolophids, two have incompletehypolophids with
no hypoconulids, and onehas a complete hypolophid withahypoconulid. Obviously, the
above characters arevariable withina sample from one locality. The hypoconulid is rare
on specimens from the Serendipity locality. The connection of the crest from the
entoconidto themesoconidispresent in threeof24teethfrom theSerendipityand Purple
Bench localities.
The type lowerjaw, 41372-179,doesnot haveaP4. Wood(1973) usedaP4ofP.junctionis
(41444-1) and a fragmentof a P ofP. sihilatoris (41372-299) for the basis of his de-
4
scription. An uneruptedP4is present on 41672-106 (fig. 1)and a fully erupted one on
-198. There is ananteriorcingulum on theP.junctionis P (41444-1)but there is onlya
4
very shortanteriorcingulum and ananteroconid oneach ofthesame teeth at the Purple
Bench and Serendipity level. A hypoconulid is present on 41672-106; otherwise, the
description ofWood (1973:24-25) is applicable toP. sihilatoris.
AP3ispresent on41672-11 (figs. 2A,B, C) andisrepresented byalveoli on41672-3(fig.
3). It is safeto assume thatskulls ofProlapsus in theearlierWhistler Squat local fauna
would also haveP3 although none is presently known.
,
It must be pointed out that in figure 8 ofWood (1973) the explanation for "F" was
inadvertently omitted and shouldread: F. LP4 41444-26. Also in the same figure, H.
,
41443-30 is not aLM3ofProlapsus junctionis, but isfrom thesame undescribed rodent
as theP4 infigure 7.
1991 Prolapsus: Sciuravidae 5
are
On 41672-3 the internalportions ofM 2 and M 3 preserved. Although the M 3 isworn,
there is still a prominentanteriorcingulum thatends lingually opposite the middleof the
protocone. In the undescribed rodent 41443-30 the anterior cingulum continues to the
anterointernal corner ofthetooth. On the M 3 ofProlapsus theprotocone is separated
from the hypoconeby a medianvalley, as inM 1and M and is stillpresent on the worn
2,
tooth. The hypocone is aprominent cusp. An isolated M 41745-305, isreferred to P.
3,
sihilatorisand is thebasisfor the following description.
TheM 3 ofP. sihilatorishas aprominent anteriorcingulum thathas its lingual endat the
anterobuccal corner of theprotocone. The protocone is larger than thehypocone and
separated from itby a prominent valley. The internal valley isalso present on 41672-3
whereonly theinternal halfofM 3 ispreserved. Theprotoloph iscomplete in41745-305
but this is a slightly worn tooth so it is not possible to distinguish a protoconule. The
metacone is smallcompared to the paracone. A short lophextendsfrom theposition of
the metaconule to the posterior cingulum enclosing a posterolingual basin. Specimen
41745-305 doesnot havenearly as complicated apattern as 41443-30 thatwas identified
as theM 3 ofProlapsus byWood(1973;fig. 8H).
Prolapsus junctionis
Figures 4, 5, Tables 5-8.
Type. TMM 41444-62,isolated RMj.
-
Material. TMM41444-1,RP -116,LMj;-59, LM -61,LM 75,LM 41443-363,RP
- 4; 2; 3; 3; 4;
-433,LP -33, LM,; -68, RM,; -71,LM,; -75,RM,; -141,RM,; -222, RM,; -371,RM,;
4;
-381,RM,; -417, RM,;-491. RM,;-496,RM,; -507, RM -557, RM -566,LM 41745
2; 2; 2;
-471,LM,; -472,LM,; -277, LM -322, LM ;-273, LM -276,LM -279,LM -280,
2; 2 3; 3; 3;
LM3; 42952-103,RP4; -82,LM,; -83,LM1,; -81,RM, orM 12; -80,RM, o1 r-2M5”,2;41444-187
(Wood’s -568),LI1; -26, LP4; -69,LM orM 2; -104,LM ; -115,LM ; RM2; -164,
RM2;-168,LM2 41443-545.RdP4;-560, LdP4;-66, LMF-434, RM1 orM2;-529, RM1
;
orM 2; -54,LM2;-65.RM2; -73, LM2;-74, LM2; -78, LM2; -215,RM2; -324,RM2; -396,
RM2; -542, RM2; -445, RM3; 41745-72,LM 1; -136,LM 1 orM 2; -269,LM 1; -270. RM1;
-110,LM2 -266.LM2 -267,LM2 -366,LM3 42952-67,LM 1 -68,LM1 -69. RM1 -70,
; ; ; ; ; ; ;
RM 1; -73,LM 1 orM 2;-74, RM 1 or M 2; -71, LM2; -72.RM2; 42953-7, RM 1 orM 2.
Stratigraphic position. Lower member of the Devil’s Graveyard Formation, basal
-
Tertiary conglomerate (41443, 41444),AguaFria area; Dogie Mountain(42952); middle
member of the Devil’s GraveyardFormation, Serendipity locality (41745), Agua Fria
area.
Emended diagnosis. Smaller than P. sihilatoris. Mesoconid prominent and lophate;
-
hypolophid extends toward hypoconid in earlier members of species and connects in
Serendipity members;entoconidmay connect toposterolophid; pseudomesolophextends
fromthehypocone tonear thecenter oftheuppermolars, mesostylesmall;protostyle not
a distinctcusp.
6 Pearce-Sellards Series No.48
Description. Size is the main distinctionbetween P. sibilatorisandP. junctionis. The
-
scatter diagramsinfigures 4and 5 imply that thereare two distinctpopulations. I here is
no apparent change in size within either the large or the small group upward in the
section. However, P. junctionis is much moreabundantin thebasalTertiary conglomer-
ate; only a singletooth,41443-70LM 1orM is identifiedasP. sibilatoris. The latteroccurs
2,
only at theWhistlerSquat quarry level.
The two species are not as distinctmorphologically attheWhistlerSquatlevelas implied
by Wood(1973), who describedProlapsus sibilatoris, the larger species, as having well
developed mesoconids, complete or nearly complete hypolophids, complexities in the
uppercheek teethand a well developed pseudomesoloph.
According toWood(1973), thesmallerspecies,P. junctionis,has mesoconids thatextend
less than half-way across the lower teeth, hypolophids that are never complete, and
entoconids that may connect to the posterolophid. The pseudomesoloph is broadly
interruptedin the center oftheuppermolars, themesostyle is small, andthe protostyle is
a distinct cusp. Additional differences noted in this study are that themesoconid in P.
sibilatorisusually occursas two slightly separatedcusps that connect with wear, while
themesoconidofP. junctionisiseithera single cusp or, morecommonly,acomplete loph.
In addition, some specimens ofP. sibilatorishavea small cusppartially separatedfrom,
and locatedimmediately lingual to, the metacone.
Although these character distinctions are valid in general, they are not consistently
present in thesample. The morphologic similaritiesbetween thesetwo species are more
readily apparentthanthedifferences,andacomparison ofM 2 in thetypesofP. sibilatoris
(TMM 41372-179) andP. junctionis(TMM 41444-59) demonstrates that some of these
specimens are nearly identical. The mesoconiddoes infact extend further than halfthe
widthofthelowermolars insomespecimens ofP. junctionis(e.g.,LM TMM41444-59).
2
The mesoconidofP. junctionis inTMM41443-557(RM2) extendstonearly onehalfthe
lengthofthetooth, and thehypoconulid is complete,although itnarrows and shallowsas
itapproachestheentoconid; thiscondition is seen inmostoftheP. sibilatorisspecimens
as well. The hypolophid is no morecomplete on P. sibilatorisM 2 (e.g.,TMM41372-179
and TMM 41372-266) than is the hypolophid of that tooth in most specimens of P.
junctionis.
TheM 2 ofP. sibilatoris(TMM41372-295) andP.junctionis(TMM41444-168) alsoshows
thesimilaritiesbetween the two species. The mesostyle is not consistently larger in P.
sibilatoris specimens. The extensionof the pseudomesoloph and theprominence ofthe
protostyle are highly variable inboth species.
Although thesizebimodality (figs. 4 and 5) remains distinct, specimensfrom higher in
thesection display subtle changes in the toothmorphology ofboth species thatresult in
an even greater similaritybetweenP. sibilatorisandP. junctionis.In the uppermolarsof
bothspecies the pseudomesoloph, extendingfromthe hypocone,is morepronouncedand
extendsfurther towardthe central part ofthetooth. The mesostyle is generallyabsent or
quite small; the protoloph is better developed, and fully or nearly connected to the
paracone; theanteroloph is reduced, and theposteroloph andpseudomesoloph are more