UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

- --------------------------------------------------------------------------------

                                  SCHEDULE 13D

                    Under the Securities Exchange Act of 1934
                               (Amendment No. 1)1

                       Pioneer Natural Resources Company*
                       ----------------------------------
                                (Name of Issuer)

                     Common Stock, par value $.01 per share
                     --------------------------------------
                         (Title of Class of Securities)

                                    723787107
                                    ---------
                                 (Cusip Number)

                            Scott B. Bernstein, Esq.
                               Caxton Corporation
                              315 Enterprise Drive
                              Plainsboro, NJ 08536
                                 (609) 936-2580
                                 --------------
                  (Name, Address and Telephone Number of Person
                Authorized to Receive Notices and Communications)

                                 August 7, 1997
- --------------------------------------------------------------------------------
                      (Date of Event which Requires Filing
                               of this Statement)

     If the filing  person has  previously  filed a statement on Schedule 13G to
report the acquisition  which is the subject of this Schedule 13D, and is filing
this schedule because of Rule 13d-1(b)(3) or (4), check the following box [ ].


- --------
1    The  remainder  of this  cover  page  shall be filled  out for a  reporting
persons's  initial  filing on this form with  respect  to the  subject  class of
securities,  and for any subsequent amendment containing information which would
alter disclosures provided in a prior cover page.

The information required on the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the  Securities  Exchange  Act of
1934 ("Act") or otherwise  subject to the liabilities of that section of the Act
but  shall be  subject  to all other  provisions  of the Act  (however,  see the
Notes.).

*    Pioneer Natural Resources Company is the successor in interest to MESA Inc.
("MESA").  The initial  Schedule 13D was filed with respect to the Common Stock,
$.01 par value, of MESA.

                                Page 1 of 5 Pages


<PAGE>


                                  SCHEDULE 13D


CUSIP No.  723787107                              Page    2    of   5   Pages
           ---------                                   -------    -----


- --------------------------------------------------------------------------------
1    NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

     Caxton International Limited
- --------------------------------------------------------------------------------
2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*              (a)  [ ]
                                                                    (b)  [ ]

- --------------------------------------------------------------------------------
3    SEC USE ONLY


- --------------------------------------------------------------------------------
4    SOURCE OF FUNDS*

     WC
- --------------------------------------------------------------------------------
5    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
     ITEMS 2(d) or 2(e)                                                   [ ]

- --------------------------------------------------------------------------------
6    CITIZENSHIP OR PLACE OF ORGANIZATION

     British Virgin Islands
- --------------------------------------------------------------------------------

        NUMBER OF             7    SOLE VOTING POWER

         SHARES                    0
                         -------------------------------------------------------
      BENEFICIALLY            8    SHARED VOTING POWER

        OWNED BY                   667,401
                         -------------------------------------------------------
          EACH                9    SOLE DISPOSITIVE POWER

        REPORTING                  0
                         -------------------------------------------------------
         PERSON               10   SHARED DISPOSITIVE POWER

          WITH                     667,401

- --------------------------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

     667,401
- --------------------------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES*
                                                                    [ ]

- --------------------------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

     .9%
- --------------------------------------------------------------------------------
14   TYPE OF REPORTING PERSON*

     CO
- --------------------------------------------------------------------------------
                      *SEE INSTRUCTIONS BEFORE FILLING OUT!




<PAGE>



                                  SCHEDULE 13D

CUSIP No.  723787107                              Page    3   of   5   Pages
           ---------                                   ------    -----


- --------------------------------------------------------------------------------
1    NAME OF REPORTING PERSON
     S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

     Bruce S. Kovner
- --------------------------------------------------------------------------------
2    CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*              (a)  [ ]
                                                                    (b)  [ ]

- --------------------------------------------------------------------------------
3    SEC USE ONLY


- --------------------------------------------------------------------------------
4    SOURCE OF FUNDS*

     Not Applicable
- --------------------------------------------------------------------------------
5    CHECK BOX IF DISCLOSURE OF LEGAL PROCEEDINGS IS REQUIRED PURSUANT TO
     ITEMS 2(d) or 2(e)                                                   [ ]

- --------------------------------------------------------------------------------
6    CITIZENSHIP OR PLACE OF ORGANIZATION

     United States
- --------------------------------------------------------------------------------

        NUMBER OF             7    SOLE VOTING POWER

         SHARES                    0
                         -------------------------------------------------------
      BENEFICIALLY            8    SHARED VOTING POWER

        OWNED BY                   667,401
                         -------------------------------------------------------
          EACH                9    SOLE DISPOSITIVE POWER

        REPORTING                  0
                         -------------------------------------------------------
         PERSON               10   SHARED DISPOSITIVE

          WITH                     667,401

- --------------------------------------------------------------------------------
11   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

     667,401
- --------------------------------------------------------------------------------
12   CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (11) EXCLUDES CERTAIN SHARES* 
                                                                   [ ]

- --------------------------------------------------------------------------------
13   PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (11)

     .9%
- --------------------------------------------------------------------------------
14   TYPE OF REPORTING PERSON*

     IN
- --------------------------------------------------------------------------------
                      *SEE INSTRUCTIONS BEFORE FILLING OUT!




<PAGE>



     This  Amendment No. 1 amends the Schedule 13D filed with the Securities and
Exchange  Commission on February 26, 1997 (the "Schedule  13D") by the Reporting
Persons,  which  Schedule  13D related to the Common  Stock,  par value $.01 per
share ("MESA Shares"),  of MESA Inc., a Texas corporation ("MESA"). On August 7,
1997,  MESA merged (the  "Merger") with and into a subsidiary,  Pioneer  Natural
Resources Company, a Delaware corporation  ("Pioneer").  Pursuant to the Merger,
MESA Shares and shares of MESA's  Series A 8% Cumulative  Convertible  Preferred
Stock  ("Preferred  Shares") were converted into shares of Pioneer Common Stock,
par value $.01 per share ("Pioneer Shares").

Item 5.  Interest in Securities of the Issuer.
- -------  -------------------------------------

     The  information  set  forth  in Item 5  ("Interest  in  Securities  of the
Issuer"),   subparagraph   (a)  of  the  Schedule  13D  is  hereby  amended  and
supplemented by adding the following information.

     Pursuant  to  the  Merger,  2,137,448  Preferred  Shares  owned  by  Caxton
International  Limited were converted into 381,601  Pioneer Shares and 2,000,000
Mesa Shares were converted into 285,800 Pioneer Shares.  Accordingly,  as of the
close  of  business  on  September  3,  1997,   Caxton   International   Limited
beneficially   owned  an  aggregate  of  667,401  Pioneer  Shares   representing
approximately .9% of the total Pioneer Shares outstanding.

                                Page 4 of 5 Pages




<PAGE>



                                    SIGNATURE
                                    ---------

     After reasonable inquiry and to the best of the undersigned's knowledge and
belief, the undersigned certify that the information set forth in this statement
is true, complete and correct.

     Dated:  September 4, 1997



                                             CAXTON INTERNATIONAL LIMITED


                                             By:            /s/
                                                -------------------------------
                                                Nitin Aggarwal
                                                President


                                             By:            /s/
                                                -------------------------------
                                                Maxwell Quin
                                                Secretary


                                                           /s/
                                             ----------------------------------
                                             Bruce S. Kovner, By Peter
                                             D'Angelo, Attorney-In-Fact


                                Page 5 of 5 Pages





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